Re: [LAD] Time & How to approach it

On Wednesday, January 20, 2010, harryhaaren(a)gmail.com wrote: In my opinion, you can solve the problem very easily with a queue. For instance, ALSA sequencer uses internally a priority queue. You put your events into a queue in time order, and it holds and releases the events when the correct time arrives for each event. Usually, MIDI APIs have functions to manage event scheduling so you only need to add time-stamped events to the MIDI queues, and the events will be delivered at the right times to the output ports. There are MIDI APIs that don't offer any facility for queuing/scheduling, because they are designed for real time use only (for instance: RtMIDI.) In this case, you have two options: use another API or implement yourself the queue, and use a timer to extract the events at their deadlines. There are many possible timers available. One of them is an audio stream: the count of audio samples provides a constant time reference. This has a shortcoming if your application is not audio-centric: you need an audio stream flowing, even when your MIDI synths are always external musical instruments. There are more time-related issues. Music has usually a "tempo" property (speed), and users may want to change it in real-time. So you need to take this into account when you are timestamping your events: it is better to use non-absolute times, in order to avoid recalculations when the user changes the tempo on the fly. Seq24 and Rosegarden use the well documented ALSA sequencer scheduling: http://www.alsa-project.org/alsa-doc/alsa-lib/seq.html#seq_ev_queue An example is the utility 'aplaymidi' (simple SMF command line player): http://git.alsa-project.org/?p=alsa-utils.git;a=tree;f=seq/aplaymidi;hb=HEAD Regards, Pedro