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?

[dariju]

There is a good article about jitter @ Digital Domain:

http://www.digido.com/modules.php?name=New...icle&sid=15

I'm looking forward to see such test... How many of you believe in "jitter effect"?

[Gigapod]

There is a good article about jitter @ Digital Domain:

http://www.digido.com/modules.php?name=New...icle&sid=15

I'm looking forward to see such test... How many of you believe in "jitter effect"?

Having just read the article you linked to, I found it a little too much "audiophile"-minded and not very clear about the technical issue of jitter. Quoting: " ...The sonic results of passing this signal through processors that truncate the signal at -110, -105, or -96 dB are: increased "grain" in the image, instruments losing their sharp edges and focus; reduced soundstage width; apparent loss of level causing the listener to want to turn up the monitor level, even though high level signals are reproduced at unity gain..."

I know quartz oscillators have a small amount of phase noise (jitter).
How much does the phase noise in a 12MHz quartz oscillator driving a DAC at 48kHz affects the final waveform?

[Zster]

There is a good article about jitter @ Digital Domain:

http://www.digido.com/modules.php?name=New...icle&sid=15

I'm looking forward to see such test... How many of you believe in "jitter effect"?

Having just read the article you linked to, I found it a little too much "audiophile"-minded and not very clear about the technical issue of jitter. Quoting: " ...The sonic results of passing this signal through processors that truncate the signal at -110, -105, or -96 dB are: increased "grain" in the image, instruments losing their sharp edges and focus; reduced soundstage width; apparent loss of level causing the listener to want to turn up the monitor level, even though high level signals are reproduced at unity gain..."

I know quartz oscillators have a small amount of phase noise (jitter).
How much does the phase noise in a 12MHz quartz oscillator driving a DAC at 48kHz affects the final waveform?

I agree with your assessment that the article is too audiophile. Here is a more scientific approach by someone who has designed ADC and DAC for MRI and military devices and makes studio quality ADC and DAC’s.

http://www.lavryengineering.com/white_papers/jitter.pdf

unfortunately jitter is very real.

[Gigapod]

...
Here is a more scientific approach by someone who has designed ADC and DAC for MRI and military devices and makes studio quality ADC and DAC’s.

http://www.lavryengineering.com/white_papers/jitter.pdf

unfortunately jitter is very real.

Thank you, that was an interesting paper.

I agree with you that jitter is real. However, the author of the paper, Dan Lavry, used FM modulation of sampling rates to simulate jitter. This is definitely not an accurate simulation. Jitter is also called "phase noise" for a good reason: it has a noise profile.

Just a note: your inexpensive PC codec uses a 24MHz clock to drive an internal PLL that itself drives the sampling rate of the A/D and D/A converters. It's next to impossible to FM modulate that internal PLL...

The second graph on page 6 is a more accurate representation of real-life jitter effects, and you can see they are below the noise floor of the signal.

Cabbagerat suggested in a post in another thread that most manufacturers don't quote jitter figures for their audio equipment gear because the effects of jitter can ultimately be measured in noise and THD figures. I think this is correct; I would only add that providing jitter figures directly would be a) difficult (because jitter is difficult to measure and where exactly do you measure it ?) and b) meaningless, as you can only hear the side effects of jitter as added noise or distortion.

Nowadays noise and distortion figures hover well below audible levels, so I confess I am not too worried about jitter, even though it's real... in a sense.