Originally Posted by Digitalchkn
Vast majority of the time the audio section is sourced by say a 25MHz at cut crystal running through some sort of a multi-purpose clock generator/buffer that spits out all sorts of clocks for CPU, memory, peripheral busses, I/O bridges, etc. It is reasonably clean, except it's PLLs bandwidths are often above the audio frequencies. The higher speed digital standards don't really care about what happens to clocks at audio frequencies. I have personally measured plenty of motherboard clocks that wander around like crazy at low frequencies -- but they pass all the jitter requirements set out by that fancy 5Gbps high speed standard.
I did a few more tests of 3 different onboard audio outputs and a few sound cards, all recording was done with the setup described here in 96/24 format.
ALC887 in a desktop PC (frequency = 11024.7515 Hz, jitter+noise = -79 dBr - this translates to 2.3 ns (RMS) of jitter, although not all of it is necessarily jitter in fact); click to zoom:
ALC270 in a laptop (frequency = 11024.5878 Hz, jitter+noise = -86 dBr; it would look somewhat cleaner at 48 kHz sample rate):
ALC850 in an old desktop PC (frequency = 12000.1886 Hz, jitter+noise = -71 dBr); this is an outdated AC97 codec, with a lot of ultrasonic imaging, aliasing, high frequency roll-off, and other problems:
Sound Blaster Live! Value - now that is some really old hardware (frequency = 12001.973 Hz, jitter+noise = -64 dBr - the reason it is so high despite the clean looking graph is that there is a high amount of very low frequency random jitter):
Sound Blaster Audigy SE, 16/24-bit samples (frequency = 11998.3397 Hz, jitter+noise = -86 dBr); note that this card auto-mutes the output when there is no signal, and that most of the peaks on the 16-bit graph are part of the JTest signal:
Xonar D1, 16/24-bit samples - finally something that is not ancient and actually performs very well (frequency = 11024.9427 Hz, jitter+noise = -103 dBr):
The frequency values are the measured frequency of the recorded JTest tone. It should be 11025 Hz or 12000 Hz, depending on whether the DAC has hardware support for 44.1 kHz sample rate. However, note that the sound card used for recording of course does not have perfectly accurate clock frequency either, so that skews all results somewhat (using a method of timing the real duration of a 10-minute tone with the NTP-synchronized system clock of the PC, I approximated it to be ~34 ppm (edit: fixed incorrect value) too "fast", but that still might not be correct).
The jitter+noise figure is the total unweighted RMS level of the signal, referenced to the test tone, around which the bandwidth is +/- 8 kHz, with a narrow (~1 Hz bandwidth) notch at the tested frequency. In some cases, much of it is not actual jitter, however.
Edited by stv014 - 9/25/13 at 4:31am