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 .
_________________
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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Mon Mar 28, 2005 6:08 am |
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Rico1210
Joined: 03 Aug 2004
Posts: 39
Location: Newcastle, UK |
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Hi,
Here's my answers for week 4. Some of these were tough 
I wasn't sure about a couple of them.
1. How does the data buffer of a Minidisc player facilitate editing.
The data buffer of a Minidisc player controls the data rate of playback. A Minidisc stores data in sections much like a Hard Disk drive does. After editing, the tracks or part of a track may not be located in order on the disc. As the data buffer controls playback, the information is read from it's location and produced with continuity by the buffer.
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).
Disc at Once records the data onto the CD all in one go, the laser records from start to finish with no stops. Other methods like Track at Once record the data at intervals, so the laser stops and starts after each track has been recorded.
A CD laser beam recorder would reject the fact that there are wasted spaces where the laser has been turned off, and refuse to cut a master for production. This is why Disc at Once is more likely to be accepted for mastering by a CD pressing plant.
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 data rate because the process of recording data takes longer. This is because some disc media involves an erase-record verify cycle during recording, that slows down the recording process and limits the number of recordings that can be made simultaneous. Playback is a simple one pass process that can be at a higher rate.
4. State and describe the FOUR possible actions of the error correction system of a CD player.
The four possible actions of the error correction system of a CD player are:
Successful error correction.
Hold - The previous sample is repeated.
Interpolation - An average is taken by adding together the previous and next bits, then dividing them to create the missing bit.
Mute - If an error cannot be corrected or concealed, the system will mute the signal at the point of the error.
5. Explain the two basic methods that a Hard Disk editor may use to create a cross fade.
The two methods that a Hard Disk editor may use to create a cross fade are:
A complicated Digital Signal Process operation can be used to create the cross fade in real time. However this uses large amounts of processor power and there is a risk of overloading the system. The second method uses the same Digital Signal Process operation but records it as a separate cross fade file. This reduces the possibility of overloading.
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 the oversampling process with regard to the design of an anti alias filter of an analogue to digital converter, is that higher frequency information can be encoded. Oversampling uses a higher sample rate. A higher sample rate means a higher Nyquist limit, which allows the filter to relax.
7. Explain the operation of a 18 bit 4x oversampled DA converter on a CD player.
An 18 bit 4x oversampled DA convertor on a CD player will have a sample rate of 176.4kHz (44.1kHz x 4). There are 3 samples added between each of the original samples whose value is created by a digital filter. The higher sample rate allows the reconstruction filter to be smoother. Two extra bits are needed above the 16 bits, one to reduce accumilative noise and one to allow for overshoot. This is where the 18 bit comes from.
8. What type of reduction does DCC use that may keep this medium in the domestic market only?
The Digital Compact Cassette(DCC) produced by Philips, uses Precision Adaptive Sub-band Coding or PASC for a 4:1 reduction in data rate. Reduction to a quarter of the PCM data rate is possible with only a small loss of quality. This is acceptable for consumer use, but is not considered acceptable for proffesional music production.
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. 44.1KHz 16bit
Audio Frequency: 44.1KHz / 2 = 22.05KHz
SQNR: 6 x 16 = 96db
b) A system utilising a sampling rate of 26KHz and 12 bit linear quantisation.
Audio Frequency: 26KHz / 2 = 13KHz
SQNR: 6 x 12 = 72db
10. What is one of the main advantages of a Rotary-Head Recorder over a stationary head?
One of the main advantages of a Rotary-Head Recorder over a stationary head is the number of data bits that can be recorded in a given space, or the density of recorded data bits.
11. What is a phased-locked loop?
A phase-locked loop (PLL) is an electronic circuit with a voltage or current-driven oscillator that is constantly adjusted to match in phase with the frequency of an input signal. PLL can be used to generate a signal, modulate or demodulate a signal, or multiply or divide a frequency.
