Signal integrity & it's possible influence on digital audio

[jkeny]

I posted this explanation from John Swenson on PFM (Place For Muppets) but as far as that lot are concerned the old dictum "ignorance is bliss" applies although I don't see much bliss on there.

The excerpt pretty much elucidates what most of us here maintained but gives some meat to the idea that when it comes to digital audio, fluctuations on the the power supply can have system wide, detrimental effects i.e can affect the A side of the D to A conversion. Now, I know the argument against this is that "competently designed" digital audio equipment should be immune to such issues - so the counter argument must be that therefore ALL digital audio devices are incompetently designed as I don't know of one that is immune. Please correct me if I'm wrong in this & can provide an example of one such device.

Executive summary: Most high-speed digital receiver chips have adaptive, digitally controlled analogue filter front-ends inside the chips (maybe DaveF can confirm this or not). These filters are role is to pre-filter the signal to eliminate noise, reflections, etc. to a certain level. As the signal integrity gets worse more layers of these filters are invoked/turned on thus consuming gradually more power as the SI gets worse. I suspect that being dynamic, these filters are dynamically drawing power rather than it being a continuous draw. This dynamic draw on power (& ground return currents) can have system wide influences with ground references which are widely used in digital audio.

Just note that J Swenson is talking about Sata cables here:

when I say the SI gets worse, I mean that when you change cables the SI at the receiver can change significantly.

High speed serial receivers really do increase their power draw a lot as the SI degrades. The "frontend" of all of these devices consists of digitally controlled analog filters. Most of them have several filter blocks that can be turned on or off as needed, and each block can have it's parameters adjusted on the fly as needed. These filters pre-process the input signal to try and get rid of the noise, reflections etc. These types of filters are what has made high speed serial interfaces viable at all.

When the SI is very good minimal filtering is needed, which uses only a small amount of power. As the SI worsens the filter parameters are changed, which increases current. As it gets worse more blocks are brought "on line" doing different types of filtering. There are control loops that constantly monitor the SI and adjust the filter system to get the best results at the "one/zero" detector.

As each of these steps occurs more and more current is required, at it's maximum the filter system can use 4 times as much current as it does at minimum. Of course the digital parts of the system take current as well, but the analog part consumes most of the power of the whole "receiver". The result is about a 3 to 1 difference in power depending on the SI of the input signal.

This SI vs. power becomes an interesting optimization strategy for some of the chips I work on. Several of them have a large number of these interfaces (hundreds), they consume a huge amount of power at lower SI, so it turns out to be cheaper to use very expensive cables ($700 cables!) so the receivers don't have to run at their full filter capability, thus using a lot less power, which means cheaper packages, smaller heatsinks, less air flow etc.

So the answer is, yes they really do "work harder" at lower SI.

Some of the lower frequency receivers don't use the analog frontends at all, they just run the input signal into a very fast ADC and use DSP to try and extract the signal from the muck. Even these change power with varying SI, but not as much as the ones with the analog frontend.

SATA is definitely in the analog frontend teritory (as is PCIe). USB HS (only 480MHz) is so slow it doesn't even use the ADC and DSP approach, although it might actually wind up being better to implement it that way today.

This obviously has wider implications as all these efforts should improve signal integrity &/or reduce system noise (although bear the underlined text above in mind :
- Better PS for PCs
- better USB PS
- software such as MQN
- cables
- EMI/RF shielding
- & this one is for you SBGK- not placing magnets close to USB cables :)

[jkeny]

And a follow up piece of data/evidence but this time related to USB cable eye patterns measured by HiFi News & their possible correlation to audibility
First USB cable group test done in July 2013 http://mvicha.sweb.cz/test%20USB%20kabelu.pdf

Second one done in July 2014 http://mvicha.sweb.cz/test%20USB%20kabelu%202014.pdf

[sbgk]

magnets were for educational purposes only

does the theory account for the lack of digital noise when silence is played ie all the instructions are running, only 0s are being copied ? the digital noise is only heard when there is a payload and seems to be variable with the frequency. Does a constant tone have noise issues or is it where frequencies change ?

[jkeny]

magnets were for educational purposes only

does the theory account for the lack of digital noise when silence is played ie all the instructions are running, only 0s are being copied ? the digital noise is only heard when there is a payload and seems to be variable with the frequency. Does a constant tone have noise issues or is it where frequencies change ?

I know the magnets were a test for the doubters to try but I doubt any of them will :) I've never tried it, btw - must give it a test.

