[linux-audio-dev] ALSA MIDI latency correction

[Clemens Ladisch]

Peter Brinkmann wrote:
> I have a sequencer application that has several MIDI output ports,
> each connected to some device that accepts MIDI input. Those
> devices may have vastly different latencies ... but of course I
> don't want to hear a time lag between those devices.
>
> I'm currently scheduling all my events through one queue (is that the
> recommended method? I've been wondering whether it would make more sense
> to have, say, one queue per output port, but I don't see how this would
> help),

Using multiple queues doesn't make sense except when you wanted to use
different timer interrupts for them.

> and the only solution I have been able to think of is to explicitly
> schedule events for faster devices at a later time. This is clumsy, and
> it's exacerbated by the fact that I'd like to schedule events in terms of
> ticks rather than milliseconds. Since latencies are usually measured in
> milliseconds, that means I have to convert them to ticks, considering
> the current tempo of the queue. There's gotta be a better way.
>
> Ideally, there are two things I'd like to do:
>     1. Assign a delay dt to each output port, so that an event scheduled
>     at time t0 will be sent at time t0+dt. Like this, I could compute the
>     maximum latency of all my devices, and the output port attached to a
>     device would get a delay of (max latency - latency of device), so
>     that everything would be in sync.

This would just move the ms-to-tick conversion into the ALSA
sequencer.

>     2. Automatically determine the latencies of the devices I'm talking
>     to. In theory, this should be possible. For instance, if timidity is
>     connected to jack, it could get jack's current latency, add its own
>     latency, and report the result. Is this science fiction?

This doesn't help with external hardware synthesizers; you'd have to
measure how soon after a note-on command its analog output shows a
signal.


Regards,
Clemens