This is a cross post from the Tech Talk section, but I thought everyone here in the Digital section would like to see it, and make comments...
As mentioned in earlier posts, a treasure-trove of jitter related papers all stems from the work of the late Julian Dunn at his colleagues in the UK.
Juergen Reis, chief engineer and designer at MBL, kindly pointed these out to me a few months ago, for which I am most grateful. While the papers are dry and technical, they do a good job of explaining the essentials. If you go through them carefully, make notes and diagram things for yourself, a great deal can be learned.
This post is an attempt to simplify and summarize a lot of details into some general observations...
One of the most important basic points to understand is that "digital" signals are really just very strange-looking analog waveforms. All the discussions about "How can there be errors in digital systems?" confuses the idealized concept (repeat, concept) of a binary digit with the reality of its electronic representation (notice I didn't say equivalent). The concept is logical, not physical: It's either a zero or one. That's great, and looks well on the chalkboard, but try making a perfect zero or one using real circuits.
Those burdened with advanced education in the field
will recal lthat a square wave in fact comprises a large number of component sine and cosine waves, of various frequencies and phases, all of which sum up to something that's got sharp corners and flat tops. When you start passing this "package" through real circuitry, the fun begins...
Where people often get off track is on the subject of how can one hear 100 picosecond timing errors? Well, of course you can't, directly. Human hearing can detect differences in arrival time of clicks to milliseconds and microseconds, but not picoseconds. However, the consequences of such timing errors are readily audible in many cases, that that they distort ordinary audio signals in various obvious ways.
One of the most insidious realities of jitter, in all its lovely forms, is that the distortions are seldom related to the natural harmonic structure of music. You could infer this from some of the comments, but I thought it was a good idea to mention it explicitly. A-harmonic distortions are very hard to hide, so to speak.
Another common-sense reality of jitter is that it affects higher frequencies more than lower, thus contributing to screechy violins, edgy transients and all that.
Basically the amplitude of the jitter sidebands relates to the magnitude of the timing slop, and the frequency offset of the sidebands relates to the periodicity of the timing error. More or less
There can be jitter in USB, in S/PDIF, in IEEE 1394/FireWire, or basically any form of interface. There are jitter artifacts caused by the bit patterns of the digital data, according to Julian. And of course during A/D or D/A sampling conversion, once again, sample clock timing errors distort the waveform.
A particularly bad situation is when the sampling clock is derived from information coming across the interface. More advanced designs use phase-locked loop devices to essentially smooth out the inevitable jitter and decouple various subsystems in the A/D or D/A.
Jitter is often pretty obvious in lower-end USB DACs, to pick on one type of component Sure, they can handle 96 and 192 kHz signals, the audio gets progressively worse. High levels of jitter have a lot to do with such issues.
With higher-quality gear, where extensive measures are taken to control and isolate various types of jitter, the reverse occurs: the sound keeps getting better and better with higher sampling rates, as one would expect.
As mentioned in earlier posts, a treasure-trove of jitter related papers all stems from the work of the late Julian Dunn at his colleagues in the UK.
Juergen Reis, chief engineer and designer at MBL, kindly pointed these out to me a few months ago, for which I am most grateful. While the papers are dry and technical, they do a good job of explaining the essentials. If you go through them carefully, make notes and diagram things for yourself, a great deal can be learned.
This post is an attempt to simplify and summarize a lot of details into some general observations...
One of the most important basic points to understand is that "digital" signals are really just very strange-looking analog waveforms. All the discussions about "How can there be errors in digital systems?" confuses the idealized concept (repeat, concept) of a binary digit with the reality of its electronic representation (notice I didn't say equivalent). The concept is logical, not physical: It's either a zero or one. That's great, and looks well on the chalkboard, but try making a perfect zero or one using real circuits.
Those burdened with advanced education in the field
will recal lthat a square wave in fact comprises a large number of component sine and cosine waves, of various frequencies and phases, all of which sum up to something that's got sharp corners and flat tops. When you start passing this "package" through real circuitry, the fun begins...Where people often get off track is on the subject of how can one hear 100 picosecond timing errors? Well, of course you can't, directly. Human hearing can detect differences in arrival time of clicks to milliseconds and microseconds, but not picoseconds. However, the consequences of such timing errors are readily audible in many cases, that that they distort ordinary audio signals in various obvious ways.
One of the most insidious realities of jitter, in all its lovely forms, is that the distortions are seldom related to the natural harmonic structure of music. You could infer this from some of the comments, but I thought it was a good idea to mention it explicitly. A-harmonic distortions are very hard to hide, so to speak.
Another common-sense reality of jitter is that it affects higher frequencies more than lower, thus contributing to screechy violins, edgy transients and all that.
Basically the amplitude of the jitter sidebands relates to the magnitude of the timing slop, and the frequency offset of the sidebands relates to the periodicity of the timing error. More or less
There can be jitter in USB, in S/PDIF, in IEEE 1394/FireWire, or basically any form of interface. There are jitter artifacts caused by the bit patterns of the digital data, according to Julian. And of course during A/D or D/A sampling conversion, once again, sample clock timing errors distort the waveform.
A particularly bad situation is when the sampling clock is derived from information coming across the interface. More advanced designs use phase-locked loop devices to essentially smooth out the inevitable jitter and decouple various subsystems in the A/D or D/A.
Jitter is often pretty obvious in lower-end USB DACs, to pick on one type of component
Sure, they can handle 96 and 192 kHz signals, the audio gets progressively worse. High levels of jitter have a lot to do with such issues. With higher-quality gear, where extensive measures are taken to control and isolate various types of jitter, the reverse occurs: the sound keeps getting better and better with higher sampling rates, as one would expect.
