early born-digital audio formats

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 = 40kHz10% margin = 44kHz

• 16bits * 6dB/bit = 96dB of dynamic range

• 44,000 samples per second16 bits per sample2 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 line25*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 11 00 10 01 11 00 10 0

1 11 00 10 01 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=101+2+x+6=10and 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)

George BloodSafe Sound Archivegeorgeblood@safesoundarchive.com(215) 248-2100www.safesoundarchive.com