On Tue, 24 Mar 2009 11:14:00 +0100 <hollunder@xxxxxx> wrote: > Hi everyone. > I was kind of curious about the real latency of my system and did a > series of tests using jdelay (http://www.kokkinizita.net/linuxaudio/). > > I share my results so others can see what to expect from such a usb > audio device and also because.. > > ! Important ! > I ran in a number of oddities that I can't explain myself. This could > mean that my results here are totally useless, so please don't take > them for real until someone can hopefully clarify those matters. > > > General Information: > ------------------- > > Audio Interface: Edirol UA-25 > http://www.rolandus.com/products/productdetails.aspx?ObjectId=704 > http://alsa.opensrc.org/index.php/Edirol_UA-25 > > It can work in 44.1, 48kHz duplex and 96kHz either record or playback, > so I couldn't (and wouldn't have anyway) tested 44.1 and 48kHz. > > There are Periods/Buffer from 2 to 42 possible, most common is I > believe 2 and 3 Periods/Buffer, I only tested these. > > My pc is rather old, mainboard is a via km400 or km400a if that helps > anything. > > On the software side there is Arch Linux with it's latest stock > kernel, 2.6.28.8, jack 0.116.2 and qjackctl 0.3.4 (the patchbay made > this a lot easier). > > Measurement Results: > ------------------- > > SR 44100 > ----- > Frames Periods F*P/SR Measured > > 64 2 2.9 8.2 > 128 2 5.8 11.1 > 256 2 11.6 15.9 > 512 2 23.2 28.5 > 1024 2 46.4 52.7 > 2048 2 92.9 102.1 > 4096 2 185.8 187.1 > > 64 3 4.4 9.7 > 128 3 8.7 14.0 > 256 3 17.4 22.7 > 512 3 34.8 30.1 > 1024 3 69.7 65.0 > 2048 3 139.3 138.6 > 4096 3 278.6 279.9 > > > SR 48000 > ----- > Frames Periods F*P/SR Measured > > 64 2 2.7 7.9 > 128 2 5.3 10.5 > 256 2 10.7 15.9 > 512 2 21.3 26.5 > 1024 2 42.7 48.9 > 2048 2 85.3 94.5 > 4096 2 170.7 171.9 > > 64 3 4 9.2 > 128 3 8 13.2 > 256 3 16 21.2 > 512 3 32 27.2 > 1024 3 64 60.2 > 2048 3 128 127.2 > 4096 3 256 257.2 > > > Questions to the wise guys: > -------------------------- > > 1) > As probably everyone has noticed, with 3 periods/buffer and > frames/period of 512, 1024 and 2048 the measured latency is smaller > than the calculated latency. I don't see how this can be possible > since, as far as I understand it, the measured latency should be the > calculated + some. > > > 2) > When doing those measurements I noticed that with 44100 the Frames > reported by jdelay always ended in .732 Frames and with 48000 always > ended in .761 Frames. > > I found out that I can change that value by changing the sensitivity > of the input or the amplification of the output. In the case the > variance is below one frame, but I find it very strange that it's > always the same at a given amplification/sensitivity. > > > 3) > Is it normal that the difference between calculated and measured > latency varies so much? Shouldn't it just always be the same? > > > 4) > Should these results be similar with any usb interface? > What does have most influence on such results? > > > I hope this can be of use after the remaining questions have been > answered. > > Best regards, > Philipp Don't rely on those results. I just had very different results for the same setting. 48kHz, 256/3 had 21.2ms, now I also got 15.9ms and 20.2ms, and I have no idea what caused that. Note that it never occurred while jdelay was running but have a look at this: 1017.757 frames ^C hollunder ~ $ jdelay 761.754 frames I absolutely don't know how this can be, what happened in these 1-2 seconds in between, what's at fault, and it looks like it could be a serious issue. Regards Philipp _______________________________________________ Linux-audio-user mailing list Linux-audio-user@xxxxxxxxxxxxxxxxxxxx http://lists.linuxaudio.org/mailman/listinfo/linux-audio-user
