[LAD] transport/timebase calculations

[James Morris]

>> Now, for your rolling transport counter code...
>>
>> 262        pos->tick += (int32_t)
>> 263            (nframes * pos->ticks_per_beat * pos->beats_per_minute
>> 264                            / ((double)pos->frame_rate * 60.0f));
>>
>> I think this suffers from truncation error.  It assumes that
>> whenever the tick changes, it is perfectly aligned with
>> frame 0 of the current cycle.  If nframes << frames_per_tick,
>> I think it'll get stuck.  To un-stick it, you may want to
>> keep track of the bbt_offset.
>>
>> Hope this helps,
>> Gabrial
>
>
> can't say i exactly understand this bbt_offset. i'd already seen it in
> the jack docs, and not really made sense of it there... but i've not
> seen it used in source code in various sequencing/timebase apps and so
> assumed i didn't need it either !???
>

I've been looking back over at those source codes...

Using a low ppqn such as 24 in my timebase code (based on
non-sequencer) utterly fails. Using a ppqn of 100 in non-sequencer
fails too. As does using a low ppqn in Dino.

(I see you've used a low ppqn of 48 in Tritium/Composite, and are
using jack_position_t::bbt_offset)

In other words, you're right about it sticking when nframes <
frames_per_tick and needing bbt_offset. Until now I did not realize
this is worked-around by using a high ppqn such as 1920 or 2520 (which
give integer results when divided by integers up to values 8 and 9
respectively (ie what the magic number means)).

So with low ppqn values, the bbt_offset integer will be ok, but what
about for high ppqn values?

Even high ppqn won't be perfect, but might not a floating point
bbt_offset improve the timing?*

*not in my case, it's some more serious problem in my code.

Cheers,
James.


>> [1]
>> http://gitorious.org/composite/composite/blobs/master/src/Tritium/Tritium/TransportPosition.hpp
>>
>>  http://gitorious.org/composite/composite/blobs/master/src/Tritium/src/transport/TransportPosition.cpp
>>
>>  http://gitorious.org/composite/composite/blobs/master/src/Tritium/test/t_TransportPosition.cpp
>>
>