如何将方法调用与 alsa 播放同步？

Question

I'm trying to write a program that will synchronize lights to playback of a basic wav file. I've struggled through all the alsa docs, the source for ffplay.c and searched around on the internet, but it's difficult to figure out how to do what seems like a common and simple task.

Basically I want to do two things, the first is to read keypress events while the audio is playing and store the offsets in a file. The second is take those queue files and load them later, this time on a different audio device like a raspberry pi.

I'm struggling with how to first account for the latency in the initial capture of the offset positions and then how to handle that latency when I playback on a completely different hardware device.

I know snd_pcm_delay() is used by the ffmpeg suite to deal with some of this, but I'm really struggling with even the basic technique. It's not a complicated playback mechanism, just a blocking write in a loop.

I'd post some code, but I don't have it with me at the moment and it's just a mess of the current hacks that aren't working.

Answer 1

So this turned out to be somewhat easy in the end, however it wasn't easy to figure out. Using snd_pcm_delay is the correct path.

The current actual audio sample being played is the number of frames written - snd_pcm_delay , this gives you a relatively exact current played frame (the frame that you should be hearing right now). This should be used to calculate the position of timestamps and as the position index as an app waits for the current position to be ahead or the same as the next cued event.

This is something that GStreamer does in a much more documented and more extensible way, building a more complicated synchronization algorithm that allows the audio to run on a separate thread while avoiding locking every time the position clock is needed. I'd use GStreamer if possible to implement any similar applications.

But if you don't need all the complexity of GStreamer and want to cut your dependencies, the basic method is the one I described above.