wip: avoiding repeated copying of audio in speedup #746
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Issue #736 demonstrates that the
speedupeffect function becomes prohibitively slow for long audio segments. This is due to the loop in lines 90,91 of the speedup function (pydub/effects.py), where the shortened chunks are appended usingAudioSegment.append, which result in repeated copying (and necessary memory allocations) of the already processed part for creating of a new immutableAudioSegmentfor each chunk.This PR shows a much more performant way by essentially copying the byte logic of
AudioSegment.appendintospeedup. For the example given in issue #736 , this completes in about 11 seconds. However, the current implementation does not currently deal with a variety of corner cases around the length of chunks and crossfading which was previously elegantly (but imperformantly) solved by abstracting throughAudioSegment.append. Also, the introduced code duplication is not ideal.Therefore, this PR draft is mostly meant to start a discussion on how to approach this best. The main problem of the old approach lies in the immutability and the corresponding copying of
AudioSegments. Maybe seperating some of the composition routines (append, overlay, fade, etc) to work on preallocated low-level buffers could be a way to avoid duplication while still getting good performance?