I have written a soft synth, which uses ALSA to get MIDI events, and Jack to do audio output. MIDI events are fetched and processed at the start of the Jack audio callback, then the Jack output buffers are filled by the synth engine. The problem I have is that, if the Jack buffer size is too large, the time resolution for MIDI events is too coarse, and this has a severe effect on rhythms.
I currently get around this by setting up Jack for a buffer size of 64 frames, and a sample rate of 48 kHz, giving a timing resolution of 1.33 msec, which appears to be adequate. However, I might not always be able to get the desired buffer size. If the Jack buffer is some multiple of 64 frames, the synth renders multiple 64 frame chunks until the buffer is filled. This takes care of audio quality. However, MIDI timing resolution suffers.
I am looking at using the event's timestamp, to decide when to process an event. For example, I might have a Jack buffer of 256 frames, so I want to work out whether to process an event right away, or after one 64 frame chunk, and so on. It would appear that, in order to make this work, I need an initial timestamp, which I get at the start of the audio callback. I then compare each event's timestamp, to determine where it should be processed.
I cannot find an ALSA function for getting a timestamp. Perhaps I am missing something, or trying to do the job in the wrong way.
Regards
Glyn