[LAD] transport/timebase calculations

[James Morris]

On 11 July 2010 16:02, Gabriel M. Beddingfield <gabrbedd at gmail.com> wrote:

>> Everything is playing too slow. For example, @ 120bpm 4/4 time, the
>> second (or third) bar starts almost an entire beat too late.
>>
>> Please could someone take a look at the calculations and see if
>> there's something obviously wrong with them?
>
> Here's some thoughts.  I haven't checked my math on these...
> but hopefully they're helpful.  FWIW, I've spent a lot of
> time perfecting transport calculations[1], so I'm not just
> spouting off.
>
> 250        pos->beat = (int32_t)(abs_beat - 251
>  (double)pos->bar * pos->beats_per_bar + 1);
>
> Your abs_beat starts at zero.  bar*beats_per_bar also starts
> at zero.  Beats in a measure start at 1.  So, I think this
> should be:
>
> 250        pos->beat = 1 + (int32_t)(abs_beat - 251
>    (double)pos->bar * pos->beats_per_bar);
>
> Now... for the tick:
>
> 253        pos->tick = (int32_t)(abs_tick - abs_beat * ppqn);
>
> Your abs_beat needs to be rounded down... because it's still
> a double and still has the fractional part.  Perhaps:
>
> 253        pos->tick = (int32_t)(abs_tick - floor(abs_beat) * ppqn);
>
> 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 !???

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
>