Does ADPCM has some sample rate?

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1

ADPCM is adaptive, so it has varible sample rate. But does it have some average rate or something? Does it have frames of fixed time duration?

2012-04-03 21:13
by Dims
The sample rate is fixed, is it not? Are you referring to the bit rate - Brad 2012-04-03 21:16
I don't know. I read ADPCM has variable quantization=sampling period - Dims 2012-04-03 21:34
Variable quantisation does not equal sampling period. Sound data is quantised in to a particular level and does not affect the actual rate of those sound levels - Cthutu 2014-06-23 19:23


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You misunderstood it here :-). "Adaptive" doesn't mean that sample rate is adjusted according to the signal it contains.

"Adaptive" means that the limited available delta steps (4Bit = only 16 possibilities to encode a sample) are adapted to the signal by prediction. It attempts to approximate from a given sample which value the next sample may have and adapts the delta steps to that.

If the signal has less change from sample to sample, the steps are chosen closer togheter than if the signal has much change. It is very unlikely that the signal goes from very oscillating to quiet from one sample to the next.

You notice that behavior if you encode a square wave with 100Hz using such algorithm and re-open it in an audio editor that makes the waveform visible. When the waveform changes from one polarity to other, the signal "speeds up" (the steps are more and more apart) until it reaches the other end and then it slows down again (The steps are more and more close togheter).

It still has a fixed sample rate. The one you will give to it. In RIFF WAVE, the sample rate is stored in the header.

2016-07-09 21:52
by user2766477
Thanks! Late but clear : - Dims 2016-07-10 08:50