AUdIoCoUrSeS
Joined: 31 Oct 2002
Posts: 2014
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1. How does the data buffer of a Minidisc player facilitate editing
2. Explain why a CD –R Master recorded using “Disc at Once” is more likely to be accepted for mastering by a CD pressing plant (over alternative methods).
3. Explain why the playback data rate of some disc based systems can be considerably higher than the maximum recording data rate.
4. State and describe the FOUR possible actions of the error correction system of a CD player.
5. Explain the two basic methods that a Hard Disk editor may use to create a cross fade.
6. What is the main advantage of the oversampling process with regard to the design of an anti alias filter of an analogue to digital converter?
7. Explain the operation of a 18 bit 4x oversampled DA converter on a CD player.
8. What type of reduction does DCC use that may keep this medium in the domestic market only?
9. Calculate the maximum theoretical audio frequency and the signal to quantisation noise ratio in dB for the following systems:
a) Compact disc digital audio system.
b) A system utilising a sampling rate of 26KHz and 12 bit linear quantisation.
10. What is one of the main advantages of a Rotary-Head Recorder over a stationary head?
11. What is a phased-locked loop?
12. Explain the principles of error concealment.
13. Explain how interleaving works.
14. Explain the limiting parameters of a hard disk drive that restrict operation in a digital audio workstation.
15. Why are some D/A converters in CD described as 18 bit operation if the CD data is 16 bit?
16. Describe the operation and the sections on a CD-Recordable WORM disk.
17. What is a RAID? What is its purpose?
18. Detail the differences between CAV and CLV optical disk drives.
19. Outline the principles of magneto-optical drives.
20. List three types of DACs.
21. A 20 bit DAC has how many levels? (16 bit = 65,536)
22. What is the SCSI system used for in A/V workstations? Outline SCSI addressing.
23. List some typical SCSI problems.
24. Shifting a sample word one step to the left or one step to the right will change the gain in dB of the sample by how much?
25. List some uses of digital filters.
26. Explain FIR and IIR.
27. The result of multiplying two binary numbers together is to create a new sample word which may have many more bits than the original, and it is common to find that digital mixers have internal structures capable of handling 32 bit words or more, comment including the word dither.
28. What is a Parity bit?
29. Comment on Improving SQNR with oversampling.
30. What is The 1630/Video U-Matic system?
31. Describe the The data pickup of a CD player.
32. Comment on The Focus and tracking system in a CD player.
33. Comment on CD master PQ editing.
34. What is CD master tape verification?
35. What is Eight Fourteen Modulation?
36. What is a Successive Approximation Register converter?
37. Describe and explain the following digital audio concepts:
• Jitter
• Simple codes
• Group codes
38. Outline Digital audio in video recording systems.
39. What is Solid state recording?
40. What is a Session file?
41. Discuss the Archiving of digital recordings.
42. Comment on some features, functions and parameters of disk recording systems .
_________________
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Tue Sep 20, 2005 4:36 am |
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rachelh
Joined: 16 Jan 2005
Posts: 35
Location: Trinidad WI |
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1. How does the data buffer of a Minidisc player facilitate editing
Minidisc [MD] has RAM [Random Access Memory] whose capability allows for about 3 seconds of data/ audio to be buffered. The buffer itself is an area in memory that stores temporary data that is being used for data transfer, the larger the buffer the faster data can be processed. The data rate from MD exceeds what audio decoders need. Procedure wise, when the RAM has reached capacity the disk drive stops sending out data but keeps on oscillating, the data itself it transmitted to the decoders via bursts of data that leaves a ¾ time period of inactivity from the disk. Keeping this in mind, the data buffer of a Minidisc player facilitates editing as this time lapse allows the disk to reposition itself between data transfers so data can be edited. [1]
2. Explain why a CD –R Master recorded using “Disc at Once” is more likely to be accepted for mastering by a CD pressing plant (over alternative methods).
A CD-R Master recorded using Disk at Once or Single Session is more likely accepted for mastering by a CD pressing plant because the disk is recorded uninterrupted, the data is written in one session and then the table of contents is written after which the disc is closed. In the duplication process this format will be followed, as it is fast and efficient and the laser does not have to recalibrate [which saves time]. Track at once and Multi Session are not as economical for the means of mastering at a pressing plant as the laser is recalibrated due to tracks being individually written which resultantly is not time efficient.
3. Explain why the playback data rate of some disc based systems can be considerably higher than the maximum recording data rate.
The playback data rate of some disc-based systems can be considerably higher than the maximum recording rate because during playback the full buffer reserve is used for this process which in turn is not as consumptive of system resources as recording is.
4. State and describe the FOUR possible actions of the error correction system of a CD player.
Four possible actions of the error correction system of a CD player are as follows:
i. Using the C1 system redundancy before interleaving. This system works by correcting single symbol errors, which allows for interleaving to occur without error. C1 also generates flags for the C2 system when large errors are encountered.
ii. Interleaving is the process by which errors which occur during bursts are spread amongst different code words, which results in error reduction. However errors due to small random errors occurring next to burst errors can impair the system.
iii. Using the C2 system redundancy after de-interleaving. The C2 decoder is used to detect and correct a single symbol error or to correct two symbols with known locations. C2 is applicable when C1 has miscorrected an error – it is second in line.
iv. Muting is the process of setting the value of the uncorrected or missing digital words to zero. Simply reducing the level to zero will produce a click thus; the level has to be gradually brought down- this is often done via the use of ‘multiplying sample values by descending coefficients, usually in the form of a half-cycle of a cosine wave’ [1]. Muting is the next step in error correction that is if the rate of the bad C2 codewords is excessive which results in the correction system being overwhelmed.
[1] [2]
5. Explain the two basic methods that a Hard Disk editor may use to create a cross fade.
The two basic methods that a Hard Disk editor may use to create a cross-fade are:
i. fade out – to proportionally decrease the gain to zero
ii. fade in- to proportionally increase the gain to the required level or full gain
A cross fade itself is a fade in and a fade out which overlaps.
