[ardour-dev] latency compensation questions

[Mark Knecht]

Paul,
   Thanks. That's helpful to many of us I'm sure.

   You seem to be very careful below in using the word 'track'. What
about buses. If I create a set of tracks with audio, do a pre or post
fader send to the bus, and then put a plugin on that bus such as a
reverb is latency adjusted for the delay through the reverb plugin?

   This is probably the most common way to place a reverb or any other
high-CPU usage plugin.

Thanks,
Mark


On Thu, 31 Mar 2005 20:23:43 -0500, Paul Davis
<paul at linuxaudiosystems.com> wrote:
> >Someone recently mentioned connection graphs in Ardour but I don't
> >recall if it was to do with latency calculations. Does Ardour adjust for
> >latencies at every connection in the system? If not, for which subset of
> >points are adjustments made?
> 
> what happens is this:
> 
>      * each track computes the total delay of a signal passing through
>        it. these delays are caused by plugins or inserts (nothing
>        inherent to the track will cause a delay).
> 
>      * plugins report their delay to Ardour. LADSPA plugins use an
>        ad-hoc mechanism that has thankfully become moderately
>        standardized. VST plugins provide a getInitialDelay() function
>        as part of the API standard.
> 
>      * the session gets the worst track delay across the entire
>        session, and tells all tracks that value. each track then
>        computes the difference between its own delay and the worst
>        value, and stores it.
> 
>      * when the transport starts "rolling", each track delays
>        "starting" by this stored value.
> 
>      * result: the worst-delayed track starts playing immediately, all
>        others follow by the appropriate amount.
> 
>      * the latency compensation is recomputed every time the transport
>        stops, every time a plugin is added or (in)activated.
> 
>      * NOTE: it is not recomputed when plugin parameters are modified.
>        If you edit a plugin in a way that changes its latency, Ardour
>        will not recompute latency compensation until the *next* time
>        the transport stops. This seems like a reasonable compromise,
>        since in general, the transport is likely to be rolling while
>        you're playing with plugin parameters.
> 
> >As an example, if I insert a gate on one track and trigger it from
> >another track (either/both tracks may be running other plugins as well),
> >can I expect the gate trigger and gate input to be in sync?
> 
> a gate has no inherent latency, so of course.
> 
> latency is caused by plugin algorithms that inevitably cause signal
> delay. the most common examples are algorithms required "windowed"
> analysis like FFT, but there are plenty of others.
> 
> --p
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