SoftwireEngineer;664286 Wrote: > I agree with most of what Soundcheck wrote. Jitter happens at source as > well as during transmission. If you have bits in memory and you clock > it out at a certain frequency, say 44.1kHz, the clock's instability is > the source of jitter. If these bits are transmitted over a cable to a > separate DAC, the reflections in the cable(due to impedence mismatch) > cause the embedded clock to be retrieved with jitter on the other end. > > > Best test, as I pointed out earlier, is to generate , say a 8Khz wav > file, play it and do a frequency analysis of the analog output. If you > see anything other than 8Khz it could only be due to the jitter. A > positive use case of jitter is FM modulation. You tune to radio at a > particular frequency and any frequency variations on this base upto > 20khz is the music signal)
Two small points: First all this talk of recovering "embedded clock" might tend to confuse readers into thinking that the dac has no option but to convert to analog at a rate determined by the clock message buried in the S/PDif signal. In fact we know exactly the rate at which the DAc should turn samples into anlog signals (ie Fs) so all we need ultimately is the data not the embedded clock. The Dac only needs to be governed by the embedded clock to the extent that it needs to match the long term average clock rate of the transmitting device so that it does not suffer data overrun or underrun. (long term here is determined by the length of buffer and latency that you can tolerate) Now unless the transmitting device is really shit, that long term clock rate shouldn't actually change much. There are of course engineering complications but nowadays dacs are not directly governed by the jitter in the embedded clock. Second, you can actually do much better than just an 8KHz tone the standard j test as you probably know is "a high-level tone at exactly one-quarter the sample frequency, or Fs/4, to which is added a squarewave at the level of the least significant bit (LSB), at Fs/192. With the twos-complement PCM encoding used by CD data, this low-level squarewave exercises all the bit transitions simultaneously, which is very much the worst case for stimulating jitter. The high-level, high-frequency tone is thus modulated by the jitter and sidebands spaced at the frequency of the Fs/192 squarewave, and its harmonics will appear in the reconstructed analog signal's noise floor. With CD data, the high-level tone has a frequency of 44.1/4kHz = 11.025kHz, while the squarewave has a frequency of 44,100/192 = 229.6875Hz." (Quoting John Atkinson A Case of the Jitters) -- adamdea ------------------------------------------------------------------------ adamdea's Profile: http://forums.slimdevices.com/member.php?userid=37603 View this thread: http://forums.slimdevices.com/showthread.php?t=84742 _______________________________________________ audiophiles mailing list audiophiles@lists.slimdevices.com http://lists.slimdevices.com/mailman/listinfo/audiophiles
