[Speex-dev] Feature request: rewindable resampler
Alexander E. Patrakov
patrakov at gmail.com
Wed Nov 19 16:20:24 UTC 2014
As you probably know, PulseAudio uses the resampler from libspeexdsp by
default. As a PulseAudio contributor, I have a feature request.
As you can see from old publications [1,2] by Lennart Poettering,
PulseAudio has a "timer-based scheduling" feature which is now active by
default. PulseAudio attempts to use as high latency as possible
(sometimes up to 2 seconds) in order to save power and reduce the chance
of dropouts. However, in order to stay responsive to events such as
software-volume changes or a new stream appearing, PulseAudio relies on
the ability to rewind over already-buffered but not-yet-played sound
data and replace them with a new version.
Also I must say that PulseAudio client API includes rewinds - i.e. any
client can say "please forget N samples that I wrote before, here is an
Unfortunately, we cannot pass such rewind requests through a resampler,
simply because no open-source resampler library has an API to do this.
I.e. there is no way to forget N last input samples, replace them, and
figure out which output samples should be replaced with what. Currently,
in this situation, we just reset the resampler, which is wrong.
Also there are situations where it ends up wanting to reask for N last
output samples corresponding to a modified input where the input
modification can be produced on-demand and logically extends to negative
infinity in time. Usually, this happens when software-volume is applied
before the resampler and a volume-change event occurs. The same kind of
bug then happens: we cannot do anything better than resetting the
resampler. End result: users complain, see e.g. .
Could you please consider designing and implementing new speexdsp API
functions that would allow PulseAudio to get rid of these bugs?
In fact, because this request is, in my opinion, specific to PulseAudio
and ALSA (in ALSA - because of the need to implement snd_pcm_rewind()
for the rate plugin), I considered writing a new resampler from scratch
with this feature. However, I will almost certainly not start within the
next 3 months due to my dayjob and because of other TODO items, and
David Henningsson has hinted me to talk to you as an upstream of speex,
because, in his opinion, this "rewind" feature might be useful for other
applications. Hence this email :)
Alexander E. Patrakov
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