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In Reply to: RE: If USB JItter specs were Superior to Spdif posted by Dynobot on April 12, 2012 at 12:28:39
The issue isn't the wire type, it's the method of flow control. The right way is for the DAC to pull data out of the transport rather than have the transport push a stream of timed samples to the DAC. With USB the right way is to use a proprietary protocol and driver for block mode transfer or to use the async protocol, which can use drivers provided by the operating system. With SPDIF the right way is to run an extra wire from the DAC back to the transport to slave the transport to the DAC's clock. If the wrong way is used, heroic measures will be needed to get good results. However, even with the right way results won't be excellent without careful implementation. A good system architecture is useless unless it is well implemented.
It's the entire system that counts and the way it is put together. It is not possible to break a system down into individual components and optimize each of these separately if one wants the best possible results. There is far too much interaction between components. A great SPDIF installation will beat a poor USB implementation and conversely. Note that when a "push" architecture is used the source, the cable and the sink are all in the signal path and hence critical to sound quality. When a "pull" architecture is used only the sink is in the signal path. The source and cable are not in the signal path and if they have an effect on sound quality it is only because of second order effects at the sink, i.e. various forms of leakage such as ground bounce and poor clock design.
Note that with both technologies, jitter on the cable is typically nearly as large as it can be to avoid data errors, since jitter is typically one of the factors limiting the maximum possible data rate. If the wire is conveying audio timing information because it is being run in push mode then the cable jitter will be 10 - 100 times larger than the jitter needed for full resolution reproduction out of a DAC. Jitter down at the level needed for audio quality is not readily measurable with any test equipment, and it certainly can't be seen on the screen of even an expensive scope. DACs that run in push mode must have a phase locked loop or other heroic measures to reduce the incoming jitter down to levels that will produce tolerable sound quality and these measures are neither cheap nor completely effective.
Tony Lauck
"Diversity is the law of nature; no two entities in this universe are uniform." - P.R. Sarkar
Follow Ups:
> > > > With SPDIF the right way is to run an extra wire from the DAC back to the transport to slave the transport to the DAC's clock.
What IF both the Dac and the transport were slaved to the same word-clock?
Dynobots Audio
Music is the Bridge between Heaven and Earth - 音楽は天国と地球のかけ橋
"What IF both the Dac and the transport were slaved to the same word-clock?"
If the word clock were external to both the DAC and the transport and separately buffered outputs were used then the jitter on the cable from the transport to the DAC would not affect the audio quality. However, the quality of the output would depend on the quality of the clock signal received by the DAC. In other words, the sound might not vary much with changes in the transport but it wouldn't be good unless the word clock was very high quality since the word clock would definitely be in the signal path. If the word clock is a separate wire that is isolated from other wires it won't have any signal dependent jitter, which is a problem with systems where the same wire is used for both clock and data. However, there will still be noise of various sorts, so it is unlikely that the word clock as received by the DAC will be as clean as the word clock at the actual oscillator. This problem can be avoided by locating the oscillator in the DAC.
Unfortunately, there is another problem with using an external word clock that affects delta-sigma DACs and other upsampling converters. These do the actual digital to analog conversion at a higher sampling rate than the word rate. If an external word clock is used then the received word clock must be multiplied up to the final master clock rate which will be some multiple of the word clock rate. There is no way to do this frequency multiplication without employing heroic measures (expensive, not completely effective).
The correct way to implement a system synchronized by a word clock is to run a high quality fixed crystal oscillator at the master clock frequency, and send this clean signal directly to the actual digital to analog converter. At the same time, buffer this oscillator signal and send it to a divide circuit (a digital counter) to divide the high master clock rate down to the output word clock. Then send this word clock out of the DAC to the transport. A DAC that works this way will have a word clock output, but not a word clock input.
I don't believe it is possible to tell whether a given DAC has implemented the clock architecture correctly from reading spec sheets and manuals. At the very least one has to open up the device and do a little reverse engineering, something that can be difficult these days due to a high degree of integration, programmable gate arrays, etc. One can look for crystals and ascertain their frequencies, etc. (I haven't reverse engineered any audio devices, but I have reverse engineered data comm devices as part of patent lawsuits, which involved a combination of various system tests, measurements, reading of chip numbers with a flashlight and magnifying glass, Googling, reading various company literature, technical, legal, and marketing.)
Tony Lauck
"Diversity is the law of nature; no two entities in this universe are uniform." - P.R. Sarkar
"the jitter on the cable from the transport to the DAC would not affect the audio quality"
True, but the cable still might affect audio quality due to commmon-mode noise or RFI differences from one to the next.
I practice, this is what happens, and I can prove it with a USB common-mode filter.
Common mode noise on the UBS cable shouldn't couple into the DAC. It should be blocked by the USB receiver circuitry and the buffering circuitry inside of the DAC that connects the USB receiver to the converter.
As to RFI, the cable might act as an antenna and radiate EMI. The DAC and other audio components and cabling should suppress this interference which isn't in the audio band.
In either case, if the cable affects the sound quality it is not the fault of the cable, it's due to a defect downstream of the cable.
Tony Lauck
"Diversity is the law of nature; no two entities in this universe are uniform." - P.R. Sarkar
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Dynobots Audio
Music is the Bridge between Heaven and Earth - 音楽は天国と地球のかけ橋
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