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Yet another look at Jitter: Clock Extraction

amirm

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#1
I hope you are not getting tired of jitter talk because I have more info to share :).

One of the common arguments made against jitter mattering is that: "the data is buffered and clock regenerated in the DAC so jitter won't be there." This makes all the sense in the world. Once we capture the data and then push it out at our will, there shouldn't be a problem. Well, there is a problem. A serious one. Buffering and clock regeneration do not deal with jitter by themselves. I have explained this in words many times but this time I am bringing in some specific data to hopefully put this myth to bed (yeh, wishful thinking :) ).

Introduction
The way a clock is "regenerated" is to have a local oscillator (clock) that we can change its frequency to eventually match and track the incoming digital stream. As you may know, S/PDIF is a serial digital connection with clock and data intermixed. By using this circuit which is called a Phase Locked Loop (or PLL for short), we are able to extract a clock that is cleaner than the incoming one. This clean up allows us to capture the digital samples reliably.

Our job is not done however. We not only need to extract the data samples but also accurately match the incoming data rate (clock regeneration) without any timing variations in it getting to our DAC. Not doing so causes jitter to appear in the output of the DAC.

S/PDIF Receiver Clock Regeneration
Let's look at a Crystal Semiconductor CS 8416. From its data sheet: http://www.cirrus.com/en/pubs/proDatasheet/CS8416_F3.pdf, we see this measurement of its rejection of incoming jitter as it regenerates the clock using its PLL:



I have annotated their graph as to make it easier to understand.

The measurement shows how much of the incoming filter is reduced in amplitude (vertical scale) at each frequency of jitter (horizontal axis).

Note as I have indicated on the graph there is absolutely no reduction of incoming jitter below 8 Khz!!! Even worse, there is actual *amplification* of jitter prior to reaching 8 Khz to the tune of 2 db. The "peaking" is due to the way the PLL filter is designed and is outside of the scope of this conversation to explain why it occurs.

Even at 20 Khz, we only have a modest 6 db reduction of incoming jitter.

The serious reduction in jitter therefore is from 20 Khz and up. Why? Because it is those frequencies which cause us to not be able to extract the incoming *data* -- the PCM audio samples. That is the "mission critical" application in IT terms. If the receiver can't decode the incoming bitstream, your system breaks and we can't let that happen. So high-frequency jitter is eliminated and with it, a ton of ultrasonic noise and interference.

Measurements
You might say, this is all theory, who knows if it is true in practice. And that would be a fair question. Fortunately I happen to have data on this :).

Here are the measurements of jitter reduction -- i.e. the same as above graph -- for all the devices that I tested for my Widescreen Review Magazine article:



Look at the mass market AVR performance when it comes to jitter reduction. Notice how there is no attenuation or slight amplification of jitter as the above data sheet showed. I show the response up to 10 Khz so the sharp attenuation in higher frequency is not there in my measurements. What is there of course, what we care about: Jitter reduction in *audible* frequency range.

Summary
We see that the common argument of audio frequency jitter being eliminated because we have buffering and clock regeneration simply does not hold water. The common/cheap implementation only gets rid of high frequency jitter so that it can reliably extract digital audio samples. It does little to filter out incoming jitter in audio band which is what we care about.

Of course the problem can be solved using skilled designers and budgets that are measured in tens of dollars as opposed to single digit.
 

Mivera

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#2
PD2 DAC.jpg
You should test the effectiveness of the USB reclocking in the DAC/Pre I'm now distributing. A friend of yours bought one of them. Took delivery today.

Here's the DAC board:
 
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dallasjustice

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#3
Blizzard,

What does this thingy do? Is this another expensive USB hub?
 

Mivera

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#4

amirm

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#6
You should test the effectiveness of the USB reclocking in the DAC/Pre I'm now distributing. A friend of yours bought one of them. Took delivery today.

Here's the DAC board:
Oh, the troublemaker is in the house. :D Seriously, welcome Mike.

The Mark Levinson DAC that showed 60 db reject of jitter in my graph was being used with the Berkeley async USB to AES/EBU. So I was showing USB reclocking in that manner. The connection though was discrete as there are two boxes. Happy to measure them anytime you want to loan one to me.
 

Mivera

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#7

Mivera

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#8
Oh, the troublemaker is in the house. :D Seriously, welcome Mike.

