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Re: [Denemo-devel] Real time playback in Denemo


From: Richard Shann
Subject: Re: [Denemo-devel] Real time playback in Denemo
Date: Mon, 02 Jan 2012 11:36:42 +0000

On Sun, 2012-01-01 at 14:41 -0600, Jeremiah Benham wrote:
> Are we going to have a release before we start this?
I think we should. Playback should mark release 1.0 I think.
What housekeeping things are there outstanding before a release
candidate is created?

Richard


> 
> Jeremiah
> 
> Sent from my Samsung smartphone on AT&T
> 
> Richard Shann <address@hidden> wrote:
> 
> >I think the next big step for Denemo will be to get the playback working
> >in its own thread.
> >
> >Below are some ideas for how to do this, first there are some notes
> >which you can cut out if you already know how the soc code is working.
> >
> >8>< 8>< 8>< 8>< 8>< 8>< 8>< 8>< 8>< 8>< 8>< 8>< 8>< 8>< 8>< 8>< 8>< 8>< 
> >I have studied the soc branch code and made the following notes:
> >
> >The playback works by using two threads, one is the
> >process_thread_func() of the backend chosen (I see alsabackend.c and
> >dummybackend.c so far) and the other is the queue_thread_func() in
> >audiointerface.c
> >
> >The process_thread_func() creates a mutex and then runs a loop:
> >the loop gets the system time g_get_current_time() and to it adds 5ms (a
> >compiler constant called PLAYBACK_INTERVAL). This time is then used for
> >a timed wait on a condition with the mutex, that is a call to
> >g_cond_timed_wait(). This means that the thread waits for the condition
> >process_cond to be signalled or until the 5ms have elapsed.
> >The g_cond_timed_wait() unlocks the mutex before the thread sleeps and
> >locks it when it continues. 
> >***Question*** is it ok that the mutex has not been locked for the first
> >time (and correspondingly, at the end g_mutex_free() is called without
> >unlocking the mutex, is that ok?).
> >
> >Next a check is made for quitting the loop - this is done by making an
> >atomic access to an int which is set by the alsa_seq_destroy() call.
> >***Question*** does that need to be an atomic access? The int in
> >question is just a boolean, so in C it just means that if any of the
> >bits are set it is true. So it really doesn't matter if another thread
> >is halfway through setting it.
> >
> >Next is code which I think is for MIDI input - I ignore this here.
> >
> >Now the playback code continues by getting the current time and
> >subtracting a value called playback_start_time. This latter value is set
> >up when starting the playing by getting the system time and subtracting
> >a value "playback_time" which is kept in audiointerface.c and controlled
> >by calls to the function update_playback_time() and initialized by
> >midi_play() to Denemo.gui->si->start_time. This last is the time in
> >seconds from the start of the piece which the user has set as the time
> >to start from (e.g. via d-SetPlaybackInterval etc).
> >So for the case of playing an entire movement, this is zero, and the
> >playback_start_time is the system time when the play was initiated.
> >
> >Back to the loop which the process_thread_func() runs...
> >
> >The difference between the current time and the playback_start_time is
> >called the playback_time and is time in seconds along the midi track
> >where the next event should be taken from.
> >
> >After a check for reset (I ignore that here) an event is read from the
> >queue. This is the call read_event_from_queue() in audiointerface.c.
> >This is called with a parameter called until_time which is set to the
> >playback_time plus 5ms (the PLAYBACK_INTERVAL value again). After
> >checking for stop play conditions (discuss later) this calls into
> >eventqueue.c for the function event_queue_read_output(). This checks the
> >midi event time is < the until_time and returns TRUE and the event if so
> >otherwise FALSE.
> >Further events are read until this call to get an event from the queue
> >fails.
> >Finally before the loop begins again the playback_time that the backend
> >has, is "sent back" to audiointerface.c via update_playback_time().
> >
> >8>< 8>< 8>< 8>< 8>< 8>< 8>< 8>< 8>< 8>< 8>< 8>< 8>< 8>< 8>< 8>< 8>< 8>< 
> >
> >This code lacks the flexibility to speed up, slow down or pause the
> >playback - once the playback_start_time has been fixed the midi events
> >following that time will be sent out by referencing the current system
> >time.
> >This could be changed so that instead of executing g_get_current_time()
> >the time is obtained from a thread which manages the requests from the
> >user to pause, change tempo etc.
> >
> >Currently, I think there are two examples of how to do this in Denemo.
> >The first was the one I wrote to allow live change of tempo - very
> >useful for getting the right tempo by adjusting the slider while the
> >music is playing. The second I wrote to allow the playback to follow a
> >user playing along with it.
> >Only the first of these is easy enough to understand that I still have
> >it in my head: when the tempo slider is moved all the elapsed times that
> >are obtained are scaled. To make this work, the playback_start_time has
> >to be re-calculated - so it is as if we had been playing since then at
> >the new tempo and had just reached the current playback time. In this
> >way we do not have to keep a history of all the tempo changes.
> >The second method of controlling the playback was for following the
> >player (or conductor). In this case the user is setting a time at which
> >the playback should pause (unless the user has updated it meanwhile).
> >I am not sure if these could be unified.
> >
> >What would the new time-control thread look like? Well, for all the
> >paused time it could add up the total of pauses so far and subtract
> >these from the time value it returns. While actually in a pause, I guess
> >it would just keep returning the same time - only when the pause is
> >ended would it increment the total amount of pause time and subtract
> >that from the time value it would otherwise return. I guess it could
> >also manage the tempo change signal, adjusting the playback_start_time
> >and scaling the times it returns. Perhaps though, it should be returning
> >not absolute times but times since start? So it would be returning 0
> >during a pause and a scaled difference of (g_get_current_time() -
> >playback_start_time) and the total_pause_time. (?)
> >
> >I would welcome comments.
> >
> >Richard Shann
> >
> >
> >
> >
> >
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> >
> >
> >
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> >
> >
> >
> >_______________________________________________
> >Denemo-devel mailing list
> >address@hidden
> >https://lists.gnu.org/mailman/listinfo/denemo-devel





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