OK message to self - make sure you don't post stupid replies without reading the message you're replying to properly. Just ignore me - which is what you all seem to be doing anyway.
On Tue, 2005-08-23 at 08:13 +0100, Mark Bennett wrote: > Hi Sean, > > This is interesting data. Do you have equivalent data when > the SB is actually playing music? > > The ongoing discussion is about the impact of different > processing algorithms on the jitter. If you can show that > the difference between play and pause is negligible then > the difference between wav and flac will surely also be > insignificant. > > Mark. > > On Mon, 2005-08-22 at 16:17 -0700, seanadams wrote: > > Measurement setup: > > > > Stanford Research Systems model SR-620 > > Squeezebox2 > > BNC connector grounded at s/pdif coax, wired to internal 11.2896 master > > clock. > > 3' RG59 cable > > > > The absolute accuracy of this unit is specced at 25 picoseconds, but > > with enough samples, it's 1ps of resolution is enough to consistently > > measure the impact of phenomena as small as CPU activity (as speculated > > in the FLAC thread). Scope probes did not give consistent readings so I > > soldered a BNC directly to the board - at this resolution even slight > > changes in the position of the probe will throw the measurement off by > > 100ps or more, so a solid connection is definitely needed. > > > > These are standard deviation for 10,000 samples measured at DAC input. > > In addition to disabling the second oscillator, I also tested some > > different power supply configurations but those did not show any > > changes in sigma. However pk-pk measurements shown on the dscope > > analyzer (for the s/pdif out) did show differences - I haven't > > investigated that disparity yet. Anyway here's the data from the > > SRS-620: > > > > Unmodified SB2, playback paused: *65 ps* > > > > SB2 with 12.2880 crystal lifted: > > > > playback paused: *55 ps* > > playing AIFF: *57 ps* > > playing FLAC: *57 ps* > > playing mp3: *58 ps* > > > > Finally: > > > > Measured at the oscillator: *43 ps* > > > > A couple screen shots below (settings are identical for both > > measurements). Here's how to read these: the vertical line indicates an > > arbitrary cursor position (rel=48.797ns). The mean measurement is > > indicating the distance from this value. BTW the nominal value here > > would be 1/11.2896/2=44.289ns. The difference does not indicate any > > imprecision in frequency, but rather that the duty cycle is not exactly > > 50%. > > > > "sigma" is the number we're most interested in. At the bottom, note the > > scale is 100ps/div. You can see the base of the curve is about 3 > > divisions wide, indicating pk-pk jitter is about 300ps. A tighter > > distribution (less jitter) is indicated by a taller, narrower > > histogram. > > > > [image: http://www.seanadams.com/jitter_pics/1.jpg][image: > > http://www.seanadams.com/jitter_pics/2.jpg] > > > > -- "The biggest problem encountered while trying to design a system that was completely foolproof, was, that people tended to underestimate the ingenuity of complete fools." (Douglas Adams) _______________________________________________ audiophiles mailing list [email protected] http://lists.slimdevices.com/lists/listinfo/audiophiles
![http://www.seanadams.com/jitter_pics/1.jpg][image:](http://www.seanadams.com/jitter_pics/1.jpg][image:){kind=link}
