so what i'm really suggesting to "fix" the timestamps in [packOSC] is
to use logical time for *advancing time* (and add some offset to put
the timestamps into the same calendar as NTP)
This corresponds to 2) in my previous mail. While this sounds simple in
theory, in practice you will encounter problems with clock drift sooner
or later. Supernova had to introduce an option to use the system time
instead of the logical time because some users had experienced issues.
For a drastic example, search for "timing drift supernova"
inhttps://www.listarc.bham.ac.uk/lists/sc-dev-2014/search/
<https://www.listarc.bham.ac.uk/lists/sc-dev-2014/search/>
In Pd there is another issue with advancing by logical time: Pd has very
relaxed realtime-safety constraints and it is very common to
occasionally drop some audio blocks, e.g. when opening a large patch or
loading a large soundfile. Naturally, the logical time does not advance
while Pd blocks, so your timestamps will be late forever.
FWIW, here's my current approach to scheduling/dispatching OSC bundles
in Pd (for the next AOO release):
1) sample the system time *once* per DSP block
2) filter the time with a DLL to get rid of jitter
3) check for dropped audio blocks and resync the time base. This is hard
to do in a plugin. The best solution I've come up with so far is to
average the last few delta times and check if it rises above a certain
threshold.
All of this steps wouldn't be necessary if Pd would
1) give access to the (estimated) system time of the current DSP tick
2) resync the system time and send a notification (e.g. a message to
"pd") when the polling scheduler has to drop a block of audio
There's still a potential problem with xruns in the audio driver. Jack
has a xrun callback
(https://jackaudio.org/api/group__ClientCallbacks.html#ga08196c75f06d9e68f9a3570dfcb1e323).
Portaudio has input/output overflow/underflow flags that are passed to
the stream callback, but it's up to the audio driver to actually provide
this information. I've read somewhere that many ASIO drivers don't do
this...
Christof
On 19.04.2021 09:17, IOhannes m zmoelnig wrote:
On 4/18/21 10:32 PM, IOhannes m zmölnig wrote:
i checked and double checked the specs but could not find anything
about this.
where do you get the idea that the OSC specs mandate wall clock time?
OSC-1.0 speaks about "NTP format" (but this is just the structure of
the 64 bits data chunk) and "the number of seconds since midnight on
January 1, 1900" (but it doesn't say whether this is supposed to be
wallclock or idealized)
i just thought that maybe we are really talking about different things
here.
so i'd like to clarify what I mean by "logical time".
Pd keeps an track of an internal monotonic value "pd_systime" that is
incremented whenever time advances within the Pd world.
the incrementation of this value is losely synched to the passing of
time in the real world ("wall clock time").
"losely" insofar as the increments do not happen at exactly the same
time, but the maximum difference between the wall clock time and the
logical time is bound to a somewhat small value (<<1sec).
wall clock time is typically provided by the OS by means of
gettimeofday() or similar.
while the two times are synched, they have different offsets.
Pd's logical time simply starts whenever the Pd instance started.
however, gettimeofday() counts seconds since the beginning of the
epoch (currently: 1970-01-01 00:00).
to complicate things, NTP-timestamps (as used by OSC) use their own
epoch that starts on 1900-01-01 00:00.
using Pd's logical time "as is" as an OSC timestamp, is of course
useless (unless you are communicating withing the same Pd-instance),
as all participants need to agree on a common epoch.
so what i'm really suggesting to "fix" the timestamps in [packOSC] is
to use logical time for *advancing time* (and add some offset to put
the timestamps into the same calendar as NTP)
so basically:
- when packOSC gets instantiated, determine the offset of the logical
time with respect to the NTP time.
e.g.:
offset=seconds_from_1900_to_1970+gettimeofday()-sys_getlogicaltime()
- when creating a timestamp, use this to calculate the ideal NTP time:
now=offset+sys_getlogicaltime()
in the actual implementation there's two issue that should be addressed:
- probably the synching of logical time to wall clock time (that is:
the calculation of the offset) should only be perfomed once for all
the [packOSC] and [unpackOSC] objects in the running instance, so they
are all tightly synched.
- make sure to handle epochs correctly (the next unix epoch starts in
2038; this is still far in the future for many, but for [packOSC]
birthday is about as far in the past)
gfmsdtf
IOhannes
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