6. What is the main advantage of the oversampling process with regard to the design of an anti alias filter of an analogue to digital converter?
The main advantage of oversampling with regard to the design of an anti alias filter of an analogue to digital converter is that oversampling is actually used to improve anti-aliasing filter characteristics. That is, oversampling increases the accuracy of the anti-alias filter.
7. Explain the operation of a 18 bit 4x oversampled DA converter on a CD player.
An 18-bit 4x oversampled DA converter on a CD player operates by:
If 18 bit corresponds to a sampling rate of 44.1 KHz then “18-bit 4x oversampled” is the same as 4 times the sampling rate … since in the oversampling process the sampling rate is multiplied by a specific factor: 44.1 kHz = 176.4 kHz. So the operation of such an oversampled DA converter on a CD player would allow for better playback due to a higher sample rate.
8. What type of reduction does DCC use that may keep this medium in the domestic market only?
DCC or Digital Compact Cassette is a tape format with a stationary head that plays analogue Compact Cassettes [it is backward compatible] as well as their digital counterpart [digital recordings can also be made]. The type pf reduction used by DCC is Precision Adaptive Sub-Band Coding [PASC], which is based on MPEG audio compression. PASC operates by reducing the recorded data rate to about one quarter of the PCM. Due to PASC tape duplication can be carried out at a fast rate due to the linearity of the tracks, which is very attractive for the domestic market as Record Companies have favour with the DCC tape format. [1]
9. Calculate the maximum theoretical audio frequency and the signal to quantisation noise ratio in dB for the following systems:
Audio Frequency = sample rate / 2
SQNR = 6 x bit rate
a) Compact disc digital audio system.
A compact disc operates at a rate of 44.1 kHz at a sampling arte of 16 bits
Hence,
Audio Frequency = 44.1KHz / 2
Thus the maximum theoretical audio frequency = 22.05 KHz
[whereas the actual audio frequency = 20 kHz]
SQNR= 6 x 16
Thus the signal to quantisation noise ratio = 96 dB
b) A system utilising a sampling rate of 26 kHz and 12 bit linear quantisation.
Sampling rate = 26 kHz, sampling rate = 12 bit
Audio Frequency: 26 kHz / 2
Thus the maximum theoretical audio frequency = 13 kHz
SQNR= 6 x 12
Thus the signal to quantisation noise ratio = 72 dB
[7]
10. What is one of the main advantages of a Rotary-Head Recorder over a stationary head?
The main advantages of a Rotary-Head Recorder over a Stationary – Head Recorder are as follows:
i. More information can be written on the rotary head as info is written diagonally across the width of the tape as opposed to parallel as a stationary head would write data
ii. The data density of recording PCM requires the rotary head as recording such information with a stationary head would be impossible
iii. Only a small portion of tape is in contact with the head at any one time
iv. Data can be retrieved at a faster rate than with a stationary head
11. What is a phased-locked loop?
A phased-locked loop or PLL is an electronic circuit equipped with an oscillator that locks onto the frequency of the input signal that is, it adjusts its value to match the signal and maintain a set frequency. PLLs take a small amount of time to adjust when the input signal changes and can act as a source of stabilization for a signal that fluctuates. PLLs can also be used to generate, modulate, demodulate, reconstitute [with noise], multiply or divide a signal. [3]
12. Explain the principles of error concealment.
Error concealment is the process by which the value of a missing sample is estimated from using the samples closest to it. Concealment can be audible if it is frequent; this is due to the fact that the concealment approximation value may not be the exact value of the missing sample so the transition from known to unknown [concealed] sample may be audible. This system works by separating odd number samples from their even counterparts pre sampling, and they are in turn recorded in separate places so, if there is an error, it would affect one section and not the other. During playback, the samples form back together and play in order. So if there was an error the erroneous sample would be split up and scattered and half of the waveform itself would be illustrated so that there is a loss of accuracy but the audio itself was salvaged. [1]
13. Explain how interleaving works.
Interleaving is a method of digital data error reduction. This system works by distributing and intermingling successive bits over a wide area on the storage media. Hence, scattering potential error sources which prevents them from being consecutive on playback. [3]
14. Explain the limiting parameters of a hard disk drive that restrict operation in a digital audio workstation.
The limiting parameters of a hard disk drive that restrict operation in a digital audio workstation are as follows:
i. Memory: Cache memory is a relatively small memory, which is located close to the calculation circuits of the processor. This memory is a form of RAM [read only memory] which is fast enough to keep up with bus speeds, cache is essential as it houses data that has been currently used with the precedent that data that is recently used will be needed again in the near future. Cache memory can be essential in DAW as it can house recently edited clips that were placed on the ‘clip board’ as it would not take long to retrieve the information thus increasing the options available to DAW users. [3]
ii. Disk Speed: RPM refers to revolutions per minute and defines the number of times per minute a computer’s hard drive would make complete revolutions. A faster rotational speed is associated with faster data retrieval. A high RPM would be beneficial in DAW as it would allow for fast data retrieval which would enhance the user’s experience. [3]
iii. Level of fragmentation [which also influences disk speed], when the drive is fragmented data is not stored contiguously so it takes a while for the disk heads to find and access the data. Which in turn slows down the process.
