Audio Channel Output Routing

SDK Feature Requests
1

We need a way to connect the output of an audio channel to another audio channel. As far as I can tell, there is not yet a feature for this, but it is essential in any real or virtual mixer system.

This would allow applying an effect (or volume/pan control) to groups of sources, while those sources can still have their own independent effects chains.

A simple example would be allowing user adjustment of BGM or SFX volume independently. Different BGM and SFX sources could need their own effects, so adding all SFX to a single SFX channel and all BGM to a single BGM channel wouldn’t work. At present, different audio channels could be associated by some means to BGM or SFX uses, and all of e.g. the BGM channels’ volume controls could be adjusted at once, but this can get complex to track. It becomes more complex when relative adjustment of different BGM channels is also desired.

Another example would be applying a delayline as an echo effect to all sounds considered to be environmental. If there are different environmental sounds that use their own effects, then the only choice is to apply them to different channels with the desired effects, then also add an identically modulated delayline to each of those channels; this is computationally expensive.

Implementation suggestion:

If the output of a channel is internally modeled as a source, or if modifications can be made to achieve this, then maybe a playdate.sound.channel.getOutputSource method could be added, and users could apply something like this:

snd = playdate.sound

local smp = snd.sample.new("SFX/PCMSample")

local syn1 = snd.synth.new(snd.kWaveSine)
local syn2 = snd.synth.new(snd.kWaveSquare)
local syn3 = snd.synth.new(smp)

local ch1 = snd.channel.new()  --syn1/syn2 don't need FX
ch1:addSource(syn1)
ch1:addSource(syn2)

local ch2 = snd.channel.new()  --syn3 has its own delay
ch2:addSource(syn3)
ch2:addEffect(snd.delayline.new(0.25))

local sfxChannel = snd.channel.new()
sfxChannel:addSource(ch1:getOutputSource())
sfxChannel:addSource(ch2:getOutputSource())

--All 3 synth sources can now be processed at sfxChannel. Vol, pan, fx...

For convenience, could also allow specifying channels directly in channel.addSource

sfxChannel:addSource(ch1)
sfxChannel:addSource(ch2)
2

In many games, opening a menu or pausing while BGM is playing will quiet or dampen the music, which can most efficiently be done with the requested routing. There are also games that when inside of a building, etc. BGM and exterior SFX are quieted or dampened.

In Hollow Knight, when the player character takes damage, a filter effect is temporarily applied to all game sounds (I think the BGM, too) but it does not affect UI sounds like menu chirps. This most likely utilizes Unity’s Send effect.

That’s another implementation approach that might be better for compatibility: implement a Send effect that can be added to a channel which creates a playdate.sound.source for the stream at that point in the effects chain and also has a setMix method like the other effects. In theory I can already sort of do this with delayline taps, but since they require delaylines on the channel of interest I believe that would be CPU-inefficient.