[sdiy] RE: [AH] control voltage was/ Keyboard in space

[Grant Richter]

> Most books say that quantisation produces some kind of hissing noise.
> This is sometimes true. But if you quantize a narrow band signal
> (all musical instruments in the decay phase) you can better
> describe this as distortion.
> 
> This is a trap some people fall into some time.

I think there was a lot of misunderstanding when binary coded technology (so
called "digital") was first introduced. This is specially true for audio
applications.

There is a difference between "noise floor" the overall flat spectral noise
component of a signal, and signal to noise ratio, which is the ratio of a
peak sine wave to the next unintended signal component. This seems to be
second harmonic distortion usually. In the case of the classic ENS-76 VCA,
the noise floor is -110 dB, but the second harmonic is at -60 dB, so
instruments will report a signal to noise ratio of -60 dB.

If this VCA is connected to a 16 bit ADC, the audio noise floor will be
RAISED to -90 db. (theoretical is -96, but real ADCs are noisier than
theoretical)

I think the confusion arose from the noisy performance of tape recording
systems. Magnetic tape can have a noise floor of -50 dB. So a noise floor of
-90 dB looks good in comparison. But the circuitry feeding into it can
easily have a noise floor lower than -90 dB.

The classic quantization noise equations just predict a noise level that is
guaranteed to be injected into the signal chain.