Audibility of Jitter, Proposing a series of tests Options

[cabbagerat]

The threshold of audibility of phase noise in ADC and DAC clocks is a fairly contentious issue in the HiFi and audiophile world. Some sources claim that jitter is clearly audible at low levels, and some claim that high levels of jitter are inaudible. The literature describes several tests, many with conflicting results.

One of the chief difficulties in testing the audibility of jitter is that it requires a complex hardware setup, which means that many listeners would be required to be present for an time consuming (and expensive) on site test. Over the last couple of months I have been thinking about organising a distributed listening test to look at the audibility of jitter in audio applications, based on algorithms for simulating the effects of jitter on signals. These algorithms are fairly well described in RF and telecomms engineering literature, and would be interesting for comparison purposes.

The kind of thing I have in mind is this:
Use samples which are accepted to sound good -> simulate jitter -> perform listening tests -> perform more tests at different levels of jitter depending on results

The purpose of this thread is to get ideas of the Hydrogenaudio community about performing these tests. Some of the things I would appreciate input on are:

• Would such a test be useful
• Would the results of such a test be valid?
• What sort of test procedure would be best?

[cabbagerat]

I would suggest you begin with a working definition of "Jitter", which you could post to the HA Knowledgebase. Is jitter "phase noise" ?

A good point. I would go with a definition of Jitter as the high frequency component of the time interval error in the clock signal. Where the cutoff of "high frequency" is must be chosen, as must the spectrum of the modelled noise. This spectrum is one very hard problem - it's likely to consist of a variety of intermodulation products, PSU noise, other noise, harmonic distortion and vary widely between different pieces of equipment.

What does it sound like? What does it do to a sine wave that goes through ADCs and DACs ? Does it matter at all ?

Jitter spreads the spectrum of a sine wave out, effectively by convolution with the spectrum of the jitter. I have no idea what it sounds like, except in some extreme cases. And whether it matters at all is one of the questions I am hoping this test will help answer.

What is the order of magnitude of jitter in PCs? and in high-end audio equipment?

I have no idea about PCs, but total jitter figures for midrange HiFi stuff is generally somewhere between 0.1ns and 100ns - a three order of magnitude range.

[Gigapod]

... total jitter figures for midrange HiFi stuff is generally somewhere between 0.1ns and 100ns ...

100ns sound a little bit high (for a 10MHz quartz oscillator that's one full cycle) but OK, let's work with that.
How would a 10MHz signal modulation of an audio signal ranging from 20Hz to 20kHz affect the signal?
It seems all the intermodulation products would be way above the audibility range, no? Or am I missing something?
I think a typical figure in an PC-grade audio codec would be 1 ns jitter (approx. 1% jitter in a 10MHz clock), which would represent approximately 0,005% jitter in a 44.1kHz clock (if my napkin calculations are right).
My a priori guess is the ABX test would be unconclusive...

(edit) I just checked the ALC882 codec datasheet and the 24MHz bit clock input is specified with a maximum jitter of 2ns.
The S/PDIF-OUT jitter is specified at 4ns max (6.144MHz clock). I think the S/PDIF-OUT clock is PLL generated internally in the codec.

This post has been edited by Gigapod: Dec 27 2006, 18:59