15. Why are some D/A converters in CD described as 18-bit operation if the CD data is 16 bit?
Some D/A converters in CD are described as 18-bit operation if the CD data is 16 bit because in practice 16 bit linear conversion is insufficient for 16 bit data. Hence the data converters themselves must have a greater dynamic range than the signal itself. The reliability of the conversion process depends on the linearity of the converter. [2]
16. Describe the operation and the sections on a CD-Recordable WORM disk.
WORM or Write Once Read Many is the process by which a laser disc is recorded and once this recording has been made the disc cannot be written on again- just read. More recently CD-R [CD Recordable] has been introduced in place of WORM. The disk may contain a thin sheet of metal on which a laser melts spots during the recording process, a hole in the metal with a thickened rim forms on the metal due to surface tension, subsequently, a low level laser can read the disk because the metal reflects the light and the holes passes the light through. [1]
17. What is a RAID? What is its purpose?
RAID or Redundant Array of Independent Disks simply denotes multiple drives, which do the work of one drive but with a level of redundancy. ‘It was originally developed as a way to organize small format disk storage devices to dramatically increase input/output (I/O) speed and improve data availability’ [3]. RAID –1: consists of two drives that store identical data. So, if one drive fails, the other will continue to function so there is no time lost for the user. RAID –1 however is not a viable means of storing data so; RAID-3 and RAID-4 and RAID-5 were devised whereby they stripe parity data across multiple drives. RAID-3 and RAID-4 store all parity data on a single drive so if the disk fails this parity data can be used to reconstruct the lost data. On the down side, the performance of RAID can be slower than one single drive at the reading and writing phases –this depends on the RAID type used. However, increased bandwidth and reliability is possible in a configured RAID system. [3]
18. Detail the differences between CAV and CLV optical disk drives.
Constant Angular Velocity (CAV) Constant Linear Velocity (CLV) are means of which data can be read or written from a rotational disk. The differences between the two mediums are as follows:
· Using an LP and a CD and contrasting them, the LP always spins at an angular rate [CAV] of 33 rpm contrasted to an CD in which the speed of the motor falls from 495 to 212 rpm [CLV]
· The surface of the LP passes over the needle at a slower linear rate as the needle makes its way toward the centre of the disk [CAV] whereas in CLV the read head moves away from the centre of the disk past the read head at a constant linear velocity.
[5]
19. Outline the principles of magneto-optical drives.
A magneto-optical drive is one, which utilises optical properties of a magnetized platter upon which data is stored magnetically, but read optically, using a laser.
20. List three types of DACs.
Three types of Digital to Analogue Converters are:
i. The weighted-resistor DAC. This DAC is rarely used in practice due to the complexicity of manufacturing resistors with sufficient accuracy.
ii. The R-2R ladder DAC. This DAC is efficiently manufactured and has a low distortion ratio.
iii. Zero-cross distortion. This DAC is the most accurate of the three.
[2]
21. A 20 bit DAC has how many levels? (16 bit = 65,536)
16 bit = 65 536 = 2 to the index of 16
Hence 20 bit = 2 to the index of 20 =1 048 576
Hence a 20 bit DAC has 1 048 756 levels
22. What is the SCSI system used for in A/V workstations? Outline SCSI addressing.
The SCSI system used for A/V workstations is??
SCSI addressing is defined below the terms in the brackets ate used to represent the name conventions use by the device pseudo file system [devfs]. Also Bus is used in preference to channel:
· SCSI adapter number [host]
· Channel number [bus]
· ID number [target]
· LUN –Logical Unit Number [lun]
[6]
23. List some typical SCSI problems.
Some typical SCSI problems are:
i. Up to eight devices inclusive of the host can be applied on a SCSI bus
ii. A terminator is needed at the end of the bus
iii. SCSI numbers have to be manually set
iv. No two devices on the bus can have the same SCSI ID
v. Hot swapping is not allowed, all devices on the bus must be turned off before cables can be disconnected or connected
vi. A fault in one device can cause the entire system to malfunction
vii. The length of the bus is to be no longer than 5 to 6 meters; the cables themselves are thick and heavy which proves another problem.
24. Shifting a sample word one step to the left or one step to the right will change the gain in dB of the sample by how much?
If a sample word were changed one step to the left or right the gain of the sample would change by ???
One quantization step = 3.0467 V
25. List some uses of digital filters.
Digital filters are used to remove high frequency content from an audio signal, an anti aliasing filter is one such low frequency filter that is used for this said purpose. Oversampeling filters are used to interpolate the signal using the impulse response system. A finite response filter is one in which the impulse response uses a finite number of points – it is used to extend the Nyquist frequency.
[2]
26. Explain FIR and IIR.
FIR or Finite Impulse Response Filter is one in which the filter operates on a finite number of samples which are known as impulse response. This impulse response has a finite duration. The FIR filter works by looking back in time at a fixed number of previous samples in order to retain a result. FIR filters can be constructed so that no phase non-leniarity would occur, hence, this type of filter is used when there is to be no effect on the signal with respect to phasing. [2][3]
IIR or Infinite Impulse Response filter is one in which the impulse response is finite and there is a level of feedback within the filter. ‘ an IIR filter can have both poles and zeros, can introduce phase shift and can be unstable if one or more poles lay outside of the unit circle’. IIR filters are incapable of creating linear phase because its impulse response is asymmetrical. Problems associated with IIR are that they are slower to implement during fixed point arithmetic, they do not facilitate decimation and interpolation and they are more susceptible to problems of finite length arithmetic such as noise generated by calculations. [2][4]
27. The result of multiplying two binary numbers together is to create a new sample word which may have many more bits than the original, and it is common to find that digital mixers have internal structures capable of handling 32 bit words or more, comment including the word dither.
White noise- a signal that includes a random distribution of all frequencies at equal levels across the spectrum, introduced in small amounts can reduce the signal-to-error and distortion in a process called dither. With the introduction of dither it is possible to encode signals that are smaller than the Least Significant Bit/ LSB of digital word that is, it is possible to encode signals less than a single quantization step. The process of introducing dither into the conversion process yields better results than if the quantization distortion that would otherwise result was left as is. [7]
28. What is a Parity bit?
A parity bit is one that is created by another bit element due to data being lost or altered when it is moved from one place to the other. [3]
29. Comment on Improving SQNR with oversampling.
SQNR or signal to quantization noise ratio is a measure of the quality of quantization. It is defied as a normalised signal power which is devided by a quantization noise signal. SQNR is measured in dB and approximates to 6 times the number of bits in the analogue to digital conversion process. [3]
