Propeller Head Plaza

RE: All PCM is digital, but not all digital is PCM

64.13.14.96

In Reply to: RE: All PCM is digital, but not all digital is PCM posted by jpvs on July 01, 2008 at 22:30:09

"Looking at this another way, is not PWM simply binary? It's output is either 1 (on) or 0 (off) for each frame of time determined by the switching rate."

Hmmm, we probably shouldn't call it binary. How about bi-level? Binary refers to the base of a number system but remember (or believe) that what's transmitted isn't a number. Take the PWM 'binary' code for the star spangled banner: 01010101010101... it isn't too different from the code for the one eyed, one horned, flying purple people eater: 0101010101...

Point being that the data is encoded in time, not numbers. But time doesn't exist without energy, or if it does, you still can't tell what time it is so we generate a voltage modulated by the data to transmit the time stream whose on/off ratio is analogous to the signal. It's just a modulation scheme.

"There may be a million set frames of time and with each the amp is either on or off with those time slots not being further dividable."

OK, but they are also not recoverable once transmitted as there is always some distortion. Let's just say that you connect them to a speaker. The mechanical, electrical, and acoustical energy couplings in the speaker will produce a varying back EMF which will sum with the signal across the impedance in the circuit and output impedance of the driver. NOW, just try to figure out to the resolution limit of the original data what the on/off ratio really is. You can't do it because there is no perfect cutting point. The saving grace is it doesn't matter, it averages out and the data is the mean. It's analog.

Your point about lossy formats is interesting. I'd say the deal is that the MP3 data transmitted is digital but that the data itself is a carefully distorted analog of the original data.

Wrapping back to the whole point, what matters is the system fidelity. I can't help but believe that all the factors involved are well known by now. What isn't well understood yet, IMHO, is their relative importance and the best possible tradeoff between any unavoidable compromises. Frankly I doubt there is one due to variations in hearing and taste. But I believe that the more perfectly that we make the system, the higher the odds that a random person will be happy with it. This forum is really about kicking the tires of those elusive residuals. That and some arguing for the sake of arguing.

Rick

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