[PD-dev] LogicalRealtime needed (was Re: [PD] packOSC TimeTag realtime vs. logicaltime)

[Miller Puckette]

Hi all,

There's no way from within Pd -- that would necessitate adding something
to the API between Pd and the audio subsystem to make a direct call to
alsa (or whatever).  Since you wrote most of the alsa code, I bet
it would be easy for you to hack it in :)

Miller

On Thu, Oct 29, 2009 at 01:58:37PM +0100, Winfried Ritsch wrote:
> Hello,
> 
> Sending audio over OSC needs a accurate time-tag in sync with Audio on the 
> sender so packages can be ordered, and re-sampling with the audio-sync on the 
> receiver, but OSC is defined to use the universal sync the Internet time 
> (realtime).
> 
> Syncing computers over Internet time is already a solved problem;using adequat 
> audio buffer there should be a stable solution. Since Audiotime is not in sync 
> with real-time.
> 
> So we need a Locgical-Realtime on the sender, that means realtime corrected 
> with the offset in the audio buffer processing, which should be fairly excat 
> enough.
> 
> The question is now:
> 
> a) How can I get the offset of the actual audio-buffer processing to either 
> the ADC-time oder DAC time, the the time at the beginning or end of the audio-
> buffer in PD ?
> 
> a)if not possible Should we invent for this purposes a LocalRealtime for each 
> audio-buffer and how ?
> 
> 
> mfG 
>  Winfried Ritsch
> 
> On Friday, 9. October 2009 05:54:25 Martin Peach wrote:
> > Wolfgang J?ger wrote:
> > > Hello,
> > >
> > > I'm using a combination of [pack~] [packOSC] -> transmission(UDP) ->
> > > [unpackOSC] and a modified version of [unpack~] to send Audio over OSC.
> > > The OSC packages are sent as Bundles so a TimeTag is generated at the
> > > sender's side. I extended the [unpackOSC] object with another outlet
> > > where I'm getting the particular TimeTag of each Bundle.
> > > In the modified version of [unpack~] I evaluate these TimeTags with
> > > regards to dropouts in the transmission.
> > > My problem is that the TimeTags are not accurate, what I ascribe to the
> > > fact, that the TimeTag in [packOSC] is generated as "real time" instead
> > > of "logical time". Using "real time" the TimeTag-"Output" is a function
> > > of the audiobuffersize (depending on the cpu load), so in my case, as
> > > the CPU-load is quite big, the values of the TimeTags are not useable
> > > for sequencing the datastream (as there are jumps in the TimeTag I
> > > always assume some packages got lost).
> > > As a workaround I established a sequence number, which is additionally
> > > transmitted in each Bundle. This is an appropiate solution, but it
> > > wouldn't be necessary if the TimeTag would be generated properly.
> > > Are my considerations conclusive?
> > 
> > At the moment the timetag is calculated when the bundle is opened with
> > the [ message. If the ] message doesn't occur at the same time the
> > timetag will be wrong as the message doesn't start to get sent until the
> > bundle is closed. Then there is the delay in the OS as it schedules the
> > packet for the network and the network card deals with possible
> > collisions with other packets, etc...
> > As well, anything [packOSC] does occurs between audio blocks (and may be
> > postponed if there is too much going on), so the timing will be jittery
> > unless your packets are being composed and sent in precise sync with the
> > audio. And if they are being sent to another machine with another clock
> > of course this is never going to happen unless you can sync their sound
> > cards via SPDIF or word clock. But that's real time.
> > Because there is no way to know what the 'logical time' is outside of
> > your logical frame, just use the sequential block number as your logical
> > time. Pack a sequence number followed by a blob, possibly resetting the
> > sequence numbers when they get too big. No need for bundle or timetag.
> > 
> > Martin
> > 
> > _______________________________________________
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> 
> -- 
> --
> - ao.Univ.Prof. DI Winfried Ritsch 
> - ritsch at iem.at - http://iem.at/ritsch
> - Institut fuer Elektronische Musik und Akustik
> - University of Music and Dramatic Art Graz
> - Tel. ++43-316-389-3510 (3170) Fax ++43-316-389-3171 
> - PGP-ID 69617A69 (see keyserver http://wwwkeys.eu.gpg.net/)
> --
> 
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