30. What is the 1630/Video U-Matic system?
The U-Matic system is a tape format that degrades slightly every time it is used. The maintenance of a U-Matic machine is expensive, for many years this system was the worldwide-accepted standard for storing and interchanging CD masters. Today, this system is still used for video editing and recording. [3]
31. Describe the data pickup of a CD player.
The data pickup of a CD player is designed to read the data using a phase contrast microscope. This microscope allows accuracy when confronted with deformities in the information layer as a phase structure. [1]
32. Comment on The Focus and tracking system in a CD player.
The focus and tracking system in a CD player keeps the laser beam on the data track. The tracking system continually moves the laser outward during playback. “As the laser moves outward from the centre of the disc, the bumps move past the laser faster -- this happens because the linear, or tangential, speed of the bumps is equal to the radius times the speed at which the disc is revolving (rpm).” Resultantly, the spindle monitor slows the speed of the CD this allows for the bumps to travel past the laser at a speed that is constant and the data in turn is read at a constant rate. [9]
33. Comment on CD master PQ editing.
PQ sub code refers to the two channels of sub code information which are contained in audio CD’s. The ‘P’ sub code is a simple music track separator flag with binary ‘0’ status and changes to ‘1’ when a selection is started and is generally used by CD players to find track start points. The ‘Q’ sub code is used for more complicated purposes and consists of track number and time data. Other sub codes such as (R, S, T, U, V, W) are also present [3]
34. What is CD master tape verification?
Master tape verification is the process by which a duplicate of a tape is compared to the original – this is done to check for errors in the duplication process and to ensure accuracy of the duplicate.
35. What is Eight Fourteen Modulation?
Eight Fourteen Modulation or EFM is an encoding technique that is used by CDs and MiniDiscs. In EFM, the data that is to be stored is broken down into 8 bits blocks namely bytes. Each block is translated to a corresponding 14-bit codeword. Hence the EFM name. “EFM requires 3 merging bits between adjacent 14-bit codewords to ensure that consecutive codewords can be cascaded without violating the specified minimum and maximum runlength constraint. The three merging bits are also used to shape the spectrum of the encoded sequence” [10]
36. What is a Successive Approximation Register converter? [11]
A Successive Approximation Register converter [SAR] is used in ADC conversion. SAR converters are available in resolutions up to 16-bits. SAR converters allow for bits to be determined by a single high speed, high accuracy comparator, this is done bit by bit from the MSB to the LSB. The analogue input is approximated via the comparison of the analogue input to a DAC that has an updated bit output that is previously decided. SAR is serial in nature and its speed is limited to a few Msps.
“An example of such a device is the MAX1132. Flash ADCs are typically limited to around 8-bits. The slower speed also allows the SAR ADC to be much lower in power. For example, the MAX1106, an 8-bit SAR converter, uses 100µA at 3.3V with a conversion rate of 25ksps. The MAX104 dissipates 5.25W. This is about 16,000 times higher power consumption compared to the MAX1106, but also 40,000 times faster in terms of its maximum sampling rate.
The SAR architecture is also less expensive. The MAX1106 at 1k volumes sells at approximately $1.51, while the MAX104 sells at roughly $398. Package sizes are larger for flash converters. In addition to a larger die size requiring a larger package, the package needs to dissipate a lot of power and needs many pins for power and ground signal integrity. The package size of the MAX104 is more than 50 times larger than the MAX1106.” [11]
37. Describe and explain the following digital audio concepts:
• Jitter
Jitter is an error, which occurs in the timing of an audio signal, which is due to fluctuations in the amplitude of the signal, which occur at a rapid pace. Jitter, also known as Sample Offset Uncertainty, is a source of distortion and creates phase discrepancies in the signal. Jitter itself acts more prominently on signals in the higher ranges than in the lower ones. [3]
• Simple codes
simple codes can be instantaneously directly processed and converted
• Group codes
Group codes are more complex to process as they utilise a lookup table to convert the data.
38. Outline Digital audio in video recording systems.
Digital audio in video recording systems operates at a sampling rate of 48 kHz. This sampling rate remains constant for the duration of the video or DVD.
39. What is Solid-state recording?
Solid- state recording pertains to data or audio that is recorded using a Solid State recording device. Solid state itself pertains to solid qualities usually crystalline. In the realm of electronics, this usually means a semiconductor of materials. [3]
40. What is a Session file?
A session file is a file in which information pertaining to a project is stored. In this file data pertaining to the name of the project, the tracks [how they are assigned], loops/ audio files used and editing which occurred are stored. Session files are usually not transportable between different systems as there is currently no universal compatibility between systems for this format. Thus, if you take one disk cartridge and load it into a disk recording system that is not the same as the one in which the data was originally stored/recorded, there will be problems with respect to reading the data if it is read at all.
41. Discuss the Archiving of digital recordings.
Digital recordings can be archived via the use of CD ROM. The CD-R disc is a read only CD that cannot be rewritten, A normal CD player can read a CD-R that the burning software specified as an Audio CD when the disc was created its capacity ranges from 80 minutes with a 700 MB capacity.
42. Comment on some features, functions and parameters of disk recording systems.
Disk recording systems allow for as many tracks and virtual tracks as the size of the hard disk allows, the buffer speed is essential is processing this data. The PC interface means that a visual is available for editing, linear fades, cross fades, splicing etc. Cache memory can be essential as it can house recently edited clips that were placed on the ‘clip board’ as it would not take long to retrieve the information thus increasing the options available with respect to processing, editing and recall. On the down side, hardware failures and system crashes can result in loss of data and compatibility among systems and programs can limit the transportability of data from system to system.
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REFERENCE:
1. The Art Of Digital Audio – John Watkinson
2. Principles Of Digital Audio – Ken C. Pohlmann
3. www.sweetwater.com
4. http://www.dspguru.com/info/faqs/iirfaq2.htm
5. http://en.wikipedia.org/wiki/Constant_Angular_Velocity
6. http://www.tldp.org/HOWTO/SCSI-2.4-HOWTO/scsiaddr.html \
7. Modern Recording Techniques – DM Huber, R Runstein
8. www.drtmastering.com
9. www.howstuffworks.com
10. http://en.wikipedia.org/wiki/Eight-to-Fourteen_Modulation
11. http://www.maxim-ic.com/appnotes.cfm/appnote_number/810 |
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Sun Sep 25, 2005 1:22 pm |
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Polarman
Joined: 24 Jun 2005
Posts: 55
Location: Barbados |
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Hi Cris! Here comes the answers...tough week this one  .
