Fluctuating USB jitter from CPU activity

[jkeny]

Next up is the same LF plots of the DACmagic Vs Halide Bridge USB/SPDIF converter - red is DACmagic, blue is Halide Bridge
Note again the high LF plot for the DACmagic & much lower for Halide bridge


And here's the Halide Bridge on it's own for various samplerates - note the 96KHz samplerate playback has lower LF plot


Looking at these LF plots & my MQN LF plots my feeling was that this lowering of LF might just correlate with better SQ but it needed more measurement examples to delve further into this hunch

[jkeny]

good to see a bit more talk on the subject all the same

funny that one of the first things to do on a pc for audio was to stop cpu throttling and set the cpu frequency
most guys seemed to prefer underclocking but it seems to be a consensus that it needs to be a constant

one thing i always noticed whether hardware, software or power supply improvements their benefit always seemed to be accumulative
or one improvement never seemed to remove the benefit of a previous improvement it was always additional

for that reason id prefer to see devices been designed from the ground up to read files and output a clean usb/i2s signal for its own or external dac rather than try to fix a poor signal from pc etc
the difference between a standard pc and highly modded pc for audio is rather large so given the cost of pc modding surely theres a market for such devices

Sure, Nige, but remember these people are only catching up with where we & others have reached some while ago - you know the obsessed ones :)
So they are just taking small steps in order to get to some understanding of what's going on. Remember that JohnW is an engineer & needs measurements in order to progress(although he does admit to hearing things he can't measure)
Also, he is developing a competing product to the Regen :) (I know it's a kind odd thing to be doing on a forum - measuring a competitors device in order to better it - I'm not saying that this doesn't happen all the time but doing it on a forum is the odd bit)

Where you are coming from with your suggestion is a big leap from where they are at currently

[jkeny]

Here's another bullet point to my argument about low frequency noise or jitter.
In the standard jitter measurements graph low frequency jitter is seen as a fattening at the base of the main spike as opposed to spikes at frequencies away from the main fundamental frequency (it's hard to explain unless you know the graphs but you'll see an example of it in the link).

This review of Audio Research Ref DAC and its network interface shows the jitter graph on page 4 along with the review notes (he hadn't seen the measurements - they are done after the review) http://www.ultrahighendreview.com/uploa ... ncedac.pdf

The graph shows a lot of low frequency jitter for the network interface of the DAC. In the review he states:

this time streamed via the network - brought me down to earth with a bump. The image lost some focus, the bass blurred and stodgy...the magic, the suspension of disbelief, all eroded.

The jitter measurements aren't shown below 15Hz where it is 2643pS - this is difficult to directly equate to the readings on the IQ-test but Jim LeSurf says this about his graphs "100 parts per billion at 0·1 Hz corresponds to a peak timing Jitter component of nearly 200 nanoseconds – i.e. almost 200,000 picoseconds!"

[jkeny]

Here's the DACmagic's standard jitter measurement from Stereophile


Note the thickening of the base of the main spike which signifies low frequency jitter. This standard jitter test is not a particularly great way to spot such LF jitter problems, is it?

[jkeny]

I posted this on the MDAC PFM thread to point out the lack of logic of some of them but no answer:

Something I would like to understand from those who hold this position - there is now an acceptance that the VBus isn't fit for purpose for powering USB audio devices & it's accepted that external power is usually better (which wasn't the view of some of these people in 2013) but yet when it comes to the data signal the USB device it is contended that the USB device should be able to deal with all possible distortion of this signal.

Why don't these people also insist that the USB device should be capable of using the VBus & dealing with all the crap on it? The crap on the VBUS & ground was well known almost from day 1 - yet this signal waveform distortion has only been known about recently (1 day )

Any good explanation?

[Clive]

It's gradual shift of their position which previously would have been "it's digital, the power supply doesn't matter" I remember interminable arguements about usb cables where I postulated the RF had to be a major factor. Of course they won't remember such comments we've made in the past...they'll just pretend they've always known the cause. Plus they'll change their spec of competently designed.....if they have one.

I posted this on the MDAC PFM thread to point out the lack of logic of some of them but no answer:

Something I would like to understand from those who hold this position - there is now an acceptance that the VBus isn't fit for purpose for powering USB audio devices & it's accepted that external power is usually better (which wasn't the view of some of these people in 2013) but yet when it comes to the data signal the USB device it is contended that the USB device should be able to deal with all possible distortion of this signal.

Why don't these people also insist that the USB device should be capable of using the VBus & dealing with all the crap on it? The crap on the VBUS & ground was well known almost from day 1 - yet this signal waveform distortion has only been known about recently (1 day )

Any good explanation?

[jkeny]

It's gradual shift of their position which previously would have been "it's digital, the power supply doesn't matter" I remember interminable arguements about usb cables where I postulated the RF had to be a major factor. Of course they won't remember such comments we've made in the past...they'll just pretend they've always known the cause. Plus they'll change their spec of competently designed.....if they have one.

Yep, how right you are - BE718 replied pretty much along these lines - that the thinking was "it's digital, the power supply doesn't matter" - suggests a lack of technological understanding about USB ground - it is still an essential connection to the PC whether the device is powered externally or not & ground is one of the chief conduits for noise.

Yes, I remember the arguments but of course the record of their posts is still extant to be recalled at any stage :)

[jkeny]

Here's an extract from on of the research papers which I found related to how we perceive low frequency modulations
"Multi-time resolution analysis of speech: evidence from psychophysics" http://journal.frontiersin.org/article/ ... 4/abstract

A growing body of research, employing various
behavioral and neurobiological experimental techniques, now points to the perceptual
relevance of both phoneme-sized (10–40 Hz modulation frequency) and syllable-sized
(2–10 Hz modulation frequency) units in speech processing

We created sentences in which the
slow (∼4 Hz; Slow) and rapid (∼33 Hz; Shigh) modulations—corresponding to ∼250 and
∼30 ms, the average duration of syllables and certain phonetic properties, respectively—
were selectively extracted. Although Slow and Shigh have low intelligibility when presented
separately, dichotic presentation of Shigh with Slow results in supra-additive performance,
suggesting a synergistic relationship between low- and high-modulation frequencies.
A second experiment desynchronized presentation of the Slow and Shigh signals.
Desynchronizing signals relative to one another had no impact on intelligibility when
delays were less than ∼45 ms. Longer delays resulted in a steep intelligibility decline,
providing further evidence of integration or binding of information within restricted
temporal windows. Our data suggest that human speech perception uses multi-time
resolution processing. Signals are concurrently analyzed on at least two separate time
scales, the intermediate representations of these analyses are integrated, and the
resulting bound percept has significant consequences for speech intelligibility—a view
compatible with recent insights from neuroscience implicating multi-timescale auditory
processing.

So what all of this means is that two mechanism in neuro-processing of audio which operates on different low frequency bands - one low frequency (2-10Hz) & a higher frequency band (10-40Hz). Although this paper is specifically related to speech processing, it makes me wonder about it's relevance when listening to music! Not that we can hear tones at these frequencies (<20Hz) but that amplitude modulations in the sound envelope in this frequency range are perceptible & important.

Actually, the paper explicitly states this:

Although this study focused on the multi-scale nature of speech (see e.g., Rosen, 1992; Poeppel, 2003; Greenberg, 2006; Elliott and Theunissen, 2009), the mechanisms that speech processing exploits to effectively analyze the multi-time-scale constitution of the signal are likely to be of a general nature rather than speech-specific. There is abundant evidence that natural sounds of many types have such a multi-scale structure that requires analysis at multiple levels (e.g., Santoro et al., 2014).