tot;216769 Wrote: > Thanks John, very informative reply and definitely not boring. > > It simply raises few other questions. > > If the recovered jittery clock drives the DA conversion in the receiver > sure, jitter can and will lead to non-perfect waveform. I would naively > think that DAC's would do something better to be immune to bit-by-bit > jitter? > > For USB, it is still packets that get send out and not bit-by-bit > stream, so jitter of the USB sender/bus/cable shouln't be a problem > because the digital stream must be clocked on the receiver to convert > packet chunks into something usable? > > Teemu
On point 1 the problem is that there are several different ways to try and do this, non of which has proven to be without drawbacks and inexpensive and easy to implement. Personally I think the one with the most promise is the digitally controlled VCXO, its still a phase lock loop in principle but with a very low jitter VCXO. The "pull range" of these is small so you still have to get things right. IF you use very low noise DAC from the digital controller to the VCXO (low noise power supply etc) the results can be very good. The beauty of this scheme is that you can easily optimize the loop time constants on the fly. Short time constant to aquire lock then long time constant during run,with the possibility to change as conditions warrent. This has been done in purely analog domain, bit its very tricky and expensive to do. Its actually fairly easy to do this with a digital controler. I knew a couple people doing it with small processors, I'm trying it out with an FPGA. On the USB issue the adaptive mode (the common one) has NO connection from the receiver to the transmitter, its purely a one way street. The the clock in the receiver has to have some way to synchronize with the frequency of the transmitted data rate or else you will either over or under run the data. If you are clocking it out slightly faster than its coming in eventually you get to the point where there is no data. If its clocking out slower than whats coming in, eventually you overflow the internal buffer. This is what the adaptive mode does, it adjusts the local clock on the fly so it stays synced to the average data rate of the incoming data. On the original subject of this thread, I've actually done some experiments with some digital cables, from basic cheap coax to some expensive "audiophile" cables. Interestingly enough the audiophile cables were actually very lousy digital interconnects, they actually added significant amounts of jitter, which with some DACs could help mask the strong spikes from the reciver making it sound better! I'm not saying ALL audiophile digital cables work this way, just the ones I tested, and no, I'm not saying who they are. True 75 ohm BNC connectors with normal good quality communications grade coax is about as good as you are going to get. Of course that means having BNC connectors on both ends, not RCA jacks. When using BNCs it important to use real 75 ohm connectors not the more common 50 ohms ones. I see a mismatch being done all over the place, just going BNC won't help if its not 75ohm BNC. Interestingly enough the other option which works extremely well is good old type F cable TV connectors. They work extremely well, only come in one flavor so you can't mess it up and are available everywhere. The S/PDIF standard SHOULD have used these. If you are going to mod equiptment to replace n RCA, consider going with type F. You might be able to get a type F in the same hole as the RCA, and you can go to Radio shack and get the cable for $4. > > For example the Benchmark DAC1 seems to be absolutely immune to input > jitter (its output measures the same no matter how jittery the input). > This is an interesting case. They use a high frequency clock (I think its something like 110MHz) to reclock the data going to the DAC. This DOES make the system almost immune to jitter on the input, but DOES effectively create jitter on the output. The clock might be low jitter, but its reclocking the 44.1 (or whatever). What hits the DAC chip is pulled in or pushed out to the nearest clock pulse of the high frequency clock. This produces distortions in the waveform exactly the same as jitter on the 44.1 clock. This is a perfect example of being immune to jitter does not necessarily mean its LOW jitter. John S. -- JohnSwenson ------------------------------------------------------------------------ JohnSwenson's Profile: http://forums.slimdevices.com/member.php?userid=5974 View this thread: http://forums.slimdevices.com/showthread.php?t=37044 _______________________________________________ audiophiles mailing list [email protected] http://lists.slimdevices.com/lists/listinfo/audiophiles