1. How does the data buffer of a Minidisc player facilitate editing?
The Minidisc player has an in built data buffer and control of the data rate of playback. This was originally built in to allow the recovery from jogging the player and skipping data. It may also be used for editing, allowing pieces to be jumped, repeated or rearranged in order without any audible effect. Any discontinuity brought about by location is smoothed out by the data.
Skipping or mistracking caused by shock or vibration is almost eliminated in a mini disc player. The minis disc uses a memory chip that acts like a buffer and can hold data up to 2 minutes before convert it to an analogue signal for listening. When you are editing on a minidisk its not the actual audio you are editing it is the TOC. Since the buffer holds enough data there will be no pause or clicks when skipping material.
http://www.minidisc.org/beyond_the_caddy.html
http://www.minidisc.org/faq_sec_5.html#_q40
http://radioworldwide.gospelcom.net/org/files/minidiscs.pdf
2. Explain why a CD –R Master recorded using “Disc at Once” is more likely to be accepted for mastering by a CD pressing plant (over alternative methods).
Mastering facilities must have perfect source data or their laser beam recorder will abort and ruin an expensive glass master. Track-at-once recording leaves "link-blocks" at the end of lead-in and at the beginning of lead-out. These are read as defects, or errors, by the LBR. Disk-at-once recording has no link blocks. Many mastering houses have learned from a rather gruesome history, and transfer information from a CD-R to 8-mm tape using methods that remove problems such as link blocks. Mastering is then conducted from the perfect image on the 8-mm tape, not from the "bad" CD-R.
http://www.mscience.com/faq63.html
3. Explain why the playback data rate of some disc based systems can be considerably higher than the maximum recording data rate.
The process to record data is longer in some disc media technology because the process involves an erase-record verify cycle. The playback process is a much simpler process.
4. State and describe the FOUR possible actions of the error correction system of a CD player.
100% correction of error
The CD player has managed to correct 100% of the errors by the error correction system. The system uses redundant information on the disc to reconstruct the lost content.
Less than 100% correction of error.
Interpolation - when data loss is too large to be handled by the error correction system, an error concealment circuit is employed to interpolate. Interpolation can be described shortly as by computing approximate values for missing data from surrounding information by calculating means from the procedding and preceeding sample values.
Hold - previous sample is hold for a period of time.
Mute means inserting a silent pause in place of an error, either with or without fade-in/fade-out of the adjacent signal.
Failure. When both error correction and concealment fail, distinct clicks will result and, in more severe cases, track skipping or stalling will take place.
Whatever method is used, error concealment - as opposed to error correction - will cause varying degrees of sound distortion, affecting sound clarity, stereo imaging and dynamics.
http://www.digital-recordings.com/cdcheck/cdmore.html
5. Explain the two basic methods that a Hard Disk editor may use to create a cross fade.
Creation of a cross fade on a hard disk editor can be done with two basic methods. The cross fade can be done I real time with a DSP operation this requires a large amount of CPU power. To save CPU power the cross fade can be saved as a separate file.
http://www.jamminpower.com/PDF/aes90.pdf
6. What is the main advantage of the oversampling process with regard to the design of an anti alias filter of an analogue to digital converter?
A higher sample rate gives a correspondingly higher Nyquist limit allowing higher frequency information to be encoded and / or easing the sonic burden of the steep cut off of the anti alias frequency filter.
Watkinson, J. (2001). The Art of Digital Audio, 3rd ed. Oxford, Focal Press.
7. Explain the operation of a 18 bit 4x oversampled DA converter on a CD player.
The existing data has 3 extra samples set to zero, inserted between each sample. This creates a new data sample rate of 176.4kHz. This information is fed into a digital filter that “rings” and creates the values for the extra samples. 18 bit refers to the need for an extra bit of overload above the 16 bit data to allow for over shoot and an extra bit to reduce accumulative noise. The process allows a gentler reconstruction filter with greater sonic performance.
Watkinson, J. (2001). The Art of Digital Audio, 3rd ed. Oxford, Focal Press.
8. What type of reduction does DCC use that may keep this medium in the domestic market only?
DCC uses a data reduction called PASC (Precision Adaptive Sub-band Coding). This is based on MPEG audio compression. The reduction of the data rate is 4:1 compared to PCM audio.
Watkinson, J. (2001). The Art of Digital Audio, 3rd ed. Oxford, Focal Press.
9. Calculate the maximum theoretical audio frequency and the signal to quantisation noise ratio in dB for the following systems:
a) Compact disc digital audio system.
Samplings frequency 44.1 KHz => With Nyquist theorem audio highest frequency 44.1KHz/2 = 22.05 KHz.
SQNR = 16 x 6 dB = 96 dB
b) A system utilising a sampling rate of 26KHz and 12 bit linear quantisation.
Samplings frequency 26KHz => With Nyquist theorem audio highest frequency 26KHz/2 = 13 KHz.
SQNR = 12 x 6 dB = 72 dB
http://www.cs.sfu.ca/CourseCentral/365/li/material/notes/Chap3/Chap3.1/Chap3.1.html
10. What is one of the main advantages of a Rotary-Head Recorder over a stationary head?
Since rotary heads have greater SNR characteristics, they offer better packing density and therefore can have narrow spacing between tracks. Stationary head devices are difficult to manufacture with narrow spacing between tracks.
http://www.ecsl.cs.sunysb.edu/tr/TR2.ps.Z.
11. What is a phased-locked loop?
Phased-locked loop (PLL) is a closed loop frequency control system. The basic function is based on detection of phase difference between the input and output signals of the controlled oscillator.
