Since all that a pcm accomplishes is analog to digital conversion, why is it called a modulation technique?
There is a message signal but no carrier. According to my knowledge, modulation requires a carrier.
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From Wikipedia's article on modulation:
In electronics and telecommunications, modulation is the process of varying one or more properties of a periodic waveform, called the carrier signal, with a modulating signal that typically contains information to be transmitted.
So all we need to do is identify a periodic waveform and a way to modify its properties by a signal. One easy way to think of this: the "carrier wave" is a dirac comb that pulses at the pulse repetition rate. This signal is modified by convolving it with a pulse whose width varies as a function of the input. This creates a pulse train.
You could say that the carrier in this case is the sample clock, the unit impulse or the combination of the two.
You don't really need to transmit the clock as long as you use the same frequency when you de-modulate it. Otherwise the sampled sound will become shorter and higher in pitch or longer and lower in pitch.