[Jack-Devel] questions about latency ranges

Markus Grabner mg.2849 at w42.at
Tue Nov 14 23:50:53 CET 2017


I have some questions about the minimum and maximum values of a port's latency 
(i.e., its latency range). The documentation states the following:

"Both latencies might potentially have more than one value because there may 
be multiple pathways to/from a given port and a terminal port."

Does this also apply to a setup with two output ports "A" and "B" 
(representing two distinct physical devices) with different capture latency, 
which are both connected to the same input port "C"?

Here are some details of an example setup (useful for demonstration purposes, 
but probably not for doing any real recording work) as reported by jack_lsp:

"A" (Focusrite Saffire PRO 14 Firewire audio interface):
firewire_pcm:00130e0402403491_1394/Out:01 (Mic/Lin/Inst:01)_in
        port latency = 337 frames
        port playback latency = [ 0 1792 ] frames
        port capture latency = [ 337 337 ] frames
        properties: output,physical,terminal,

"B" (Line6 PODxt Live guitar effects processor synchronized by zita-ajbridge):
        port latency = 867 frames
        port playback latency = [ 1792 1792 ] frames
        port capture latency = [ 867 867 ] frames
        properties: output,physical,terminal,

"C" (Ardour track):
ardour:track 1/audio_in 1
        port latency = 0 frames
        port playback latency = [ 1792 1792 ] frames
        port capture latency = [ 867 867 ] frames
        properties: input,

Under the assumption that the physical inputs of "A" and "B" represent the 
same time scale, the correct answer for the port capture latency at port "C" 
would be [ 337 867 ] and not [ 867 867 ].

So my questions are:
*) What is the rationale behind jack reporting the larger of the two latencies 
of "A" and "B" both as the minimum and maximum latency of "C", and ignoring 
the smaller one?
*) Is it possible to provide additional information to jack to state that the 
signals at "A" and "B" originate from the same physical source (or from 
different physical sources sharing the same time scale), such that both 
latencies of "A" and "B" are taken into account for the latency range of "C"?

	Thanks & kind regards,

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