Bring out the shotguns, here is a hopefully not too boring attempt at going over jitter, interfaces, audio, USB and anything else I can think of to throw in here. (BTW non of this is new or uniquely my own, its based on reading a lot of stuff and my own experiments, measurements and listening)
Jitter, what is it? In a nutshell its the variation in timing between signal edges. In particular we are dealing with the jitter on a clock used to clock out data in a DAC. The clock is supposed to be converting the data at precise intervals (44.1 KHz, 96KHz etc) If the time from one edge to another varies from one clock pulse to the next, there WILL be distortion in the analog waveform. Jitter is usually refered to in the hifi press in terms of a single number, such as 250 pico seconds. Unfortunately that does not characterize jitter very well. You find out that the one clock might be off from perfect by 50ps, the next by 135ps, the next by 600ps etc, it varies all over the place. So if you want to use time numbers you have to use averages, or other statistical means to try and come up with something meaningful. What this means is that "500ps" from one device can have very different actual jitter than "500ps" from another device. Most technical people use the term "phase noise" to describe jitter, this is usually displayed as a frequency spectrum of the clock. This seems to have much better correlation to audibility than just the single time number, but its much harder to understand and compare, so I understand why the press doesn't use it. Why is this important? Because the frequency of the jitter relates directly to sidebands in the recovered audio. If you have jitter that is varying in a nice repeatable pattern that shows up in a spectrum, that will produce sidebands in the audio signal. (its not quite that simple, but I really don't want to get into that here!) These sidebands are NOT harmonically related to the "signal" so they are not easily masked. Listening seems to indicate that "random jitter" which looks like noise on the phase noise spectrum is much less audible than phase noise at specific frequencies. The S/PDIF receivers in common use are famous for their narrow frequency jitter spectrum. Very narrow specific frequencies but at fairly high levels. Other systems such as some types of USB receivers LOOK much worse, but they are much more random with the jitter spread out over broad frequency ranges. Since I'm on S/PDIF lets go breifly into that. As has been mentioned by others, the issue is NOT data integrity, its the jitter in the recovered clock coming from the S/PDIF receiver. As I mentioned earlier the spectrum of the recovered clock contains some strong "spikes" which seem to be audible under some circumstances. This is using the traditional "transport is in control" methodology, the DAC has to synchronize itself to what is happening in the transport. It is possible to break this relationship,but no tvery many DACs actually do that. One way is to have a separate low jitter clock which "clocks out the data", the problem is its not synchronized with the input stream, you will either drop samples on the floor or duplicate samples, this of course DOES cause waveform distortion. You can stick a buffer in, but in order to handle any situation the latency of the buffer gets to be somewhat long, people don't like pushing play and waiting several seconds for the sound or having a lag time when pushing buttons on the transport. The latency also makes it really difficult to use such a DAC with video! There are some very fancy buffer management techniques that can alleviate some of these issues, but very few companies have tried to deal with that. There are a whole host of other methods to try and make a DAC less sensitive to jitter coming from the receiver. Some more effective than others. I've done an experiment where I look at the phase noise of a recovered S/PDIF clock and tried different cables, connectors, moving the cable etc and all this things DO make changes in the phase noise of the recovered clock coming out of the S/PDIF receiver. It was really interesting to watch the spectrum change as I moved the cable around! (this means that you might actually have vibrations of the cable changing the clock jitter!) Now on to USB, there are three official USB audio "modes", all of these are called "isochronous" this means the bus reserves space for these packets, nobody else using the bus can interrupt them. There is synchrounous, adaptive and asynchronous. Synchronous sticks a PLL directly on the USB data stream, the jitter is terrible and extremely sensitive to what happens on the bus. It has the same problems as S/PDIF, the source is in control. Very few modern devices use this. Some early ones did which is where you here about USB having 1500ps jitter or some such, again this is not the norm today. Almost everything today uses adaptive mode, again the Source is in control, but the reciver has its own clock generator which is slowly changed to match the average data rate coming across the bus. These are MUCH better than synchronous, but there is still a variable frequency clock generator in there. The different implementations vary quite a bit with the best having quite low jitter without large spikes in the spectrum. All of these are quite sensitive to implementation, board layout, supply bypassing, power supply noise, jitetr of the clock feeding the frequency synthesizer all can make huge differences in the jitter even with the same chip using the same value components! Implementation makes BIG difference. Again there is NO feedback to the host it just sends out packets at what it thinks is the right speed and the DAC has to synch with it. There is also asynchronous mode, here the data is clocked out by the local oscillator when the buffer starts to over or underflow it tells the host to slow down or speed up the data stream. This is the only mode where the DAC is in control. The host is still sending the packets isochronously, there is no packet by packet handshake or anything like that, but there IS a path back from the reciver to the host. This is almost never done. There are only one or two USB chips that support this mode and ALL of them require re-writting the firmware inside the chip, not an easy task. I only know of one person who has ever done that. I tried for a long time and gave up. (thats a whole nother story). There a few devices that DO have the DAC in control (such as the EMU 0404 USB) but they do not use the official USB audio spec, they came up with their own packet protocol which means they have to write their own drivers. Back on S/PDIF and external DACs, the jitter and power supply noise of the signal driving the S/PDIF transmitter CAN show up as increased jitter in the recovered clock, this is why various "digital mods" actually can make a difference. BUT you still have the jitter from the reciver circuit, these two (source jitter and receiver jitter) interact in interesting ways. It is definately possible that the changes in the source jitter can be swamped by the receiver jitter, in such a case the "digital mod" would hardly make a difference, in a different reciver circuit the source jitter can be much more prominant, and in other receivers both types can be diminished significantly. Thus if you don't hear it in your system, doesn't mean it won't be audible in someone elses. If you can' hear it doesn't mean that every one that can is deluding themselves, and if you CAN hear it, it deosn't mean that everyone that can't is deaf. There are so many different combinations of devices and interactions between them that its impossible to make blanket statements about jitter audibility based on your system. OK I hope I didn't bore too many people. I get carried away sometimes and this thread was too big an invitation. 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
