DaveF wrote:I havent read the entire thread as I've been away from hifi for several months but is the motivation behind this dedicated PC that you'll minimise or eliminate all sources of ground noise that might otherwise pollute the USB connection down to the DAC?
I take it that belief is that the bits extracted/packaged by the PC and sent to the external DAC are always the same no matter what PC improvements or JPlay versions are in use? That the technical explaination for any real improvements is the above?
Dave,
The jury still hasn't given their verdict on this yet as evidence is still being presented both anecdotal & empirical evidence.
As Tony says, a number of us have heard the incremental improvement in audio that comes with the improvement in the power supplies to various parts of the computer.
The empirical evidence is beginning to be teased out also - John Swenson has claimed (& I've no reason not to believe him) that he has measured different noise levels/spectrum on the ground plane (in USB DAC ?) form different playback software.
There is a good series of articles by John Swenson on this topic posted on Audiostream here:
Part 1
http://www.audiostream.com/content/qa-j ... at-digital
Part 2:
http://www.audiostream.com/content/qa-j ... -just-bits
Part 3 to follow.
You probably know all or most of this but it is very well presented & Part 3 will be the most interesting, I believe?
From Part 2 I have quoted some relevant bits in answer to your question:
So now the crux of the matter, how can what goes into the USB receiver affect any of this? In several ways: packet jitter, edge jitter, PLLs. I’ll go over each of these.
Packet jitter is the difference in the arrival time of packets to the receiver chip. USB packets are transmitted over the bus at either 1000 per second (full speed mode) or 8000 per second (high speed mode). Every time one of those packets hits the receiver a lot of activity happens inside the receiver chip. This creates lots of noise inside the chip and on the ground plane. This causes a lot of jitter on the outputs from the chip. The spectrum of this noise and jitter has a VERY strong component at either 1KHz or 8KHz, both of which are directly in the audio range. Any changes in the arrival time of the packets will change the spectrum of this packet noise. In the next installment I’ll cover what causes this packet jitter.
Next is edge jitter, this is traditional jitter of the individual edges on the bus. As with everything else I have been talking about, jitter on the input can cause noise in the chip and on the ground plane that is related to the spectrum of the jitter. This is where things like different cables can have an effect on what is happening.
Next is PLLs. Every USB receiver chip has at least one if not more PLLs. These PLLs are affected by both the previous types of jitter and since almost all the circuitry inside the chip is clocked by these PLLs, the jitter on output signals and ground plane noise is going to be significantly affected by the noise spectrum coming out of the PLLs. This is filtered by the PLL loop filter, but there are still major components related to the input jitter.
I remember Marcin answering me with one word when I emailed him asking for help with best computer spec for an audio PC - that word was "stability" & I believe it encompasses all of the above succinctly. Latency, which used to be considered a desirable feature for audio PCs is actually, I believe a kind of surrogate (but inaccurate) measurement for stability.
