Originally Posted by DougWinsor
The load on a processor and the processor its self would be irrelvant to how it sounds.
I wish what you were saying were true but unfortunately the situation is far more complex than it seems on the surface. The explanation requires total system understanding both at digital and analog levels.
The load on any current CPU would be less then 1% for playing back a MP3 file. The new intel quad core CPU's only use 7% for playing back blu ray movies.
What you say here is true but not material to the current discussion per-se.
Let's first look at how a digital system works. A CPU does work on every clock cycle (the Gigahertz refers to this). So billions of times per second, something happens inside your machine. Even when the CPU is "idle" it is executing something (the kernel idle code). Unless you suspend your machine and put it to sleep the thing is running around the proverbial squirrel cage, whether there is something to do or not. So the fact that the CPU is working 1% or 7% is not relevant because the CPU is not shutting off between the intervals (and if it did, it might cause other audible problems).
The CPU and associated digital components connected to it require power to perform work. The circuits are digital meaning they jump from zero to the supply voltage in a very short amount of time. Recall basics of electronics. To generate a perfect transition from zero to supply voltage, requires infinite power. Real life signals are a bit more gentle than this but we are still talking about pretty strong impulses, given the 30 to 70 watt of the CPU and rest of the circuits connected to it. Imagine pulsing a 70 watt light bulb on and off billions of times per second. Do you think it generates noise? You bet. So much so that you can pick it up over the air on a radio many feet from it!
Since the power supply doesn't have infinite capacity, it winds up having its output voltage modulated rapidly by these pulses (manifesting in noise due to high frequency at work). Its voltage sags with the transition and then goes back up shortly thereafter. Yes, we have filters and decoupling caps on power supply rails to help with this but we can't filter out all the noise. You can easily verify this with a scope on the power line, with or without the CPU running. With me so far?
Now here is where it gets interesting. The sound subsystem is also sitting on the same power supply line (even with post regulators). The DAC master clock gets its power from the same line as does the reference voltage. Vary either one of these a hair (picoseconds in case of master clock) and the sound changes. Remember, the DAC is not a digital device. It takes digital samples. But what it outputs is analog both in value and timing. The only way it can be accurate in its output is if its input is accurate (clock and reference voltage).
Further, look up DAC power supply rejection ratio rating. You will find specs as low as 20db. Meaning anything noise higher than this will leak into the DAC, distorting its output.
In summary, the CPU is connected through the power supply to the most sensitive parts of the audio reproduction system. Its activities (or lack thereof) have the potential to impact the audio fidelity because it changes the parameters which the DAC sees. As long as you share a connection between the two parts, you can't eliminate the impact of the CPU at the extreme. You can build a PC that is fairly immune to these problems (say, by having independent power supplies and total isolation) but I suspect none of you have such a system. Putting even a good sound card in a PC subjects it to the issues discussed so far.
So as you see, there can be an impact. What that is in real life in the PC you use is unpredictable due to complexity of the system and the many variables. And of course, how sensitive your ears are in picking up the variations is also a major factor.