Transcript Early Born-Digital Audio Formats

Early born-digital audio formats
Compiled by George Blood
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George Blood Audio, LP
Safe Sound Archive
First Commercially Available
Formats
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PCM-1
•
PCM-10
•
PCM-F1
•
PCM1600/1610/1630
•
DAT
Resources
“The Dawn of Commerical Digital Recording”
Thomas Fine, ARSC Journal (Spring, 2008): 1-17.
Principles of Digital Audio, Ken Pohlman
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
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
•
white
black
Video monitor showing digital audio being played
QuickTime™ and a
H.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
0
0
1
1
0
0
1
0
1
0
1
0
1
0
1
1
0
0
1
1
0
1
0
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]
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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)
George Blood
Safe Sound Archive
[email protected]
(215) 248-2100
www.safesoundarchive.com