Event Queues-
-Creation of a queue-
Creating a queue is done usually by calling snd_seq_alloc_queue. You can create a queue with a certain name by snd_seq_alloc_named_queue(), too.

// create a queue and return its id
int my_queue(snd_seq_t *handle)
{
        return snd_seq_alloc_named_queue(handle, "my queue");
}

These functions are the wrapper to the function snd_seq_create_queue(). For releasing the allocated queue, call snd_seq_free_queue() with the obtained queue id.

Once when a queue is created, the two queues are associated to that queue record in fact: one is the realtime queue and another is the tick queue. These two queues are bound together to work synchronously. Hence, when you schedule an event, you have to choose which queue type is used as described in the section Time stamp.

Setting queue tempo
The tempo (or the speed) of the scheduling queue is variable. In the case of tick queue, the tempo is controlled in the manner of MIDI. There are two parameters to define the actual tempo, PPQ (pulse per quarter note) and MIDI tempo. The former defines the base resolution of the ticks, while the latter defines the beat tempo in microseconds. As default, 96 PPQ and 120 BPM are used, respectively. That is, the tempo is set to 500000 (= 60 * 1000000 / 120). Note that PPQ cannot be changed while the queue is running. It must be set before the queue is started.

On the other hand, in the case of realtime queue, the time resolution is fixed to nanoseconds. There is, however, a parameter to change the speed of this queue, called skew. You can make the queue faster or slower by setting the skew value bigger or smaller. In the API, the skew is defined by two values, the skew base and the skew value. The actual skew is the fraction of them, value/base. As default, the skew base is set to 16bit (0x10000) and the skew value is the identical, so that the queue is processed as well as in the real world.

When the tempo of realtime queue is changed, the tempo of the associated tick queue is changed together, too. That's the reason why two queues are created always. This feature can be used to synchronize the event queue with the external synchronization source like SMPTE. In such a case, the realtime queue is skewed to match with the external source, so that both the realtime timestamp and the MIDI timestamp are synchronized.

For setting these tempo parameters, use snd_seq_queue_tempo_t record. For example, to set the tempo of the queue q to 48 PPQ, 60 BPM,