12. Explain the principles of error concealment.
Errors can occur in the digital recording of binary data, a bit is either correct or incorrect with no middle ground. Dropouts or burst errors can occur, which cause a large number of bits in one place to be incorrect. Whether an error is audible depends on which bit of the sample is involved. If the Least Significant Bit(LSB) is in error in a loud piece of music, the effect would not be heard as it would be masked. If the Most Significant Bit(MSB) is in error in a quiet piece of music, the effect would clearly be heard. Error correction works by reversing the state of the binary number, this works because binary only has two states. If the error can't be corrected then the process of Error Concealment is used. Concealment is a process where the value of the missing sample is estimated from nearby samples. The nearby sample may not necessarily be the same as the original, which in some cases can cause the concealment to be audible. Concealment is made possible by shuffling the sample sequence to separate odd and even numbers prior to recording. The odd and even samples are recorded in different places and recombined into their natural sequence upon replay. Therefore if there is a burst error, only either even or odd numbers will be lost, the result being that every other sample is correct. Only half of the waveform can now be described, but it can be reproduced, though with a little loss of accuracy.
13. Explain how interleaving works.
Interleaving is a process that assists error correction by breaking up burst errors into smaller errors that can be more efficiently corrected. This is achieved from recording samples out of their normal sequence by taking columns from a memory, which was filled in rows. Upon replay the process is reversed putting samples back into their regular sequence, but any burst errors will be broken up into smaller errors.
Interleaving and de-interleaving causes delay.
14. Explain the limiting parameters of a hard disk drive that restrict operation in a digital audio workstation.
One of the limiting parameters of a hard disk drive that restricts operation is the speed of the disk. The number of tracks of which a system may be capable depends on how fast the disk can access data, and how fast the rest of the system can process it. Hard disk recording systems can play back much slower than normal speed, but only a little faster because they are restricted by the speed that data can be accessed from the hard disk. It is easy for the data to be read at a slower rate than normal, but reading data faster than normal is difficult and can cause delays or audible errors during playback.
Another limiting factor of a hard disk drive is the size. The size of the hard disk will limit the number of tracks that can be recorded onto the disk and the sample rate of the tracks
It is not advisable to use a hard disk for long-term storage, as they are a mechanical medium that contains perishable parts. If a part of the hard disk e.g. the arm were to seize, then the data contained on the disk would be lost.
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 because 2 extra bits are needed above the 16 bits, one to reduce accumilative noise and one to allow for overshoot. This is where the 18 bit comes from.
16. Describe the operation and the sections on a CD-Recordable WORM disk.
A CD-Recordable WORM(Write Once Read Many) disk can only be recorded once. Once a recording has been made on a WORM disk it can't be edited or erased. There are several types of WORM disk. One type writes the data to the disk by melting spots on a thin layer of metal with a powerful laser. Surface tension creates a hole with a raised edge on the metal layer that can be read by a low-power laser. The laser is reflected by the metal, but is able to pass through the hole. Another method uses a very thin layer of metal on the surface of the disk, which is heated by the laser causing a raised spot to appear. The plastic beneath the metal layer is heated to the point of decomposition producing a gas that raises a bubble on the surface. A third method uses a chemical dye that darkens with the high radiation of a writing laser, creating a pattern of dark areas that can be read by the read laser. Once a pattern has been written to disk using one of the above methods, it is permanent and cannot be reversed.
17. What is a RAID? What is its purpose?
A RAID is a Redundant Array of Inexpensive Disks. A RAID is used to ensure data integrity in a system where data is valuable. It works by spreading data across a number of drives, so if one drive fails the error correction is powerful enough to correct an error in the block. The drive can then be replaced as they are designed to be hot-swappable, and the error correction system will rewrite the new drive with the data which was lost.
18. Detail the differences between CAV and CLV optical disk drives.
Constant Angular Velocity(CAV). The angular velocity is the speed that the disc spins.
Constant Linear Velocity(CLV). The linear velocity is the speed that bits are read.
In a CAV drive the disc maintains a constant speed, with a linear velocity that increases as it moves towards the outside edge of the disc. The maximum angular velocity is determined by physical constraints.
A CLV drive has a constant linear velocity and an angular velocity that changes. As there are more bits in the circumference of the outside edge of the disc than on the inside, the drive needs to spin slower to maintain a constant linear velocity.
