To MP3 and beyond!  or  The story of a compiler test program gone large. James D. (jj) Johnston Chief Scientist DTS, Inc. A bit of Chronology. 1976 – 16 kHz, 28 bit ADPCM in analog hardware 1977 – 32kHz, 212 bit ADPCM in analog hardware
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James D. (jj) Johnston
Chief Scientist
DTS, Inc.
Well, this was my first signal processor:
gnd
r
+
ADC
r

r
r
Yes, that’s an analog
DPCM encoder.
Yes.
Really!!!
DAC

rp
C
Well, for the next trick, we added an analog divider in front of the ADC and an analog multiplier in front of the DAC. This allowed us to change the step size in the ADC/DAC pair.
As the divider and multiplier had exponential inputs for the “control” voltage, this conveniently allowed us to implement Jayant/Gersho adaptive quantization, i.e.:
Delta (t+1)= delta(t)^(b)*M(q(t))
Where Delta is the step size, b is a “leak” factor that allows the system to converge on startup or system error, M() is a set of multipliers based on the absolute value of the quantized value, and q(t) the quantized value. ‘t’ is the time index.
In the hardware, this was implemented by the equation
d(t+1)=b*d(t)+m(q(t)), where each (except b) are logs of their respective values.
(time passes, working on other things) coder
*Warning, sarcasm included.
 or 
Quote from Allan Sherman, “Peter and the Commissar”