AUdIoCoUrSeS
Joined: 31 Oct 2002
Posts: 2014
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| Week 4 - Digital recording systems (and some) |
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Digital recording systems
You have a very busy week here and I'm looking for some in depth answers, which will require significant research on your part. Nevertheless you should enjoy diving into these topics, lots to get your teeth into here. Good luck.
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 Sep 27, 2004 12:57 pm |
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iNSTiNCT2765
Joined: 05 Nov 2003
Posts: 60
Location: Denmark |
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Week 4 – Digital Recording Systems
There's some questions I didn't know how to answer or couldn't find any information on. Did my best with the rest 
1. How does the data buffer of a Minidisc player facilitate editing?
Since the data stored on the Minidisc is non linear, it is accessed in the same way as data on a hard disk. Data is stored in segments so editing is made easier. If you have 10 tracks on the Minidisc and erase track 4, which was a 3 minute song and then decide to record a new song that is 4 minutes long, the first three minutes of the track will be stored where track 4 used to be and the rest will be placed at the next available empty segment, which is after track 10. When playing back the new track, the read buffer keeps the track seamless while the Minidisc accesses the first segment and then the second.
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).
This is because the majority of CD pressing plants’ mastering software doesn’t interpret other modes of CD recording properly. But now more facilities are being updated to interpret multisession discs so CD-R Masters can be recorded using ‘Track at Once’ and ‘Multisession’ modes. Using the ‘Disc at Once’ mode eliminates the 2-second gap between each track, which is essential if tracks have smooth overlapping transitions between them.
3. Explain why the playback data rate of some disc based systems can be considerably higher than the maximum recording data rate.
This is because the recording is done in linear real-time and therefore cannot be speed up but during playback it is possible for the system to access the data at a much faster rate because it isn’t real-time.
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?
The main advantage is that the Nyquist frequency moves farther above the audible 20Hz-20KHz frequencies and this gives a smoother more linear phasing because the filter is more relaxed.
7. Explain the operation of an 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?
DCC uses Dolby B noise reduction, which only has a one-band filter for reduction working only at 1KHz and above. This makes it ineffective with high-quality recorders in comparison to other reduction systems.
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.
Audio frequency = ½ the Nyquist Frequency (Sample Rate)
So since the sample rate of a CD is 44.1KHz, the audio frequency will be:
44.1KHz / 2 = 22.05KHz
SQNR = bit rate x 6
16 bit x 6 = 96dB
b) A system utilising a sampling rate of 26KHz and 12 bit linear quantisation.
Audio frequency = 26KHz / 2 = 13KHz
SQNR = 12 bit x 6 = 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 is the extremely high packing density (the number of bits that can be recorded in a given space). This directly translates into the amount of time available for playing digital audio for a specific size of the medium.
11. What is a phased-locked loop?
This is a circuit containing a VCO whose frequency or phase can be adjusted to keep it in sync with a reference source. In digital terms, the ADC will have this circuit so the input signal coming into it will be the same constant frequency when it leaves.
12. Explain the principles of error concealment.
Error concealment is a technique used to reduce the effect of a digital error if the error cannot be corrected using error correction. It involves making a smooth transition from the last good block and first good block before and after the error respectively. This is done using some form of crossfading. This is the reason why a digital copy may not be an exact duplicate of the original master.
13. Explain how interleaving works.
Interleaving is the process of scattering data around the medium so if a large section is damaged, it results in many small manageable data losses that can be recovered using error correction.
14. Explain the limiting parameters of a hard disk drive that restrict operation in a digital audio workstation.
Limiting parameters of a hard disk drive include rotation speed, which will determine the speed at which the information on the drive is located. The size of the hard disk drive will also restrict the number of tracks you can record and edit simultaneously as well as the sample rate at which they can be recorded. Higher sample rates take up more disk space so the number of tracks will obviously decrease as the sample rate increases.
