early born-digital audio formats compiled by george blood george blood audio, lp safe sound archive
TRANSCRIPT
Early born-digital audio formats
Compiled by George Blood
• George Blood Audio, LP• Safe Sound Archive
First Commercially Available Formats
• PCM-1
• PCM-10
• PCM-F1
• PCM1600/1610/1630
• DAT
“The Dawn of Commerical Digital Recording” Thomas Fine, ARSC Journal (Spring, 2008): 1-17.
Principles of Digital Audio, Ken Pohlman
Resources
Quantization
• “The process of converting analog signals to digital.” syn: digitization
•Pulse Code Modulation: PCM
Sine Wave
Sine Wave Quantized
PCM≈TIFF
• TIFF congruent to PCM
• DPI congruent to kHZ
• Range of color congruent to range of volume
Other quantization methods
• PWM: Pulse wide modulation
• Delta-Sigma: sum of change
• Delta-Modulation: change in value (used in SACD’s “direct stream digital”)
1’s & 0’s
• light on light off
• positive voltage negative voltage
• positive magnetic flux negative magnetic flux
• lands (light reflects) pits (light doesn’t reflect)
Nyquist formula
• the highest frequency that can be captured in PCM is exactly one half the sample rate
fN = (fs/2)
where fN is they Nyquist frequency and fs is the sampling
frequency
Nyquist in Action
44kHz 16 bits
• 20kHZ target upper limit *2 = 40kHz
10% margin = 44kHz
• 16bits * 6dB/bit = 96dB of dynamic range
• 44,000 samples per second
16 bits per sample
2 channels (stereo)
44,000*16*2 = 1,411,200Hz (1.4MHz)
Tape Head - Side View
Tape Head - Side View
• “How can we increase the size/length of the signal relative to the head gap?”
- We could move the tape faster.
- Or we could move the head in relation to the tape!
Helical Scan
How 44,000 became 44,100
• First video recorders used were PAL (European) format– Frame rate is 25 (instead of 30 for NTSC)– Lines per frame is 625 (instead of 525 for NTSC)
• 37 lines reserved for sync, overhead, headers• 588 active lines for audio data
• 3 samples per line
25*588*3 = 44,100
NTSC: 30*490*3 = 44,100
• light on light off
• positive voltage negative voltage
• positive magnetic flux negative magnetic flux
• lands (light reflects) pits (light doesn’t reflect
1’s & 0’s
• white black
Video monitor showing digital audio being played
QuickTime™ and aH.264 decompressor
are needed to see this picture.
• Color video doesn’t run exactly 30 frames/sec.– 29.97 frames/sec– NSTC Color: 29.97*490*3 = 44,056
• 44,100 comes out of the A to D converter• Video is locked to the incoming signal• Video is played back by the internal crystal running at 29.97• Audio is clocked at 44,056
CHAPTER 2: Organizing the data
• .wav
• Header
• data block
• Header/control track (metadata)
binary
1 1
1 0
0 1
0 0
1 1
1 0
0 1
0 0
1 1
1 0
0 1
0 0
1 1
1 0
0 0 1 0
11001100 10101010
• LRLRLR
or• LLLRRR
Chapter 3:Error Correction
1 0
• LRLRLR
or• LLLRRR
Cyclic Redundancy Check Code (CRCC)
• x6+y3+z+1
• assume two values are correct, solve for third:
1+2+3+x=10
1+2+x+6=10
and so on..
Further reading“Google Search Terms”
• Dual Reed-Solomon [error correction]
• Cyclic redundancy check codes [CRCC]
• Block structure• Control Track (Metadata)• Interleaving• Error correction• ETF (eight to fourteen transform)• Sync pulses• etc• etc• etc
Playback challenges
1. Hardware obsolescence
- finding a machine is many times more difficult than playing any given tape
2. Fragile Carriers - very old and/or very fragile video formats (typically U-Matic, consumer Beta or VHS)
3. Experienced operators
- many apparently catastrophic playback problems are due to simple, easily corrected causes
4. Marriage to video carriers mean you get all of video’s problems too
- drop outs (drop out compensation makes matters worse)
- time base errors (slow tape speed vs. high frequency)
- tracking errors (fuzzy 1s and 0s, including error correction data)
- media deterioration (such as Sticky Shed Syndrome)
