If you will allow me, Id like to elaborate on the external clocks use.
Something that should be realized about using a "super-precision clock" is that the stability of the clock in ppm (parts per million) does not have anything to do with improving short term instabilities, also known as clock jitter; all it affects is playback speed stability. In other words, if there is a deviation (plus or minus) of the clock frequency, you will hear increased or decreased playback speed, nothing else. Lets say the audio master clock is 22,000,000Hz (or 22MHz) specified at the extremely high +/-100ppm frequency deviation, then you do the math what percentage is 100 parts per million from this frequency and you will find out what will be the speed deviation. Simply, this is something you cannot hear and hardly measure.
When it comes to digital audio, the jitter (or period jitter) is whats important for best sound quality, not the frequency deviation/stability, which is negligible even at 100ppm.
External clocks are made for synchronization of multiple digital audio/video devices in studios and TV stations. The external clock is usually converted to DC voltage that controls a VCO (voltage controlled oscillator). While this synchronizes everything connected to the house clock (the external clock) and locks the frequency, it has nothing to do with clock jitter. Not to talk about the fact that, usually, the VCO is used as a reference to a multi-clock generator that is PLL based, so the jitter can sometimes exceed 400pS, although the frequency deviation is locked to some really low figure as 0.00005ppm. Compare to a non-PLL clock oscillator with 2pS of jitter, although its frequency deviation is specified at 50ppm or more.
Hope this helps you understand digital audio clocking even better!
Best wishes,
Alex Peychev
www.aplhifi.com