[Matheus290]

This adds a WSOLA pitch change filter, interchangeable with rbpitch and with no external dependencies, and makes it a second pitch compensation method for timewarp, as discussed.

What it does

• New audio filter wsola in the plus module. It takes the same parameters as rbpitch (octaveshift, pitchscale, animatable, and stretch) and, like it, keeps the number of samples and the frequency, so it works on its own and the two can be swapped.
• It changes the pitch in two steps: a sinc resampler resamples the audio by pitchscale, which changes the pitch, and WSOLA then time stretches it back to the requested length. Both are ports from Chromium (media/base/sinc_resampler and media/filters/audio_renderer_algorithm with wsola_internals, BSD licensed; the license is kept in src/modules/plus/wsola/). The algorithms, constants and index arithmetic are unchanged; only Chromium's support types were replaced, and the resampler's AVX2 path was left out.
• producer_timewarp: new property warp_pitch_method. With warp_pitch=1, wsola selects the new filter. Any other value uses rbpitch exactly as before, so existing projects are not affected. Both filters get pitchscale = 1/speed.

Testing

• New test_wsola (Qt test): pitch preserved through timewarp at 0.5x, 2x and 3x; the filter on its own shifts a tone by pitchscale 0.5, 1.5, 2 and octaveshift -1, with the requested number of samples and no gaps; a seek does not change the pitch.
• Measured latency (onset of a blipflash blip, with and without the filter): 22 ms at pitchscale 0.5 to 72 ms at 2 (rbpitch: 25–67 ms). Through timewarp: about 60 ms at 0.5x and 2x. The delay is constant over time.
• On a 1 kHz tone, the energy outside the tone stays more than 100 dB down at every scale tested.
• clang-format 14 clean; the .yml files validate against metaschema.yaml.
• ctest: everything passes except TestProperties::RadixRespondsToLocale, which fails the same way on master on this machine (no comma-decimal locale).

How to listen

Any clip with speech or music works (close the melt window to stop):

melt timewarp:2.0:clip.mp4 warp_pitch=1 warp_pitch_method=rubberband
melt timewarp:2.0:clip.mp4 warp_pitch=1 warp_pitch_method=wsola
melt clip.mp4 -attach wsola pitchscale=1.5
melt clip.mp4 -attach rbpitch pitchscale=1.5

To run the automated test: ctest -R wsola --output-on-failure.

Kdenlive has said it wants to offer this; a matching merge request there will select the method per clip.

This was written with the help of Claude (AI).

🤖 Generated with Claude Code

[bmatherly]

What is the typical latency for this service? Also, can you check what the typical latency is for rubberband? I wonder how they compare?

[Matheus290]

Thanks for the review, and sorry for the delay. I've pushed changes for all three points:

plus module. The filter is now in plus, with the Chromium code in src/modules/plus/wsola/ like ebur128, and copyright Meltytech. MOD_WSOLA is gone.

Same parameters as rubberband. wsola is now a regular pitch change filter with the same interface as rbpitch: octaveshift, pitchscale (animatable) and stretch, keeping the number of samples and the frequency. It does not know whether timewarp uses it. To change the pitch without an external library, I also ported Chromium's sinc resampler (same BSD license): the audio is resampled by pitchscale, which changes the pitch, and WSOLA stretches it back to the requested length. timewarp now sets pitchscale = 1/speed for both filters, so the two are interchangeable there too. The earlier commit that made wsola pass through audio from anything but timewarp is superseded by this.

Latency. Measured end to end with blipflash (onset of the blip with and without the filter, at 48 kHz):

pitchscale	wsola	rbpitch
0.5	22 ms	55–67 ms
0.75	35 ms	43–51 ms
1.5	47 ms	43–47 ms
2	72 ms	25–27 ms

Through timewarp: at 0.5x, wsola 63 ms and rubberband 25 ms; at 2x, wsola 58 ms and rubberband between 91 and 126 ms (it varies). So they are in the same range. wsola's delay grows with the pitch scale, because the resampling step stretches WSOLA's own look-ahead, while rbpitch tends to go the other way. wsola's delay is also constant over time.

On a 1 kHz tone, wsola keeps the energy outside the tone more than 100 dB down at all of the scales above, where rbpitch measured 7–53 dB. Both hit the right pitch.

[Copilot]

🟡 Changes recommended

Unresolved sample-accounting, overlap-selection, and pitch-range defects can produce silence or incorrect audio, while filter destruction leaks resources and the seek test fails to exercise nonzero seeks.

5 open issues

• Fall back to rbpitch when WSOLA cannot support the speed ratio · New
• Close the parent service to prevent leaks · New
• Allow FullSearch to select negative similarity scores · New
• Restrict passthrough to an exact 1.0 playback rate · New
• Honor the requested starting seek in the audio test · New

What changed in this PR

Adds a dependency-free WSOLA pitch filter to MLT’s plus module and makes it selectable for timewarp pitch compensation, while retaining Rubber Band as the default.

Changes:

• Adds Chromium-derived sinc resampling and WSOLA time stretching.
• Introduces warp_pitch_method and registers the new filter with its metadata.
• Adds Qt tests for pitch shifts, timewarp compensation, sample counts, audio gaps and seeking.

File	Description
src/​tests/​test_wsola/​test_wsola.cpp	Adds standalone filter and timewarp audio tests.
src/​tests/​CMakeLists.txt	Enables WSOLA tests when plus is built.
src/​modules/​plus/​wsola/​wsola_algorithm.h	Declares audio buffers and time-stretching interfaces.
src/​modules/​plus/​wsola/​wsola_algorithm.cpp	Implements WSOLA with optimized similarity searches.
src/​modules/​plus/​wsola/​sinc_resampler.h	Declares the sinc resampler interface.
src/​modules/​plus/​wsola/​sinc_resampler.cpp	Implements sample-rate conversion for pitch shifting.
src/​modules/​plus/​filter_wsola.yml	Documents filter parameters and latency reporting.
src/​modules/​plus/​filter_wsola.cpp	Integrates resampling and time stretching into an MLT filter.
src/​modules/​plus/​factory.c	Registers WSOLA and its metadata.
src/​modules/​plus/​CMakeLists.txt	Builds WSOLA sources and installs metadata.
src/​modules/​core/​producer_timewarp.yml	Documents pitch-compensation method selection.
src/​modules/​core/​producer_timewarp.c	Selects and switches pitch-compensation filters.

_{Review effort: Balanced}