[alsa-devel] Tempo event in a Midi sequencer...

Ru Vuott vuott at yahoo.it
Wed Jun 20 22:17:09 CEST 2012

I am finally able to make a simple midi sequencer with the Gambas language. It works, but I have some little problem about changing the metronomic Tempo within a midi song.
Since I used the method of sending an Echo event to send Midi Messages groups to ALSA, I thought I'd solve the problem of an eventual return of metronomic Time by sending an echo and backing from Alsa custom (Type: 90). At the time of receipt of the echo by the Alsa function:

int snd_seq_event_input(snd_seq_t *handle, snd_seq_event_t **ev)

the code sends the value of the tempo to a routine, where there are these Alsa functions:

...obviously with teir parametres.
I noticed, though, unfortunately (incomprehensibly) that often the change is a bit 'earlier than required. I do not think there is a problem of Delta Time, since the Delta Time of metronomic Tempo event is treated the same way as that relating to other MIDI events.
Looking for a solution, I had a look at the code of your Aplaymidi, and I saw this statement:

case 0x51: /* tempo */
    event->type = SND_SEQ_EVENT_TEMPO;

so, I thought that the change of Tempo occur simply by sending an event by identifying it with precisely the type number: 35. I performed the test, checking the result is listening to the song, seeing both as reported by Aseqdump.
I guess it Aseqdump me signals a correct delivery, because it gives me back this response:
   queue + a number depending on the value sent with the Tempo event.

Instead, listening to the midi track, the change does not happen!
I am writing to ask you, if my analysis is correct, and if it is sufficient to send only time that event, or if some function of Alsa is absolutely necessary


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