Subject: Re: [linux-audio-dev] XAP: a polemic
From: Tim Goetze (tim_AT_quitte.de)
Date: Mon Dec 16 2002 - 02:43:03 EET
Tim Hockin wrote:
>> by definition, time isn't flowing when the transport is
>> stopped. a delay in stationary time can only be a zero
>
>umm, I know that a requirement for me is to be able to stop the sequencer,
>and still play MIDI and have delay lines etc still delay. Are you saying
>that this can't be done in your model? In my opinion just because transport
>has stopped does NOT mean time has stopped. On the contrary, ticks are
>still happening, the tempo is still in effect. Just because the SEQUENCER
>stopped sending events doesn't mean the rest of the studio did.
to allow for everything you mention here, you need to
start counting a different time -- you've stopped the
transport, so transport time isn't flowing any more.
at least that's what i do, calling it 'virtual time'.
the thing is that you need to keep time well-defined
and controllable at one point, for the whole network.
if you don't, things like synchronization and transport
control are tough to get right.
>Standing proposal:
> Host processes blocks of 'n' samples. Events are delivered with a
> timestamp that says 'actuate this event at this time within this buffer'.
> This is exactly what user-supplied automation is, totally randomly timed
> events. Some plugins need to sync to tempo or music-milestones. They
> indicate this need and receive tempo, meter, ticks events. It is
> responsible for tracking changes.
drop the tick events and it starts to sound reasonable.
buffer-relative timestamps are a definite no.
reason: you need to be able to schedule events far
ahead (streaming, prequeueing). calculating a buffer-
relative time in this case requires either knowing
all future buffer sizes or updating these events at
every cycle. the latter is too awkward, and the former
enforces a guarantee that severely limits the system's
synchronization capabilities.
tim
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