The Mark Levinson DAC that showed 60 db reject of jitter in my graph was being used with the Berkeley async USB to AES/EBU. So I was showing USB reclocking in that manner. The connection though was discrete as there are two boxes. Happy to measure them anytime you want to loan one to me.
Shouldn't be any trouble around here. Unless low jitter is a problem :)

Well this is the best USB audio implementation I've ever seen. It was a lot of work getting it to this level. The DAC master clock is derived from the reclocked USB clock, which is reclocked after the galvanic isolation. And the DAC chip is a very short distance away from the clocks. Not much chance for jitter to sneak in. The coaxial SPDIF and Toslink inputs are upsampled to 24/192 and reclocked as well to eliminate jitter.

A friend of yours has one in your neighbourhood. He told me he was going to see if he could borrow an amp from you until his is delivered.
 

Purité Audio

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#9
Mike Hi, I had a look at your site but details are a little sparse, who makes it ?
Keith
 

Mivera

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#10
Mike Hi, I had a look at your site but details are a little sparse, who makes it ?
Keith

Yes I'll be updating it soon. It's the Phison PD2. Made in Denmark. It's a very nice fully modular DAC/PRE with 100% discrete class A gain stages. No opamps in the signal path. The discrete gain stages and servo's are on modular boards. It's designed to never need to replace the case. If a new DAC upgrade comes out, just replace the single DAC board. Even the DAC output stage/discrete DSD filter board is modular and separate. Has optional phono stage as well, but not really pushing that option :)

IMG_4080.jpg
IMG_4082.jpg
PD2.jpg
 
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Thomas savage

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#11
IMG_2086.jpg
it lifts the earth like my TAD, interesting it only uses one transformer rather than two. how dose it compare with the dac's you put together your self mike?

i found this when looking it up but the power supply is different..
 

Mivera

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#12
it lifts the earth like my TAD, interesting it only uses one transformer rather than two. how dose it compare with the dac's you put together your self mike?

There's only 1 transformer, but there's dozen's of LDO's all over the place powering each section of everything with ultra clean power. The DAC board alone, without the discrete output/filter board has 15 separate ultra low noise/high PSSR LDO regulators.

It's the best I've heard, which is why I decided to distribute it. I also believe in the highly modular/upgradable platform. The engineer behind it did the DAC/preamp boards for AAVIK. The introductory price is only $7500 due to distributor direct sales.

Yeah that's the old transformer, but after further testing it was swapped out with this one in my picture. I'm not sure why he shared that picture instead.
 
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Mivera

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#14
Nice unit. Let's create a separate thread for it Mike. Will they be at AXPONA and do they have any measurements for it?
Thanks. Yes it's quite nice. No AXPONA is a bit too soon. Too much work to do. Theres a matching amp that won't be ready until April 15th. Unit #1 in my picture is what sold to your friend in Seattle.

Okay made a new thread.
 
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iridium

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#15
CLOCKS FAST:
Tiny optical frequency clock measures time accurately to 270 quintillionths of a second
optical clock that's just 1 cubic centimeter—small enough to fit on a standard silicon chip—and can track time intervals with precision to 270 quintillionths of second. (One quintillionth is equivalent to 1 times 10 to the negative 18th power, or 0.000000000000000001.)

Read more at: http://phys.org/news/2016-05-tiny-optical-frequency-clock-accurately.html#jCp
 

RayDunzl

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#16
Jitter Rejection Ratio is not a measurement listed in my DAC manual.

Is there a way to calculate (ballpark) what it would be based on the levels of jitter-induced distortion or noise?
 

Cosmik

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#18
Just to re-iterate, though... I don't think it can be stressed enough that all of this refers only to systems where the DAC is slaved to the source of the data.

None of this applies to asynchronous USB.
 

NorthSky

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#19
http://www.thewelltemperedcomputer.com/KB/USB.html
http://archimago.blogspot.ca/2015/05/measurements-usb-hubs-and-8khz-phy.html
________

I would love to watch all my 3D Blu-ray movies (high def pic + hi res multi sound) with a USB connection. ...And 4K Blu-rays too...later on.
I would easily throw all my HDMI cables and components in the trash can.

* A multi triple separated USB cable:
1. Power supply USB
2. Audio only (multi hi res sound)
3. Video only (multi hi def pic)

It would have three separated ends, and would connect from the source to the pre/pro via triple USB ports.
And jitter would be smaller than 0.001ps

Any taker? :)
 
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