3.6.0: AudioChannel.Speed has no effect

[ivan-mogilko]

In 3.6.0 setting playback speed in script (using AudioChannel.Speed property) has no effect whatsoever.

The script channel's speed value is being set correctly, which in turn sets OpenAL's source AL_PITCH param.
But apparent problem is that mojoAL is missing pitch implementation completely. It has it applied to a data structure, but I found no code that would use this value.

[ericoporto]

Probably related issue there is icculus/mojoAL#2

[ivan-mogilko]

For the future reference, mentioned in that issue, people in another project added their own pitch impl to their copy of mojoal:
supertuxkart/stk-code@33459e8

[ivan-mogilko]

On the other hand, it might be that OpenAL's "pitch" by concept is not a direct equivalent to the playback speed.
In this, and any case, we may also implement resampling in the engine code, and pass altered sound data into mojoal.

[ericoporto]

MojoAL now has a pitch implementation. :)

[ivan-mogilko]

Aha. Commit for the reference: icculus/mojoAL@47b22d0

EDIT: from the description it seems like this is not an equivalent to Speed though. I will have to test how it works.

[ivan-mogilko]

So, I tried it out, and mojoAL's Pitch is not what we need for Speed, it's a separate kind feature. As mentioned in the commit description, setting pitch only adjusts the sound samples, but it does not change the sound duration.

When reading OpenAL specification though, here what it states, in a paragraph for AL_SEC_OFFSET:

This value is based on byte position, so a pitch-shifted source will have an exaggerated
playback speed. For example, you can be 0.500 seconds into a buffer having taken only
0.250 seconds to get there if the pitch is set to 2.0.

Unfortunately, the paragraph for AL_PITCH itself does not mention anything about this.

[ivan-mogilko]

As mentioned by @ericoporto elsewhere one may e.g. pass different frequency into the alBufferData call to get the speed changing result.
UPDATE: unfortunately this particular method does not allow to change the frequency for a playing sound dynamically. Only if you set the different freq at the start.

[ivan-mogilko]

I made an experiment with resampler, which changes frequency before passing to OpenAl. So OpenAl thinks that the frequency is same, and sound ends up playing slower and faster; result matches the one of 3.5.1 from what I mat tell.
ivan-mogilko@40af1c2

There are couple of issues with this, first the resampling is done per chunk, so there may be lags in speed change. Also position calculation is based on what openal returns, and so far not precise.