[openal] Filtering a single channel in a stereo source

[Chris Robinson]

On 04/06/2016 09:07 PM, Ian Reed wrote:
> I'd like to apply a filter to a single channel in a stereo sound source.
> Is this possible?
> If not, is there a good way for me to start each channel as its own mono
> source and somehow keep them in sync?

There's currently no method to set a specific source channel's filter 
properties, just for all source channels. It would theoretically be 
doable, with the right method (having a source method to tweak specific 
properties for specific channels would actually be nice beyond just 
filters), although I have concerns. Specifically, filters can effect the 
phase of the output, which if they can be set separately for separate 
channels, may cause the channels to sound slightly out of sync, or worse 
cancel out so the lower frequencies are cut in the middle. It might be 
something worth testing, at any rate.

You can play two mono sources separately in sync with each other. 
alSourcePlayv allows you to play multiple sources at once, with the 
guarantee that they start at the same time. Just ensure they have the 
same offset to start with when calling it. If you want to change any 
pitch-related properties while it's playing, such as AL_PITCH or 
AL_VELOCITY, you need to make sure the changes are batched using the 
AL_SOFT_deferred_updates extension, or hoping alcSuspendContext and 
alcProcessContext will make sure property changes get batched.