Advice on Event Synchronization with Portaudio

[Eamon Kavanagh]

Hey everyone,

I'm trying to write some software that synchronizes song beats with other events and I'm a bit confused on the correct way to approach this. I've read through this PDF and tried to implement a prototype of the "Synchronising Graphical Display" section as follows:

- Start filling a callback buffer with a fixed BPM click track and save the outputBufferDacTime as the start_time of the track.

- Listen to the output and try to click my mouse in time

- Whenever there's a click event, calculate the song_time as Pa_GetStreamTime(...) - start_time

- Use fmod(song_time, beat_duration) where beat_duration = 60 / BPM to determine how far off the beat I am.

Is this the right approach? When I try this I get fairly poor results and I'm usually off the beat by ~150ms. What I don't quite understand is if I instead calculate song_time each callback as  outputBufferDacTime - start_time + Pa_GetStreamInfo(...).outputLatency, I get much better results and am off the beat by ~20ms.

Some additional information if it's helpful:

Sample Rate: 48000

Output Latency: 0.048

Frames per buffer: 256

Let me know if there's any other information I can provide.

Thanks,

Eamon