19. Outline the principles of magneto-optical drives.
Magneto-optical drives are based on the principles of magnetism, hence the name 'magneto'. A magneto-optical disk is initially magnetised in one direction only. Changing the direction of this magnetism on certain areas of the disk makes a recording. This is achieved using a magnetic field and a finely focused laser. When the magnetic material is heated above its Curie temperature, it is demagnetised and can assume the magnetisation of the applied field upon cooling. The current of a large coil, which can be reversed in order to erase the data on the disk, creates the opposite magnetic field. Erasing the disk uses the same principle of demagnetising the area by the laser, and taking on the magnetic direction of the coil. As the materials used have a Curie point low enough to be reached, they are highly corrodable by air, consequently needing to be kept in a sealed protective case. Sony MiniDisc uses the principles of magneto-optical recording.
20. List three types of DACs.
Three types of DACs are: Pulse Width Modulator, which is the simplest type of DAC. Segmented DAC which is the fastest and most expensive DAC. Binary Weighted DAC which is one of the fastest types of DAC but suffers from poor accuracy.
21. A 20 bit DAC has how many levels? (16 bit = 65,536)
2 to the power 16 = 65,536
2 to the power 20 = 1,048,576
A 20 bit DAC has 1,048,576 levels.
22. What is the SCSI system used for in A/V workstations? Outline SCSI addressing.
In A/V workstations, the Small Computer System Interface(SCSI) system is used to transfer data to external storage devices at a high rate.
This is the four level hierarchical SCSI addressing scheme
SCSI Adapter Number [host]
Channel Number [bus]
ID Number [target]
Logical Unit Number [lun]
In brackets are the terms used by the device pseudo file system (devfs) <host, bus, target, lun>
23. List some typical SCSI problems.
Some typical SCSI problems could be electrical noise on the bus or termination voltage that is too high or low. This can be caused by defective cables or cables that are too short or too long, or the terminators on the ends of the SCSI bus may be terminated incorrectly. Also, each device should have a unique address to avoid an address conflict.
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?
Not sure how this works.
25. List some uses of digital filters.
Digital filters can be used to filter high frequencies (High Pass Filter), low frequencies (Low Pass Filter), or a range of frequencies (Band Pass)
26. Explain FIR and IIR.
Finite-Impulse Response(FIR) and Infinite-Impulse Response(IIR) are the two main classes of filters. FIR filters respond to an impulse once and are always stable as they only have a forward direction, whereas IIR filters respond to an impulse indefinitely and are not stable as they have a return from the output back to the input. IIR filters are sometimes known as recursive filters, and are not phase linear.
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.
Not sure I understand this one. Read a bit about it but don't know what ir's asking.
28. What is a Parity bit?
A Parity bit is an extra bit that is added to a character or block of bits to determine whether even parity or odd parity is used. A Parity bit is used to check for errors in data transfers between computers. The extra bit can be either a 1 or 0 depending on which parity is desired.
29. Comment on Improving SQNR with oversampling.
Signal to quantisation ratio(SQNR) can be improved by oversampling as the increased sample rate widens the nyquist bandwidth, which reduces the noise density
30. What is The 1630/Video U-Matic system?
The 1630/Video U-Matic system is a CD mastering recorder produced by Sony. The PCM-1610/1630 uses a U-Matic Video Cassette Recorder(VCR) specifically for CD mastering.
31. Describe the data pickup of a CD player.
The data pickup of a CD player is a near-infrared 780nm laser that reads the data off the pits and lands of the disc. The laser shines through the plastic of the CD and reflects off the metal layer back to be read by a photosensor in the head. It reads the start and end of a pit as 1, and all other areas as a 0.
32. Comment on The Focus and tracking system in a CD player.
The laser in the CD drive refracts through the polybycarbonate layer over the metal film, allowing the laser to be finely focused. The laser can focus 800um at the bottom of the polycarbonate, to 1.7um at the metal surface. The effects of dust and scratches are reduced as the focus width of the laser is reduced when it hits the CD.
