[LAD] non-sequencer tempo drift

[Gabriel M. Beddingfield]

Hi Nick,

On Sun, 18 Oct 2009, nick bailey wrote:

> I've been playing with Non-sequencer over the last few days, love the 
> simplicity and uncluttered approach to the programme, however I'm 
> finding if run alongside Hydrogen or Ardour, the tempo drifts out of 
> sync, this occurs playing any song  longer than a couple of patterns. 
> The obvious answer to this is to have ardour as jack transport master, 
> Non-sequencer does not like this however and crashes. I post the

Since Non is a pretty immature program, it's inflexible about the 
Transport.  When initializing, it always wants to be the transport master, 
and doesn't take action of another program takes over.  That's why it's 
unstable.

> question here rather than Linux Audio Users because I plan to write 
> applications utilising Jack MIDI myself and am wondering the best 
> approach to implement tempo in absence of alsa sequencer, which I found 
> quite nice to use but still jittery even with high res timers / real 
> time kernels.

The transport is the way to go.  One way would be to take it upon yourself 
to fix Non's transport.  You will learn a lot of the in's and out's that 
way.

The JACK Transport is a Good Thing.  Some things about it that aren't easy 
to understand:

     * The program who is transport master can *lose* this
       status at any time without notification.  You
       must not write your program with logic that says
       "If I'm the master... calculate this way, but if
       I'm a transport slave.... calculate some other way."

     * Because of that, always program your audio code as
       if it's *always* the transport slave.  Set up the
       Transport Master code elsewhere.

     * When you supply the Bar:Beat:Tick info, it needs
       to come evenly.  Your ticks-per-beat and beat-per-
       bar needs to align.

     * Beware of frames-per-tick roundoff error.  It's
       natural to calculate frames-per-tick, round it to
       and integer, and use that as a basic building
       block.  However, it will result in tempo drift.

HTH,
Gabriel