A clock can be obtained from a fixed frequency oscillator such as crystal but many audio operations requires synchronizing the clock to an external source. The PLL makes this possible. Here the oscillator can run according to a voltage applied by a control. This is called Voltage Controlled Oscillator (VCO).
Watkinson, J. (2001). The Art of Digital Audio, 3rd ed. Oxford, Focal Press.
12. Explain the principles of error concealment.
When recording digital audio errors can occur. Either it can be a small noise impulse that can cause some individual bits to be in error or it can be dropout that causes a large number of bits to be in error. This bigger error is called burst error. The audibility of a bit error depends of course which bit of the sample is involved. If it is LSB on a loud passage the error would not be noticeable but if it is the MSB in a quite passage you could hear the error quite clearly. In digital audio the amount of errors that can be corrected depends on the amount of redundancy. Corrected samples are indistinguishable from the original. If the errors are more then your redundancy can handle a correction is not possible. But to do as small damage as possible error concealment is used.
Prior to recording the sample stream the odd and even samples are separated. They may even be recorded in separate places to avoid that a burst error affects both the odd and the even samples. On replay the odd and even samples are put together as the original sequence. If an error has occurred you will at least have every other sample correct. The incorrect samples are recalculated. If sample 7 and 9 are correct and sample 8 is incorrect you can calculate sample 8 as (7+9)/2.
Watkinson, J. (2001). The Art of Digital Audio, 3rd ed. Oxford, Focal Press.
13. Explain how interleaving works.
Interleaving is a error correction system used to get rid of burst errors. The system spreads out the burst error into many smaller errors. Interleaving and de-interleaving causes delay.
In interleaving samples are recorded out of their normal sequence. This is done by taking the columns from the memory which was filled in rows. On the replay the process is reversed so the sample comes in the original sequence. .
Watkinson, J. (2001). The Art of Digital Audio, 3rd ed. Oxford, Focal Press.
14. Explain the limiting parameters of a hard disk drive that restrict operation in a digital audio workstation.
Size, today maybe the size is not a big problem. But still one minute 44.1 kHz/16 bit takes up around 5.3 Mb. If you are recording 24 tracks it will use 5.3x24=127 Mb. If you use higher samplings frequency or bit resolution the amount you have to store rises drastically.
Access time for write/read it’s critical since multitrack recording involves a lot of reading and writing to and from the disk. To reduce the amount of read and write operations its important to defragment your hard after each session.
Noise, if you have your computer in the room you are recording in you have to have very quite hard drives. To cover your drive can build up extra heat.
Storage time, hard drives are mechanical so to store files for archives it’s not a good idea its better to back on DVD.
Buffer cache, when choosing hard drive for audio recording it’s good to have as big buffer cache on the hard drive as possible. A cache is basically a local memory for the hard drive. The hard drive stores information in its memory, so that, if the same information is requested again, it can be read from cache memory faster than it would take to re-read it from the disk. Higher capacity improves performance, particularly where the repeat information is constantly being accessed. This is an essential feature for stability and reducing wear and tear due the constant and rigorous demands of audio processing.
15. Why are some D/A converters in CD described as 18 bit operation if the CD data is 16 bit?
CD players with 18-bit or even 20-bit converters cannot yield a greater dynamic range since the information on the CD is in the form of 16-bit words.
These converters just makes sure that the dynamic range is not impaired by additional resolution errors. 2 extra bits are needed above the 16 bits, one to reduce accumilative noise and one to allow for overshoot.
http://www.rohde-schwarz.com/www/appnotes_files.nsf/ANFileByANNoForInternet/661D72DBD9A2299EC1256B6000468789/$file/1GA21_1E.pdf
16. Describe the operation and the sections on a CD-Recordable WORM disk.
WORM stands for Write Once Read Many. As this says it can only be recorded once. The recording cannot be edited or erased. There are several types of WORM disks. Certain WORM disks can be read in conventional CD player and are thus called CDs or CD-R.
Watkinson, J. (2001). The Art of Digital Audio, 3rd ed. Oxford, Focal Press.
17. What is a RAID? What is its purpose?
One way to ensure against disk failure and data loss is to use Redundant Array of Inexpensive Disks (RAID). RAID also gives a higher data transfer performance to disk. To set up a RAID you need to combine two or more drives. There are different levels of RAID most common in data storage centrals is level 5 which give good performance and fault tolerance.
On music system level 0 is most common. That is not a real RAID in that sense because it is not fault-tolerant. Failure of just one drive will result in all data in an array being lost so it is very important to keep a recent back up all the time. There are some advantages though. It gives good performance when data is striped across multiple controllers with only one drive per controller. No parity calculation overhead is involved. It’s a very simple design and easy to implement.
http://www.acnc.com/raid.html
http://www.webopedia.com/TERM/R/RAID.html
Watkinson, J. (2001). The Art of Digital Audio, 3rd ed. Oxford, Focal Press.
18. Detail the differences between CAV and CLV optical disk drives.
CAV, short for Constant Angular Velocity, is a technique for accessing data off of rotating disks. With CAV, the disk rotates at a constant speed regardless of what area of the disk is being accessed. CAV discs are arranged into tracks (mostly magnetic media such as floppy discs) This means that sectors at the outside of the disc pass under the head much faster than those at the centre, and thus the data is more spread out and this wastes space on the disc. On the CAV the transfer rate is variable.
This differs from Constant Linear Velocity (CLV), which rotates the disk faster for inner tracks. This simply means that the drive adjusts the rotational speed to ensure that the throughput is constant from the inner track all the way through the outer track. The data transfer rate on CLV is fixed. CLV discs have a single, spiral track.
http://www.pcguide.com/ref/cd/constSpindle-c.html
http://telcontar.net/Misc/maths/CAVvsCLV.php
19. Outline the principles of magneto-optical drives.
A magneto-optical drive is a kind of optical disc drive capable of writing and rewriting data upon magneto-optical discs. One example is MD.
