Hi guys, Last few days I've been trying to get a sense of jitter measurements. As many know, magazines like Stereophile (http://www.stereophile.com/content/case-jitters) publishes these measurements using the Julian Dunn J-Test based on the original paper http://www.nanophon.com/audio/diagnose.pdf. Basically, this test consists of a primary signal at Fs/4 with a square wave LSB at Fs/192. The intent of this signal is to STIMULATE (not simulate) jitter-induced distortion in the analog signal as sidebands around the primary frequency (Fs/4).
Personally, I'm really not sure how much of a problem this is in actual music playback since studies have suggest audibility in real music is on the order of many nanoseconds (rather than the picosecond results the test seems to imply). In any case, since we're interested in "high fidelity", ideally we wouldn't want any of this distortion at all... For my tests over the last couple days, I used the standard Touch setup I had before... Touch with EDO firmware --> TosLink/Coaxial --> ASUS Essence One --> XLR --> E-MU 0404USB. FFT analysis was with WaveSpectra 1.40E (http://www.ne.jp/asahi/fa/efu/soft/ws/WS140.ZIP). FFT parameters: 131072 points / Blackman-Harris / focusing on 8kHz - 15kHz, display "peak only". I'm going to show here the results from the 24-bit/48kHz J-Test signal as described by Dunn in the paper above, the only difference is the one I got has the 12kHz at -6dBFS rather than his -3dBFS in the paper. Note that this is different from the 44kHz one that Stereophile usually uses but I think the results are quite similar. As I noted before, I'm an amateur using an E-MU USB audio device, so the results will not be anything as accurate as John Atkinson's AP! The reason I stayed with 24/48 is mainly because of convenience and the E-MU ADC I think works best at 96kHz so I figure I'll use an even multiple as in 48kHz signal. Furthermore, I honestly have no idea how worthwhile it is to report jitter as a single number like "500ps". Instead I think I'll just show the pictures and let you see the difference depending on test conditions & cables. Here's the Touch analogue output itself: 14445 Pretty good! I don't see any sidebands at all; if present, they must be below the noise floor. How useful the J-Test is when utilized in a one-box solution may be questionable since I think Dunn's paper was looking at the interface itself and the combination of interface + data jitter. Which brings us to using the Touch as a transport for the ASUS Essence One and what happens when the SPDIF interface + cable gets uses - here's TosLink: 14444 It's sidebands time! The tallest sidebands are at +/-0.25kHz at an amplitude of -120dB. I believe these are called "data related sidebands" given the multiple of Fs/192 (250Hz for 48kHz sampling rate). Here's coaxial with proper shielded Acoustic Research cable (~$20): 14442 Better than TosLink. I can't see any symmetrical sidebands. A few of those spikes I believe are due to the ASUS/E-MU. Touch coaxial interface looks clean! Here's coaxial with crappy RCA zip chord (pack in 'free' cable from an old DVD player): 14443 This is what jitter looks like when you have a poor coaxial cable connecting the Touch to the Essence One! Obviously this is close to "worst case scenerio" with poor impedance matching and no shielding to speak of. Interesting all the sidebands, largest peak up at -112dB or so. There ya go... Some results I've gotten with the Touch so far. I'm still trying to understand the test better so would appreciate any pointers and suggestions! >From what I see so far: 1. Touch analogue out is really clean! But the noise floor is not as low as with the Essence One as DAC as previously measured. 2. TosLink is more jittery than a decent coaxial cable at least with the Essence One as DAC. Nonetheless, at 24/48, these sidebands are down at -120dB. 3. A decent coaxial cable can look very clean! Again, I don't know how generalizable this is beyond my Touch-E1 combo. 4. What cable you use with the coaxial output DOES make a difference. The (probably) "worst case" scenario with a toss-away RCA cable has measurably worse jitter performance based on what I found. Nonetheless, the sideband amplitude is still way down below -110dB! +-------------------------------------------------------------------+ |Filename: Touch_Analogue.jpg | |Download: http://forums.slimdevices.com/attachment.php?attachmentid=14445| +-------------------------------------------------------------------+ ------------------------------------------------------------------------ Archimago's Profile: http://forums.slimdevices.com/member.php?userid=2207 View this thread: http://forums.slimdevices.com/showthread.php?t=97950 _______________________________________________ audiophiles mailing list [email protected] http://lists.slimdevices.com/mailman/listinfo/audiophiles
