[LAD] SMPTE and jackd

Robin Gareus robin at gareus.org
Tue May 8 13:50:44 UTC 2012

On 05/08/2012 05:07 AM, Egor Sanin wrote:
> Hi folks.

Hi Egor,

> I've got a question about SMPTE frames and jack period size.
> In the "worst case" of 30 frames/second, each frame spans 1/30 seconds.
> Does that mean that if jackd is called such that (period size)/(rate)
> ≥ (1/30) seconds, we will drop SMPTE frames?


JACK does not know anything about SMPTE, does it? It counts audio-frames.

JACK-transport timecode (as returned by jack_transport_query())
increases in steps of the period size. It depends on the application
what to make with this.

jack_frame_time() OTOH estimates the time and includes the offset
jack_frames_since_cycle_start(); but I don't know how accurate this is.

@Paul, Fons,..: does it use a PLL or simply return the difference from
clock_gettime() since the beginning of the cycle?
IIRC Fons reworked some of jack timing functions not too long ago when
writing zita-a2j, zita-j2a, but don't know if they also affect the
transport API.

> Large period sizes are rare these days, but this is still a possible
> scenario and I want to understand it properly.

Where does the SMPTE come into play? MTC?


More information about the Linux-audio-dev mailing list