Re: [linux-audio-dev] jack transport

On Fri, Mar 18, 2005 at 11:34:34PM +0100, Jens M Andreasen wrote: actually i beleive the answer is mostly no, although perhaps its best to consider smpte and midi as paralle rather than being supported by jack-transport. http://jackit.sourceforge.net/docs/reference/html/transport-design.html the job of jack-transport is to sync clients in position (eg 1min, 25.026secs). (Actually strictly speaking there is postional information in jack without it, but jack-transport extends this, and provides the commands neccesary to make use of it.) Without this you only have speed sync (clients are sample synchronous and can be mixed together without resampling). For example, to bounce an audio track from Rosegarden to Ardour, you need to be able to tell both clients to locate to *exactly* the same position and play from there. smpte is analogue so is not useful on a computer except for compatibility with other systems. For a jack plugin or client to generate a smpte signal to act as a master is relatively easy, but extracting the clock from a sampled analogue smpte signal to facilitate slaving is a non-trivial dsp task. I do beleive Fons Adriaensen was working on this. Theoretically, smpte lock between 2 stable systems could be quite tight, but not only is it not as tight as Jack, it is way too complicated and cpu intensive. at least Midi Spp is digital, but as its an asynchronous message, it suffers from jitter, and cant acheive a tight lock. Control commands such as MidiContinue make more sense, but not all jack clients will want to support midi. Its mostly useful for supporting external or uncooperative systems. so while many people will claim they dont need sample accurate positioning, jack-transport appears to be both the best and easiest solution to control of intra-box Libre applications. And hopefully will remain so when Jack gets extended to the LAN. the reason i asked about jack-transport in relation to bio2jack is that there are several unasked question on the mplayer lists re how to get this working. People dont realise that bio2jack only provides "jack 1.0 simple" or whatever you want to call it. Syncing video playback to Ardour would be a great example of the usefullness of jack-transport. Unfortunately, i guess the fact is that the design of mplayer means it can never be a real jack client. Perhaps a jack client could control mplayer via its slave mode, for use with non-keyframe based video formats, but thats a bit of a hack. hope that helped:-) -- Tim Orford