15. Why are some D/A converters in CD described as 18-bit operation if the CD data is 16 bit?
The error in a 16-bit converter relies on the MSB. Therefore, a higher bit rate (18-bit or 20-bit) ensures the accuracy of the MSB and also adds more quantisation levels improving the SQNR.
16. Describe the operation and the sections on a CD-Recordable WORM disk.
WORM (Write Once, Read Many) CD’s can only be written to once and the data cannot be erased after. The data is written to the CD-R by a laser that heats up tiny sections on the disc.
17. What is a RAID? What is its purpose?
RAID stands for Redundant Array of Independent Disks and it’s a disk drive category that utilizes two or more disk drives in combination for ‘fault tolerance’. Fault tolerance is the ability for a system to respond gracefully to unexpected hardware and software failures. RAID disk drives are used frequently on servers but not on personal PC’s.
18. Detail the differences between CAV and CLV optical disk drives.
CAV (constant angular velocity) drives such as hard disks and floppy disks spin at a constant speed whether the inside or the outside of the disk is being read.
CD-ROM’s have to have the same amount of data going past the read heads all the time. To accomplish this, the disc will spin faster when the read heads are on the outside of the disc and slower on the inside. This is called CLV (constant linear velocity).
19. Outline the principles of magneto-optical drives.
MO drives come in 3.5” and 5.5” and write magnetically and read optically. Magnetic material has characteristic temperature called the Curie temperature above which they lose magnetization and in the case of a MO disk, loses its data. The coercivity of the material decreases as the Curie temperature is reached and is zero after passing that temperature. The Curie temperature of the MO system is in the order of 200 degrees. To read the data on the disks, polarised light is used.
20. List three types of DACs.
1) CD Player
2) Soundcard with analogue outs
3) Minidisc Player
21. A 20 bit DAC has how many levels? (16 bit = 65,536)
A 20 bit DAC has 1048576 levels. (2^20)
22. What is the SCSI system used for in A/V workstations? Outline SCSI addressing.
The SCSI system is used for storage in A/V workstations. SCSI drives have relatively large block transfers of sequentially stored data, which is essential for A/V as the input and output data needs to appear seamless after digital to analogue conversion.
Linux derived a 4 level addressing scheme for SCSI devices which is:
SCSI adapter number [host]
channel number [bus]
id number [target]
lun [logical unit numbers]
23. List some typical SCSI problems.
ID number conflict
Hardware defect affecting the SCSI chain
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?
This will shift the gain by 6dB. ???
25. List some uses of digital filters.
Delay
LFO
Gain
Room Simulation
26. Explain FIR and IIR.
An FIR (finite impulse response) filter is one with a finite duration as where an IIR (infinite impulse response) filter has an infinite response (theoretically).
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.
If the digital mixers were unable to handle 32 bit words, then after the multiplying of two binary digits, the word would have to be cut down to 16 bits again or be dithered down to 16 bits. When dithering the sample, a low level noise is being added before rounding down to 16 bits to avoid distortion. This noise still compromises the sound so high end digital mixers have the capability of handling 32 bit words or more to eliminate compromises in sound quality due to dithering.
28. What is a Parity bit?
A Parity bit is a single bit added to a binary data transmission indicating whether the 0’s or 1’s in the data are odd or even.
29. Comment on improving SQNR with oversampling.
With oversampling, the signal to noise ratio due to quantisation on a CD, which is 96dB, can be spread over a larger bandwidth meaning the noise in the 20Hz-20000Hz frequency range is reduced.
30. What is the 1630/Video U-Matic system?
Couldn’t find much on this other than it was an old Sony system.
31. Describe the data pickup of a CD player.
The data pickup of a CD player tracks, focuses and reads digital data that is aligned in rows of pits and flats at speeds of 1.4 m/sec.
