USB jitter - why does it matter?

[Sound Hound]

hi folks,

 

I'm putting together an 8 channel system with DDX amps for experimenting with ambisonics and multiway active setups.

since my background is computers and I've only recently forayed into audio, I'm a bit mystified by jitter and precision clocking.

I get that galvanic isolation and a separate, clean power source are important to the audio bits beyond the computer.

but I don't understand why the USB connection between such needs to have more than a reliable/accurate transfer of data.

does the jitter transmit stray signals into the latter stages?  if not, then the only place high precision clocks are warranted is in driving the DAC or DDX stage.

surely any USB implementation is sufficient with the data adequately buffered.

reclockers?! iPurifiers?!   pah - audio voodoo!

I'm cynical but ready to be enlightened!

thanks....

[mansr]

I think you have the right idea.

[sandyk]

47 minutes ago, Sound Hound said:

I'm cynical but ready to be enlightened!

 

 mansr needs to be too, but it would be a waste of time trying to enlighten him.

 

Items such as the USB Regen, when used in conjunction with cleaner power , and USB cables with improved isolation between D+ and D-, and the SMPS derived +5V supply, do result in improvements in SQ, but you need to read other areas of the forum, not just rely on advice from the usual sceptics in the General area of the forum ,where most sceptics appear to hang out and argue incessantly about the existence of God, Philosophy etc. , and very little to do with Computer Audio.

 1,000s of C.A. members use devices such as the USB Regen etc. and higher quality USB cables and power supplies for them, and report improved results with USB Audio.

 

[davide256]

No one has it really nailed for an authoritative scientific explanation yet why what is supposed to be perfect( digital playback)  isn't but the last 2 years have seen a steady improvement in sound using better USB converter tech and lower component noise, cleaner power for hardware renderer solutions. Unfortunately there are always foot draggers  who insist that if you can't provide/test the science now, what you hear can't be real. Science is often misused by marketing and sales as a way to close a sale... I've learned to listen first and ask questions about the science only if you hear something you like.

[fas42]

The explanation is actually very simple: digital playback requires high levels of electrical cleaniness for audible artifacts not to be part of the picture. The sorry truth is that such is just the way that it is, because the designers, etc, haven't gone the yards to ensure that there is never an issue - until all the key participants realise this, grok the importance of getting this right, then there will be waving of hands, saying "Ahh, that's good enough ... the textbooks reckon I've dotted all the i's and crossed all the t's ... nothing more is necessary!" ... and, it still won't sound "good enough" ...

[JohnSwenson]

1 hour ago, Sound Hound said:

hi folks,

 

I'm putting together an 8 channel system with DDX amps for experimenting with ambisonics and multiway active setups.

since my background is computers and I've only recently forayed into audio, I'm a bit mystified by jitter and precision clocking.

I get that galvanic isolation and a separate, clean power source are important to the audio bits beyond the computer.

but I don't understand why the USB connection between such needs to have more than a reliable/accurate transfer of data.

does the jitter transmit stray signals into the latter stages?  if not, then the only place high precision clocks are warranted is in driving the DAC or DDX stage.

surely any USB implementation is sufficient with the data adequately buffered.

reclockers?! iPurifiers?!   pah - audio voodoo!

I'm cynical but ready to be enlightened!

thanks....

Hi Sound Hound,

I have been working on this for years, I'm getting close to a complete end to end measurement, but test equipment to properly measure this stuff doesn't exist, I'm having to design and build my own as I go along. I can measure pieces of the chain now and the rest hopefully coming soon. Part of the slowness was getting laid off and retiring and moving to a new state. I now have a working lab again and am working on the next piece of test equipment.

 

The hypothesis goes thusly:

 

ALL crystal oscillators exhibit frequency change with power supply voltage change. This is known and well measured. A cyclical change in voltage causes a cyclical change in frequency which shows up in phase noise plots. For example if you apply a 100Hz signal to the power supply of the oscillator you will see a 100Hz spur in the phase noise plot. 

 

A circuit that has a digital stream running through it will will generate noise on the power and ground planes of the PCB just from the transistors turning on and off that are processing that stream. This effect is very well known and measured. Combine this with the previous paragraph and you have jitter on the incoming data stream producing varying noise on the PG planes that modulates the clock increasing its jitter.

 

The above has been measured.

 

But shouldn't ground plane isolation and reclockers fix this? At first glance you would think so, but look carefully at what is happening. What is a reclocker? A flip flop. The incoming data with a particular phase noise profile goes through transistors inside the flip flop. Those transistors switching create noise on its internal PG traces, wires in the package and traces on the board. This noise is directly related to the phase noise profile of the incoming data. This PG noise changes the thresholds of the transistors that are clocking the data out thus overlaying the phase noise profile of the local clock with that of the clock used to generate the stream that is being reclocked. This process is hard to see, so I am working on a test setup that generates a "marker" in the phase noise of the incoming clock so it becomes easy to see this phase noise overlaying process.

 

This process has always been there but has been masked by the phase noise of the local clock itself. Now that we are using much lower phase noise local clocks this overlying is a significantly larger percentage of the total phase noise from the local clock.

 

Digital isolators used in ground plane isolation schemes don't help this. Jitter on the input to the isolator still shows up on the output, with added jitter from the isolators. This combination of original phase noise and that added by the isolator is what goes into the reclocking flip flop, increasing the jitter in the local clock. Some great strides have been made in the digital isolator space, significantly decreasing the added phase noise which over all helps, but now the phase noise from the input is a larger percentage, so changes to it are more obvious.

 

The result is that even digital isolators and reclocking don't completely block the phase noise contribution of the incoming data stream. It can help, but it doesn't get rid of it.

 

For USB (and Ethernet) it gets more complicated since the data is not a continuous stream, it comes in packets, thus this PG noise comes in bursts. This makes analysis of this in real systems much more difficult since most of the time it is not there. Thus any affects to an audio stream come and go. Thus just looking at a scope is not going to show anything since any distortion caused by this only happens when the data over the bus actually comes in. To look at anything with a scope will take synchronizing to the packet arrivals. Things like FFTs get problematic as well since what you are trying to measure is not constant . It will probably take something like wavelet analysis to see what is really happening.

 

The next step in my ongoing saga is to actually measure these effects on a DAC output. Again I have to build my own test equipment. The primary tool is going to be an ADC with a clock with lower phase noise than the changes which occur from the above. AND it needs to be 24 bits or so resolution. You just can't go out and buy these, they don't exist. So I build it myself.

 

I have done the design and have the boards and parts, but haven't had time to get them assembled yet. Then there is a ton of software to make this all work. Fortunately a large part already exists, designed to work with other systems but I can re-purpose it for this.

 

So it's not going to be right away, but hopefully not too off in the future I should be able to get to actually testing the end to end path of clock interactions all the way to DAC output.

 

John S.

 

 

 

 

[Ralf11]

does phase noise sound like IM distortion?

 

"smear"?