33. Comment on CD master PQ editing.
PQ editing refers to 2 of the 8 subcode areas of a CD, P and Q. The P and Q areas are defined by the CD audio standard. The P and Q areas contain start and end IDs, number of channels, ISRC (International Standard Recording Code), they determine whether a track is data or audio, and specify the Digital Copy Permitted and Pre-emphasis flags.
34. What is CD master tape verification?
CD master tape verification is the comparison of a duplicate copy to the master, used to identify any errors.
35. What is Eight Fourteen Modulation?
Eight Fourteen Modulation(EFM) is the channel coding used in compact discs and Minidiscs. The control code has a binary word with 14 bits known as channel bits. A group of 8 data bits is represented by a code of fourteen channel bits, which is where the name ‘eight to fourteen modulation’ comes from.
36. What is a Successive Approximation Register converter?
A Successive Approximation Register converter is a counter circuit used in analogue to digital conversion(ADC). It counts by trying all values of bits starting with the most significant bit(MSB) and finishing with the least significant bit(LSB). The register monitors the output, and adjusts the bit values if the binary count is less or more than the signal input.
37. Describe and explain the following digital audio concepts:
• Jitter can be defined as time instability of samples that causes noise and distortion. Jitter is overcome by having samples read into a RAM buffer, then the samples can be produced independently.
• Simple codes code each data transition separately.
• Group codes use a code translation or lookup table to convert input data bits to output words.
38. Outline Digital audio in video recording systems.
Digital Audio in video recording systems is recorded in stereo at a sample rate of 48KHz. The sample rate of 48 kHz is adopted from the entire film, video and broadcast industry and also DVD is based on the standard of 48/96 kHz.
39. What is Solid State recording?
A Solid State recording is a recording made on a Solid State storage device. Solid State storage uses integrated circuits to hold data, rather than a magnetic or optical medium. The advantages of this are that everything is done electronically, which gives a higher data transfer rate. As there are no moving parts, the life of the medium is increased significantly. Flash memory and USB devices are examples of Solid State media.
At the moment Solid State media is lagging behind electromechanical storage in terms of storage capacity. A Solid State device can store 1 or 2 gigabytes, whereas a hard disk can store hundreds of gigabytes.
40. What is a Session file?
The session file contains information including the project title, audio files used, any edits, track assignments and mix and plug-in data.
Session files can be transported between different systems provided the systems are compatible. For instance, if a session file is recorded and edited on a system with 16 inputs and outputs and mixed with a particular set of plug-ins. It will be very difficult to recreate the session on a system with only 8 inputs and a different combination of plug-ins. Session files are highly incompatible between manufacturers.
41. Discuss the Archiving of digital recordings.
Digital data can be archived to such optical media as a Compact Disc(CD). Optical storage is considered to be a safe option for long-term storage. Unfortunately the space available for storage is only 700MB on a standard CD-ROM.
42. Comment on some features, functions and parameters of disk recording systems.
Disk recording systems commonly have features and functions that include disk and track editing, title input, synchro-recording, optical in/outs, PC interface via USB. The Minidisc and CD recorder are disk recording systems.
References
The Art of Digital Audio - John Watkinson
http://www.totse.com/en/technology/science_technology/cd_audio.html
http://en.wikipedia.org/wiki/Digital_to_analog_converter
http://www.cdrfaq.org/faq05.html#S5-22
http://www.linuxdocs.org/HOWTOs/SCSI-2.4-HOWTO/scsiaddr.html
http://www.beicorp.com/kb5/htm/solvingtypicalscsiproblems.htm
http://www.ibiblio.org/obp/electricCircuits/Digital/DIGI_13.html
http://www.mcgoodwin.net/digitalaudio/digitalaudio.html |
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Sun Apr 03, 2005 4:10 pm |
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AUdIoCoUrSeS
Joined: 31 Oct 2002
Posts: 2014
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Some good answers there RICO, keep it up!
quote:
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?
Not sure how this works.
6dB
quote:
7. 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.
Not sure I understand this one. Read a bit about it but don't know what ir's asking.
This is all about how some software will truncate, or "round-off" binary words, and some will do this well or badly. Dither goes some ways to randomising the problem and increasing percieved quality.
_________________
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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Thu Apr 07, 2005 7:47 am |
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