32. Comment on the focus and tracking system in a CD player.
Since CD’s are not the same size, the lens in the pickup must move up and down to achieve focus for separate discs. The focusing mechanism has a magnetized pole piece surrounded by a focusing coil. Electrical current passes through this coil to achieve focus. To read data without significant error, the lens has to be moved very precisely. A tracking coil that is wound around the same pole piece as the focusing coil handles this movement. This means the lens can move approximately 0.15 inches either way from its point of rest. The biggest difference between the focusing coil and the tracking coil is the force applied to them, which is vertical and horizontal respectively.
33. Comment on CD master PQ editing.
PQ editing entails editing the subcode area P and Q on a CD. The information included in this subcode area is start IDs and end IDs, indexes, ISRC codes (International Standard Recording Code, which contains owner of the copyright, release #, record label...), emphasis and copy protection of the CD.
34. What is CD master tape verification?
This is a byte-to-byte comparison with the original master to insure an exact duplicate.
35. What is Eight Fourteen Modulation?
Magnetic media stores data using 8-bit bytes but optical media such as CD-ROM’s store data using 14-bit bytes due to the way data is stored and read by the lasers. When the data is transferred from magnetic to optical media the 8-bit byte is modulated to a 14-bit byte. Then when the computer reads the CD-ROM, an interface card demodulates the 14-bit code back to an 8-bit code.
36. What is a Successive Approximation Register converter?
It’s a counter circuit on an ADC that counts all values of bits starting from MSB and ending with the LSB. The register monitors if the binary count is greater or less than the analogue input signal and adjusting the bit value accordingly. This gives much faster results.
37. Describe and explain the following digital audio concepts:
• Jitter – this is time-base error caused by varying timing delays in the circuit path from component to component.
• Simple codes –?
• Group codes –?
38. Outline digital audio in video recording systems.
?
39. What is Solid State recording?
Solid state recording is recording to media that have no moving mechanical parts. Instead everything is electrical.
40. What is a Session file?
A session file can be the file you load into your sequencer that stores the title of the project, fader positions, automation, the edits done to audio files, effects settings such as EQ, Send/Return, the arrangement of all recorded audio etc. Basically, it stores your mix exactly the way you left it the last time you saved.
41. Discuss the archiving of digital recordings.
Digital recordings can be archived to CD-R’s. CD-R archiving is a very stable and reliable method, taken that the CD-R’s are taken care of and stored in a safe place. The downside is the small amount of space available on a CD-R (700 MB).
42. Comment on some features, functions and parameters of disk recording systems.
Disc recorders such as the Minidisc come with features such as track editing (renaming, moving, deleting), digital bass, playlist editing, functions such as repeating songs or entire discs. Many new models can also be connected to computers using USB and editing of parameters can be done through the PC. |
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Sun Oct 03, 2004 2:35 pm |
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AUdIoCoUrSeS
Joined: 31 Oct 2002
Posts: 2014
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| Answers |
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Ok well done with some of those, they are answered well.
However I want these loose ends tidied up.
Perhaps reach for the library, or trying searching in different ways or different engines on the web.
quote:
4. State and describe the FOUR possible actions of the error correction system of a CD player.
This needs an answer
5. Explain the two basic methods that a Hard Disk editor may use to create a cross fade.
This needs an answer
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 is that the Nyquist frequency moves farther above the audible 20Hz-20KHz frequencies and this gives a smoother more linear phasing because the filter is more relaxed.
7. Explain the operation of an 18-bit 4x oversampled DA converter on a CD player.
This needs an answer, you kinda of answered it in question 15.
8. What type of reduction does DCC use that may keep this medium in the domestic market only?
I want you to look at this again. I am interested in DATA reduction, not noise reduction.
20. List three types of DACs
hmm re think this.
30. What is the 1630/Video U-Matic system?
Need this answered
37. Describe and explain the following digital audio concepts:
• Jitter – this is time-base error caused by varying timing delays in the circuit path from component to component.
• Simple codes –?
