[LAD] ALSA Sequencer timestamp on event without scheduling

Clemens Ladisch clemens at ladisch.de
Mon Sep 19 12:27:59 UTC 2016

Felipe Ferreri Tonello wrote:
>  * We want to deliver these events *ASAP* to the application -
> scheduling adds latency, a lot;
>  * Timestamps are in the past relative to the central.
> But I still need the timestamp information. Why? The spec doesn't
> explain, but it makes sense to believe it is used to have a predictable
> latency, so if the central device wants to layout these MIDI message,
> they little or no jitter in between.

MIDI is a real-time protocol, and the sequencer assumes that events are
delivered in real time.

If you want to minimize jitter, you have to impose a certain fixed
latency, and schedule all events that arrived too early.

If you want to minimize latency, you have to deliver the events

> Thus, to be MIDI compliant

Compliant with what?  The MIDI specification does not say anything about
timestamps associated with events.

> we need to set this timestamp some how on the event. And I think the
> simplest way, is to use snd_seq_real_time_t on ev.time.time.

But nobody will read it.  And without a queue, the timestamp does not
make sense.  And applications that care about the time of received events
tell the sequencer to overwrite the timestamp with the actual delivery
time anyway.


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