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In Reply to: RE: Jitter and music software posted by Mercman on June 02, 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.
Follow Ups:
Hey Cics,
Unfortunately the string thing was lost on me. It seemed like the starting point what deep inside the dac chip, but the endpoint was BEFORE the chip?
Can you unconfuse me? :)
And the big question is how does software affect what happens in the dac chip if the bits are perfect?? It clearly does IME but some cant get past the "if the players are bit perfect than they must sound the same" mentality.
How does software impact the "DAC input signals (other clocks, control and data)" and is this the main reason there are differences?
Re your questions:
- "string" represents a critical value chain delivering the bitstream (to either internal or external DAC)
- bits remain unchanged but sonic differences can occur when changing sw (os & player). we get the same bits out but different sound due to sw induced jitter which is measurable. how sw manifests at the physical level needs to be understood. this is not some abstraction that happens without any consequences (esp. power consumption, resource conflicts resulting in arbitration, timing and error handling), i.e. there's no such thing as "digital physics" just physics. strictly for the enthusiasts: suggested reading material provides excellent insight into what sw actually does (there are several books covering a few thousand pages so i would recommend reading "Volume 1" and the "Optimization Reference Manual")
- "bits r bits" mindset is a common misconception when jitter distortion is incorrectly (or not) understood. using a wavecrest to measure clock jitter before entering the DAC is not measuring jitter distortion. hence any jitter conclusions from this (apart from ensuring a clean clock signal) is invalid
I agree with you on the jitter because I have proven it to myself through a series of experiments, some with reclocker and some without. Some using Wi-Fi/networked playback and some using USB/Firewire and S/PDIF.
The thing is that comparisons like these should show ALL playback software sounding as good as it can when a reclocker is used. This is not always the case however, particularly with iTunes. Even after the jitter is minimized with a reclocker that totally isolates the computer clock, Amarra still makes an improvement.
This can only mean that something in addition to jitter is at work in this case. I'm not saying that Amarra does not reduce jitter, because I believe it does, however it sounds better than iTunes even with a reclocker used for both.
Steve N.
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