• Group codes –?
38. Outline digital audio in video recording systems.
?
CD PLayer
http://www.tc.umn.edu/~erick205/Papers/paper.html
Cross Fades
http://www.sonicstudio.com/PDFs/White_papers/wp_aud_Hard-DiskEditofDA.pdf
DCC
http://www.screensound.gov.au/glossary.nsf/Pages/DCC+-+Digital+Compact+Cassette?OpenDocument
DACs
http://en.wikipedia.org/wiki/Digital_to_analog_converter
Video
http://www.chumpchange.com/parkplace/Video/DVPapers/dv_formt.htm
_________________
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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Sun Oct 03, 2004 4:48 pm |
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iNSTiNCT2765
Joined: 05 Nov 2003
Posts: 60
Location: Denmark |
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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 are:
(1) Parity Checking – good for catching small error but doesn’t work if the parity bits are wiped out as well, due to a big chunk of data being damaged.
(2) Interleaving – used to reconstruct interleaved data using algorithms the player understands. Since the data is scattered, if a big chunk is damaged or unreadable, the player can determine what the missing bits should be by using the algorithms.
(3) Interpolation – the player will determine an average using the good bits before and after the bad bits. This can result in an inaudible continuation of the audio or a slight glitch.
(4) Muting – if the player encounters an error, which cannot be fixed using the other means, it will mute the output for the duration of the error burst. Fast fade ins and outs are executed to avoid pops and glitches.
5. Explain the two basic methods that a Hard Disk editor may use to create a cross fade.
The first method is called a ‘memoryless fade’. This is done in real-time and can be done using faders. This type of cross fade cannot be edited since it is a one-time action and has to be done over if not satisfactory.
The second method is called a ‘precomputed fade’. Here, a series of edits are scanned and all the fades are noted. The sounds that accompany the edits are read into RAM memory while the computer applies the fades to them. On playback, the data switches from the direct audio on the hard drive to the edited sounds in RAM and back again after the fade is complete.
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 here is to avoid distortion and get a better linear phase when sampling into the digital domain.
7. Explain the operation of an 18-bit 4x oversampled DA converter on a CD player.
The converter uses the 2 extra bits to create a longer word ensuring the accuracy of the MSB and by oversampling 4x, the linear response of the low pass filter gives an output with better SQNR.
8. What type of reduction does DCC use that may keep this medium in the domestic market only?
DCC uses Dolby B noise reduction, which only has a one-band filter for reduction working only at 1KHz and above. This makes it ineffective with high-quality recorders in comparison to other reduction systems.
DCC uses Precision Adaptive Sub-band Coding (PASC) for a data reduction of 4:1. This means that the sound quality will be perceived as CD quality but will be lacking the dynamic range needed for recording original music in professional studios. Just like a Minidisc player, the DCC is good for consumer use to play back music where attention to detail is not absolutely critical but it is not acceptable in a professional studio environment.
20. List three types of DACs.
1) Pulse Width Modulator
2) Oversampling DAC
3) Binary Weighted DAC
30. What is the 1630/Video U-Matic system?
This is a system developed by Sony, which records digital audio onto a U-Matic videocassette. It was the standard medium for CD master tapes.
37. Describe and explain the following digital audio concepts:
• Jitter – this is time-base error caused by varying timing delays in the circuit path from component to component.
Channel coding creates the actual modulated signal from the original signal that is stored on the media. There are two types of codes:
• Simple codes – each data word from the original signal is taken and encoded into a separate channel code.
• Group codes – use code look-up tables to convert groups of input words to patterns of output words.
38. Outline digital audio in video recording systems.
Digital audio can be recorded as either 2 stereo 32KHz, 12-bit non-linear channels or as one 16-bit stereo channel at sample rates of 48KHz, 44.1KHz or 32KHz onto the audio sector of a DV. |
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Tue Oct 05, 2004 11:59 am |
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