The disc consists of a ferromagnetic material sealed beneath a plastic coating. There is never any physical contact during reading or recording. During reading, a laser projects a beam on the disk and according to the magnetic state of the surface, the reflected light varies due to the magneto-optical Kerr effect. During recording, the light becomes stronger so it can heat the material up to the Curie point in a single spot. This allows an electromagnet positioned on the opposite site of the disc to change the local magnetic polarization, and the polarization is retained when temperature drops.
Magneto-optical drives by default check information after writing it to the disc, and are able to immediately report any problems to the operating system. This means that writing can actually take three times longer than reading, but it makes the media extremely reliable, unlike the CD-R or DVD-R technologies upon which data is written to media without any concurrent data integrity checking.
Watkinson, J. (2001). The Art of Digital Audio, 3rd ed. Oxford, Focal Press.
http://www.answers.com/topic/magneto-optical-drive
20. List three types of DACs.
Pulse Width Modulator, the simplest DAC type.
Oversampling DAC The oversampling technique allows for the use of a lower resolution DAC internally.
Segmented DAC, This is perhaps the fastest and highest precision DAC architecture but at the expense of high cost. http://www.answers.com/topic/digital-to-analog-converter
21. A 20 bit DAC has how many levels? (16 bit = 65,536)
20 bit has 2 raised by 20 levels = 1048576 levels
22. What is the SCSI system used for in A/V workstations? Outline SCSI addressing.
SCSI is used to transfer data to external storage devices. SCSI is the ability to build a huge multi-drive array.
SCSI adapter number [host]
channel number [bus]
id number [target]
Logical Unit Number [lun]
http://www.tldp.org/HOWTO/SCSI-2.4-HOWTO/scsiaddr.html
23. List some typical SCSI problems.
Cabling
Cabling can be too long, too short, be of poor quality or simply defective.
Termination
There are two types of termination, passive and active. An external terminator is a cap that fits on the last device in the chain. The internal chain is also terminated on the last device, however, termination may be provided by either a jumper setting on the last device (typically a disk drive), a set of removable resistors (either tape or disk), or by an external terminator block which is specially designed to terminate the last device on an internal SCSI bus.
Bus Termination Power
Power to the terminator is either supplied by the adapter card or is provided by a SCSI peripheral device, e.g. disk drive. Termination of the last device and termination power are not the same thing! Another common cause for corruption is when both the adapter card and a SCSI peripheral device are set to provide termination power to the bus.
SCSI Address Conflicts
Each device on an adapter card requires a unique address. A device's unique address is either set by jumper settings or dipswitches on the device itself or by means of an address clicker when housed in an external cabinet. When adding a new device in an external cabinet which is set properly for the address externally but fails to be recognized by the adapter card or the system fails to boot, remove the cabinet cover and verify that the clicker connector is actually connected to the address pins on the device and check the termination status to verify that is set correctly.
http://www.ultrabac.com/kb8/ubq000092.htm
24. Shifting a sample word one step to the left or one step to the right will change the gain in dB of the sample by how much?
I am not really sure what this question mean…but
One bit is 6dB. If you are working with 4 bits.
Let say we have the word 0110
Shifting left – 0011
Shifting right – 1100
Answer: + - 6db
25. List some uses of digital filters.
Any medium through which the music signal passes, whatever its form, can be regarded as a filter. Some uses for digital filters are equalizers, reverberators, echo devices, phase shifters, anti aliasing filter, oversampling filter etc.
It is important to realize that a digital filter can do anything that a real-world filter can do.
http://ccrma.stanford.edu/~jos/filters/
26. Explain FIR and IIR.
FIR and IIR are the to main classes for filters.
Filter Impulse Response (FIR)
These filters responds to a impulse once since they only have a forward path. These filters are always stable and the time for which the filter responds to an input is finite, fixed and readably established. The same is valid for tese filters in the spatial domain. These filter can be made totallt phase linear. Filter for oversampling and conversion falls in this category.
Infinite Impulse Response (IIR)
This category of filters are not necessary stable and responds to impulses indefinitely. The reason for this is that to output on these filters are connected to the input. For this reason they are called recursive filters. The IIR filters are not phase linear since the impulse response is not symmetrical. Similar to analog tone controls.
Watkinson, J. (2001). The Art of Digital Audio, 3rd ed. Oxford, Focal Press.
27. The result of multiplying two binary numbers together is to create a new sample word which may have many more bits than the original, and it is common to find that digital mixers have internal structures capable of handling 32 bit words or more, comment including the word dither.
I am not really sure what this question mean so I will answer to what I think it means…
Some software or hardware says they working long bit words. Like Izotops plug-ins. They work with 64-bit digital precision. The problem is that when you later want to convert your music to a 16 bit 44.1 KHz CD the software or hardware must be able to convert from there word length to 16 bit. There are two possibilities for this either they just truncate to 16 bit or they automatically use dither. So probably there are good and bad software/hardware regarding this.
28. What is a Parity bit?
A parity bit is added to the binary array to make the sum of all of the bits always odd or always even as a form of error checking during the transmission of information.
http://home.intermec.com/eprise/main/Intermec/Content/About/GlossarySubpages/Glossary_OP
http://kb.iu.edu/data/afdh.html
29. Comment on Improving SQNR with oversampling.
SQNR = 1.76 + 6.02 B
Every doubling of the sampling frequency improves the SQNR with 3 dB.
http://web.it.kth.se/~ana/Thesis_Report_Final_KTH_Craig.pdf
30. What is The 1630/Video U-Matic system?
The first video formats to contain the videotape inside a cassette was the U-matic. This is a videocassette format developed by Sony in 1969,
But it has such a lasting appeal as a cheap, well specified, and hard-wealmost every television facility the world-over still has a U-matic recorder. Nearly 36 years after it was developed, the format is still in daily use for the menial tasks of the industry, being more highly specialized and suited to the needs of production staff than the domestic VHS.
