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Computer Audio Asylum Music servers and other computer based digital audio technologies. |
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Computer Audio Asylum Music servers and other computer based digital audio technologies. |
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In Reply to: Jitter and music software posted by Mercman on June 2, 2009 at 12:58:34:
Jitter is a complex subject but fortunately is well defined in mathematical terms allowing us to analyse, measure and reduce its nasty effects. Both DACs and ADCs suffer from jitter distortion. This is unique to digital audio, i.e. analogue audio is jitter free (but not free from having good interconnects, EMI etc.).
For starters we know there's a well defined end-point where jitter distortion arises. This part of our string is located deep inside the DAC chip where the master clock marshals each sample point (both left & right channel) to its "rightful position" in the output analogue voltage. Unfortunately, this "right" position is constantly fluctuating in time causing analogue voltage signal changes out of time (either earlier or later). This is jitter distortion. Its correct measurement can only be determined at the analogue outputs. This was discussed in the past: Jitter Research paper.
Jitter measurement made at the DAC's clock input using for example the Wavecrest analyser is misleading. Whilst this measure is correct in providing clock jitter data, it is not jitter distortion as defined. What is being measured here is clock jitter as it enters the DAC chip. But this is not where signal coupling takes place that affects actual analogue voltage changes, instead we find this happening deep within the DAC chip. This aspect has large bearing on jitter distortion actually experienced by the listener.
DAC chips are complex integrated circuits where the seemingly pure clock signal entering it suffers damage (for example substrate noise). The quality of remaining DAC input signals (other clocks, control and data) are noise pollution sources that significantly affect jitter. The chip itself consumes power, performs computations and has other complications such as buffers which also contribute towards jitter. Hence measuring jitter distortion can only be done at the DAC's analogue outputs.
It's also important asking the question: how much jitter distortion in audible? Playback jitter levels needs to be below 8.5ps (Jpp RSS, assuming no recorded Jitter!) to be inaudible (if human hearing is capable of 22 bits resolution). A very complex task indeed.
Some vendors (like dCS, AudioEngr, Gordan) design computer audio with the assumption that incoming computer bitstream is highly jittered and noisy. The solution applied here involves reclocking the data (this includes Asynchronous USB Audio). Such designs attempt to create a "firewall" against incoming jittered data but has a drawback: jitter levels are that of the device in use (intrinsic jitter) and any superior bitstream sent to it is largely wasted. In such cases, this is the other string endpoint.
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Follow Ups
- How long is a piece of string? (part 1) - cics 10:03:08 06/03/09 (3)
- RE: How long is a piece of string? (part 1) - Dawnrazor 22:45:06 06/04/09 (2)
- part 2 will cover sw (here's some info) - cics 14:31:54 06/05/09 (1)
- RE: part 2 will cover sw (here's some info) - audioengr 18:01:51 06/05/09 (0)
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