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Re: Research Paper

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Quick summary then details:

on current system (SB3, I2S out, synchronous reclock to DAC) the computer makes no difference.

When running USB input DAC everything makes a difference.

EMI from computer in close proximity to stereo system is much worse than anything else.

Details:

I have slimserver running on 6 different machines, from old slow laptops to just put together screaming fast machines, running windows and linux, I cannot hear any difference whatsoever on the sound as long as the data stream doesn't get massively interrupted. (unplugging the cable, some program takes the whole machine down with it etc) Wifi or cable does make a difference, but not in sound "quality", my neighborhood has so many wifi networks its tough to get a reliable continuous data stream. Even the SB3's large buffer isn't always enough, I get dropouts and disconnections from slimserver, this doesn't happen when hardwired so thats the way I go.

USB DACs are a completely different story. If the USB receiver does not have a good reclocking PLL after it they are quite sensitive to pretty much anything that goes on in the computer. Some USB DACs like the Hagerman Chime have a good VCXO based secondary PLL that significantly cut down on the sensitivity to interface effects (but they do not completely eliminate it)

For ones that do not have good reclocking just about anything can cause a change in the sound. Whats running on the processor, fragmented files, power supply, how much other junk is running off the computer, the USB cable used, the temperature of the chips inside, the mechanical construction of the case etc. ANY of those things can change or modulate the noise and jitter of the USB data stream which can cause changes in the jitter spectrum of the recovered clock in the DAC. One interesting effect is aciustical feedback (commonly refered to as microphoinics), the sound from the speakers causes vibrations in the computer which causes changes in the jitter of the USB signal which causes changes in the recovered clock. Laptops are quite susceptible to this, that large open screen makes a quite decent acoustical transducer which couples nicely into the relatively light weight processor module. Its rather interesting to hook up a non-reclocked USB DAC to a laptop in the listening room and then try putting the screen at different angles and rotating the laptop at different angles.

Computers designed as "quiet" frequently have various forms of acoustical damping in the case which can help reduce this microphonic effect. Most towers and desktop cases are terrible in this regard, they are made out of thin sheet metal that rings like a bell.

Whats going on in the computer also affects the sound with a regular USB DAC. I can hear a change in sound when I'm moving the mouse! For me the biggest problem is being connected to the net, there are processes on the computer that are constantly receiving and sending packets even if no application on the computer is actively "browsing the web". On my system disconnecting the computer from the netowrk makes a significant improvement in sound when using a regular USB DAC. Don't even think about running games or spreadsheets on the same computer.

Temperature also makes a significant difference, the warmer the computer gets the better it sounds, up to the point where it gets so hot it stops working of course. I've done quite a bit of testing on this and it does correlate quite well. As the chips get hotter the sound gets better. Thus all the compulsive "cool the computer down at all costs" thinking tends to counteract good sound! Fortunately specialty items such as fanless computers tend to run pretty hot so they are good candidates for music playback.

The USB cable also makes a significant difference, I'm a known proponent of the optical USB cable, especially with a decent linear power supply on the DAC end. Length of cable makes a significant difference as does cable "brand" to a smaller degree. The USB cable specs are tightly regulated by the USB specs so there is not a large difference between them. I'm not sure what aspect of cable construction makes a difference but my guess is its either the dialectric quality or the size of the ground wire. The shorter a cable gets the better the sound, but see below. That is why I like the optical cable.

The single biggest effect on sound has nothing to do with jitter etc, its emi emitted by the computer which gets picked up by the stereo system. If the computer is sitting next to the stereo system this is a FAR greater effect than anything mentioned above. Different computers spew out different amounts of EMI so its hard to make absolute pronouncements, but I would not put any computer right next to a stereo system. When using the USB DAC my computer is 25 feet away from the system with the long optical USB cable. If the computer is only a few feet away its audible effects are quite noticeable. Laptops tend to have lower emmisions than other types but even they can be heard. The ultra quite computers are no better than anything else in this regard, I would seriously think twice about putting ANY computer in the rack with your other stereo components. Ultra low EMI computers can certainly be built, but almost everything on the market is shielded just to the point of meeting the FCC regs and no better.

I have not done extensive testing with sound cards so I don't know how these things work with sound cards. I while back I did some comparisons with several sound cards including a Lynx and a couple RME cards and I did not like them as much as my home grown USB DAC. Of course it might have been the EMI from the computer completely swamping the "native" sound quaility of the cards. When running the cards the computer was pretty close to the stereo system, I did not have any long good quality interconnects in order to place the computer on the other side of the room.


SO to sum it up: systems like the squeeze box do a very good job of decoupling you from computer issues, as long as the server can keep up with the data demand, anything will do with whatever else you want going on.

A USB DAC with a good reclocker cuts down a lot on sensitivity, but finding the right one is not easy or cheap.

For a USB DAC without reclocker keep the computer as simple as possible with as few things running on it as you can possibly get. An optical USB cable lets you put the computer well away from the rest of the system.

Keep the computer away from the stereo system, period.

Well thats about it for now.

John S.




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