http://www.answers.com/main/ntq-s-U%252DMatic
31. Describe the The data pickup of a CD player.
The pickup assembly of a CD player focuses on, and reads digital information that is aligned in rows of pits and flats. It uses a laser that reads data. While performing this task, the pickup's laser beam encounters disc manufacturing flaws and errors that develop during the normal course of use.
http://www.digital-recordings.com/cdcheck/cdmore.html
32. Comment on The Focus and tracking system in a CD player.
The primary job of the CD player is to focus the laser on the track of bumps. The laser beam passes through the polycarbonate layer, reflects off the aluminum layer and hits an opto-electronic device that detects changes in light. The bumps reflect light differently than the "lands" (the rest of the aluminum layer), and the opto-electronic sensor detects that change in reflectivity. The electronics in the drive interpret the changes in reflectivity in order to read the bits that make up the bytes. The hardest part is keeping the laser beam centered on the data track. This centering is the job of the tracking system. The tracking system, as it plays the CD, has to continually move the laser outward. As the laser moves outward from the center of the disc, the bumps move past the laser faster -- this happens because the linear, or tangential, speed of the bumps is equal to the radius times the speed at which the disc is revolving (rpm). Therefore, as the laser moves outward, the spindle motor must slow the speed of the CD. That way, the bumps travel past the laser at a constant speed, and the data comes off the disc at a constant rate.
http://electronics.howstuffworks.com/cd6.htm
33. Comment on CD master PQ editing.
The ability to specify track and index markers when writing a Red Book audio CD is often referred to as "PQ editing" because that information is contained in the P and Q subcodes. A disc has three main regions: the lead-in area, the program area, and the lead-out area. Subcode Q mode 1 data in the lead-in is used to hold the table of contents (TOC) for the disc. The TOC is repeated continuously in the lead-in area in case of damage In the program and lead-out area, mode 1 contains track numbers,index numbers, time within the current track, and absolute time. Index 0 marks the start of a pregap (pause) before the audio in a track begins, index 1 marks the start of the music, and indexes 2 through 99 are usually not set but can be added if desired.
http://www.faqs.org/faqs/cdrom/cd-recordable/part1/
34. What is CD master tape verification?
This is the last stage prior to manufacturing, includes a printout of an error analysis of the tape (data verification) plus a frame accurate log sheet. It can also include a PQ burst if requested by the client. The PQ Burst contains the "Table of Contents" information for the finished CD. It is a short burst of digital data which is compiled and placed at the beginning of the master tape.
http://www.audiomechanics.com/www/services.html
35. What is Eight Fourteen Modulation?
Eight-to-Fourteen Modulation (EFM) is an channel coding technique used by CDs and Minidisks. The control code is a binary word of 14 bits. A group of eight data bits are represented by a code of 14 channel bits.
36. What is a Successive Approximation Register converter?
A Successive Approximation Register Converter is special counter circuit used in analogue to digital conversion ADC. Instead of counting up in binary sequence, this register counts by trying all values of bits starting with the most-significant bit and finishing at the least-significant bit. Throughout the count process, the register monitors the comparator's output to see if the binary count is less than or greater than the analog signal input, adjusting the bit values accordingly. The advantage to this counting strategy is much faster results then a digital ramp.
http://www.faqs.org/docs/electric/Digital/DIGI_13.html
37. Describe and explain the following digital audio concepts:
Jitter
The term 'jitter', in the context of a digital audio system, refers to the timing uncertainty of the data or clock signals. There is always a small degree of frequency modulation caused by noise which finds its way into the sensitive clocking circuitry. One way to get a clock stable is to use PLL. http://www.weiss.ch/hydra/hydra.html
Simple codes
Simple codes, which take one data word from the original signal and encode one channel code for it.
http://www.ozmediasolutions.com/content/research/dsp.php
Group codes
Group codes use more complicated methods such as using code look-up tables to convert groups of input words into patterns of output words.
http://www.ozmediasolutions.com/content/research/dsp.php
38. Outline Digital audio in video recording systems.
In video recording systems the sampling rate 48KHz is used.
39. What is Solid state recording?
Solid state recording is a recording made on a solid state recording device like Marantz PMD 671 or HHBs new Flash mic DRM 85.
40. What is a Session file?
A session file stores information of plugins used, audiofiles used, miditracks, edits, project name, master track changes etc. Mostly the sessions files are software specific. Then you have to save all your work as wav file for each track to be able to change platform. One open standard is omf that makes it able to convert between the platforms.
41. Discuss the Archiving of digital recordings.
Archiving digital information is always under debate. Nobody knows how long a DVD or CD will last before the data gets corrupt. The safest medium today is DVD.
http://www.music.columbia.edu/%7Ececenter/CenterSite/digitalarchive.html
42. Comment on some features, functions and parameters of disk recording systems
The control computer where and when samples will be stored and retrieved it also sends instructions to the disk controller to make the drives to read or write and transfers data between the disks and the memory. The data rate of a disk is often 10 times higher then the sampling rate. Since the disk cannot stop a continuous data stream must be provided when the disk is to be written during each block. Audio disk recording systems must have a quantity of RAM for buffering to be able to accept a steady audio sample stream for storage and return it in the same way on replay. One high quality audio channel requires round one megabit per second that equals that one gigabit offers about three hours of monophonic audio. Its also possible to assign to audio channels so you can record 90 minutes on one gigabit.
One advantage of hard disk recording is that editing doesn’t take up more space on the disk since the editing is done in the memory of the control system.
Watkinson, J. (2001). The Art of Digital Audio, 3rd ed. Oxford, Focal Press. |
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Sun Sep 25, 2005 7:27 pm |
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AUdIoCoUrSeS
Joined: 31 Oct 2002
Posts: 2014
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Very tidy, and I appreciate the work that went into that week!
You might want to start to practise a few diagrams to help explain some of these concepts for the exam, interleaving is one such example, drawing a grid with how data is read and written to springs to mind.
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It's all in the ears. - Learn the concepts not the software.
Audio Courses is a way into the music business for you
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