The concept of signal integrity applies to all signals including digital silence - the better the signal quality, the less work being done at the receiver. But I can surmise that if the digital audio source is a dynamic system (PC) then the quality of the signal (the waveforms) can be varying - more varying under load than when not under load. This varying waveform signal will then be transmitted to the receiver & cause another dynamic load adjustment to deal with the dynamic waveform being encountered. The complex relationship that is communicated by this waveform disturbance is pretty difficult to predict unless some real-time waveform capturing at the DAC could be achieved with monitoring between the two systems?

[DaveF]

Executive summary: Most high-speed digital receiver chips have adaptive, digitally controlled analogue filter front-ends inside the chips (maybe DaveF can confirm this or not). These filters are role is to pre-filter the signal to eliminate noise, reflections, etc. to a certain level. As the signal integrity gets worse more layers of these filters are invoked/turned on thus consuming gradually more power as the SI gets worse. I suspect that being dynamic, these filters are dynamically drawing power rather than it being a continuous draw. This dynamic draw on power (& ground return currents) can have system wide influences with ground references which are widely used in digital audio.

Yes, I've worked a few high speed rx's where there is adaptive equalisation on the incoming signal, but we'd be talking about signals in the GHz region. Digital filters tend to be running all the time from any of the implementations I've seen but perhaps there are designs out there that switch in and out different filters depending on the scenario. I have to say though that I've never come across extra power draw or fluctuations from any of this especially if something just to filter out noise above a certain frequency.
Signal integrity is something I've gone through quite a bit of in the last few months but more on the digital and FPGA side of things.

[jkeny]

Yes, I've worked a few high speed rx's where there is adaptive equalisation on the incoming signal, but we'd be talking about signals in the GHz region. Digital filters tend to be running all the time from any of the implementations I've seen but perhaps there are designs out there that switch in and out different filters depending on the scenario. I have to say though that I've never come across extra power draw or fluctuations from any of this especially if something just to filter out noise above a certain frequency.
Signal integrity is something I've gone through quite a bit of in the last few months but more on the digital and FPGA side of things.

Thanks Dave for pitching in.
I don't think we are talking just filtering out a noise level at a certain frequency - I believe it may be more dynamic than this i.e. varying levels of noise at various frequencies would be normal to emanate from a PC. Also variations in the square wave waveform structure, rise, fall times may well be of significance

Just some stumbling ideas, really.

[DaveF]

Also variations in the square wave waveform structure, rise, fall times may well be of significance

Never seen the waveform structure vary at all regardless of what load was on the system and I've scope out all sorts of boards over the years. The PCB designers generally do extensive testing on the signals or if not they really should. If rise/fall times/skew dont seem to be correct then component changes usually sort that out. But variances, never seen it. The exception to that is jitter.

[jkeny]

Also variations in the square wave waveform structure, rise, fall times may well be of significance

Never seen the waveform structure vary at all regardless of what load was on the system and I've scope out all sorts of boards over the years. The PCB designers generally do extensive testing on the signals or if not they really should. If rise/fall times/skew dont seem to be correct then component changes usually sort that out. But variances, never seen it. The exception to that is jitter.

Dave, did you look at the two files I linked to at the beginning of the thread showing eye diagrams for the same signal transported through different USB cables? For those who don't know what an eye diagram is - it's simply an oscilloscope shot of a USB square waveform overlaid with the next shot, overlaid with the next shot, etc up to an agreed number of shots. The resulting image looks like an eye & the ideal eye diagram would be a single pixel-depth line that traces the image. This is not possible as some fluctuation in the waveform always happen giving a bit of a fuzzy line. The greater the fuzziness, the more fluctuations there are & the worse the signal integrity is. USB standard will allow a certain degree of fuzziness. This works fine for digital but is it fine for digital audio (for the reasons stated in previous post)?

Dave, I know USB is a far lower speed than you are handling & you are probably not testing cable transmission but rather pcb transmission line traces so your experience may be different but what do you think of those USB eye diagrams & the differences between them?

[nige2000]

hope i didn't misunderstand the thread title

in the audio pc we lower power consumption and throttling in the bios, although you have to realise it needs what it needs so there is a tipping point.
where possible give individual pc components its own power source especially usb cards, dacs, operating system drive etc, this helps contain/isolate the noise

removal of the main switching power supply (atx) with direct quality linear or even better battery supplies, this reduces noise and improves esr i guess (as bass and dynamics get a big boost from this mod)

other examples were improved sq by powering xo's by an extremely clean supply (gives a more fluid and detailed playback)
signal cable length and construction

[nige2000]

i see some of the better usb cables in those reviews using solid silver cores in dielectric and in a spiral formation :)