This CL breaks out the output sample rate calculation from
webrtc::AudioMixerImpl. A new OutputRateCalculator interface is added
to make the sample rate configurable. There are at least three reasons
for this change:
1. The mixer will be used for an internal project, in which no
resampling is done after the mixing. There the sample rate should
be static. Currently, it can differ across mix iterations and
depends on the number of audio sources. If there are no sources,
the WebRTC mixer behavior is to produce silence at 48 kHz.
2. A planned change to WebRTC will make audio processing steps
happen at constant sample rates. A configurable sample rate
calculator will make the transition simpler for the mixer.
3. The current mixer design is a single large file. Behavior is not
always simple to change (e.g. as in this case to mix at a
constant rate), unrelated behavior can be broken, reusing the
mixer in internal projects is tricky. Using DI for the sample
rate calculation solves parts of these issues.
Changes:
The protected mixer c-tor now takes
unique_ptr<OutputRateCalculator>. The current output rate calculation
is moved to DefaultOutputRateCalculator. A new factory method
AudioMixerImpl::CreateWithOutputRateCalculator is added. The old
factory method passes the default rate calculator.
BUG=webrtc:6346
Review-Url: https://codereview.webrtc.org/2557713006
Cr-Commit-Position: refs/heads/master@{#15472}
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