[LAU] jdelay - measured latency of a usb interface

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