Raspberry Pi I2S output as Asynchronous USB-to-I2S - Page 2 - diyAudio
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Old 4th April 2013, 06:23 PM   #11
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You have still not addressed the issue of buffer over- and underruns.

You will find it impossible to achieve a substantial improvement in jitter in the complex environment offered by a RPi. Low jitter (SOTA jitter) already requires custom layouts of the utmost simplicity, pared down to the minimum required to enable functionality.

The answer to this problem is not an elaborate buffering scheme and reclocking. The answer is a DAC with a low-jitter local clock and a small buffer and a data on demand delivery scheme to achieve low-latency.

Put your efforts into devising a way to achieve this, instead of a scheme which merely attempts to put a bandaid on a fundamentally broken system.

There is very little to no chance that your university will have an audio test set. Universities have generic equipment. Test sets are industry and production line specific.
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Old 4th April 2013, 09:56 PM   #12
Tinco is offline Tinco  Netherlands
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Hey counter culture,

So you are saying this is something the WaveIO achieves and the Raspberry could not? If so then I shall just order the WaveIO and give up on trying to use the RPi
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Old 5th April 2013, 12:41 PM   #13
Tinco is offline Tinco  Netherlands
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So I can imagine my university doesn't have something like this: AP High Performance Audio Analyzer & Audio Test Instruments : APX525 because it doesn't research audio signals as far as I am aware. But I2S is a digital signal, this means any digital analyzer should be able to get some data on it right? There is a whole lab that researches optical circuits that have loads of analyzers like this (I had a tour of it there, the researcher was very proud of the expensive equipment :P).

So what should I measure? I could measure the clock signal that is sent by the Pi, that's just a sinus so I could plot an FFT of it and it should just have 1 peak and the rest should be as low as possible right?

The data channel is more complex, what data should I make the Pi generate? Could I for example make it send a pattern of 1's and 0's that alternates in some pattern?

Do I have to measure the data channel? Doesn't the clock just drive the data acquisition, making it the only source of jitter?

Last edited by Tinco; 5th April 2013 at 12:43 PM. Reason: spelling
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Old 5th April 2013, 01:48 PM   #14
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What you are interested in is the timing purity of your word clock when it comes to jitter. Dedicated AP hardware will do it and will make it possible to get useful information out without a lot of knowledge of what you are trying to fundamentally measure. That's the beauty of dedicated boxes like the AP units. But you can surely do the same sort of measurements with a fast tektronix digital sampling scope, or an RF spectrum analyzer. Your jitter in your word clock will show up as changing sidebands in an RF spectrum analyzer I believe. You'll have to do some math to get it back into the time domain, but it's not like you have a lot of control of the jitter unless you are planning to do new layouts and hardware designs.

Also remember that your measured signal isn't really a sine wave, it's a square wave, and has lots of harmonics. All the audio ASICs that I know of, use or derive a master clock, and then divide that down to get the bit, word and L/R clocks. So you could measure the master clock. But at the end of the day, only the word clock is what triggers the D to A conversion. And that is an amplitude based threshold, so if your waveform is somewhat ratty and changes over time, you created jitter yourself. That's why proper termination on the clock lines is very important (the word clock especially).

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Old 5th April 2013, 03:47 PM   #15
Tinco is offline Tinco  Netherlands
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Thanks for the clear explanation Sheldon!

You are right that I don't have much control of the jitter. The Pi allows for an external clock, but from what I gather from the reactions here even if I pick a high quality clock it will still be disturbed by the Pi's circuitry, with no way to separate it from that influence.

So my goal is just to evaluate how big that influence really is. Is it terrible enough to throw in the towel and just output the data over usb to a simpler better architected circuit like the WaveIO or is it in the same quality range as the WaveIO?
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Old 5th April 2013, 04:08 PM   #16
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I think people are saying that if you are concerned with your timing to the point of using an external clock, you need more understanding of all aspects of the circuit. You really need to be doing a complete design. It's very possible to take the I2S signals from the rPI and buffer them and build a very respectable DAC. The question that remains is this more or less work than starting from scratch? Clearly things like Tek Scopes, AP measurement systems can have very complex digital sides (modern Tek scopes are windows PC's), and still have very good analog performance. But people did a lot of work to understand both sides and their interactions.

I stream all my music these days, and I'm using Airport Expresses into my own DACs with asyncronous sample rate conversion. If I were doing what you would like to do, I'd use the I2S output to a sample rate converter chip (CS8421) and then to a good DAC chip and analog stage. I'd pay attention to the power supplies in the clock circuit of the ASRC and all the voltages in the DAC chip as well as the transmission of the clocks between the chips.

For a look at my DAC see: Sheldon’s World Blog Archive Another Audio Digital-to-Analog (DAC) is Born

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Old 5th April 2013, 07:06 PM   #17
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Quote:
Originally Posted by Tinco View Post
But I2S is a digital signal, this means any digital analyzer should be able to get some data on it right?
A typical logic analyzer will tell you nothing about jitter. Right?

You will get more information if you ask some questions and stop making assertions. Most of your assertions so far have been inaccurate, and suffixing them with redundancies such as 'right' is just plain annoying. Right?
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Old 5th April 2013, 07:54 PM   #18
Tinco is offline Tinco  Netherlands
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Quote:
Originally Posted by counter culture View Post
A typical logic analyzer will tell you nothing about jitter. Right?

You will get more information if you ask some questions and stop making assertions. Most of your assertions so far have been inaccurate, and suffixing them with redundancies such as 'right' is just plain annoying. Right?
I'm sorry, it wasn't my intention to be annoying, I did not suffix my assertions with right redundantly, I did so because I was uncertain of them. I shall try to formulate my assertions more as questions from now on.

I started this topic out of curiosity about wether I could replace the WaveIO with a Raspberry Pi. The answers raised questions that obviously go over my head. I did not fully understand (and am still a bit unclear on) why the WaveIO would give a more precise signal when it gets its data over USB than the RPi would even if it itself was the source of the signal.

Since no one made final judgements on the RPi's amount of jitter I thought I could measure it and see for myself, but it seems that measuring it is also a hard thing to do.

Perhaps I should just buy them both and hear if there's a difference in sound quality.
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Old 5th April 2013, 08:44 PM   #19
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Originally Posted by Tinco View Post
Perhaps I should just buy them both and hear if there's a difference in sound quality.
Better to try and get to hear them side-by-side without buying them.

There have been lots of attempts to show how much jitter is audible. There's no doubt that as it gets worse, at some point it will become audible, but most USB DACs use a PLL based clock linked to the data frame rate, which is /millisecond. So the PLL has to interpolate 441 clock ticks per 10 data frames, an odd fraction and quite a high number. So this clock almost certainly has poorer jitter than the crystal clock in the computer by a not inconsiderable amount. Despite this, the evidence is that a Behringer UCA202 at $30 is barely distinguishable from the very best of DACs, and is perfectly enjoyable to listen to for most people in the majority of circumstances.

Take a look at the nwavguy's blind test, it will give you some perspective on the issue.

NwAvGuy: DAC Listening Challenge Results

Another person with a sensible approach to reproduction is Ethan Winer, who provides a number of online examples for people to educate themselves about the audibility or otherwise of various 'flaws' which are used to in a inverted sense to sell audio equipment.

Artifact Audibility Comparisons
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Old 6th April 2013, 12:12 AM   #20
Tinco is offline Tinco  Netherlands
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Thanks for giving me some perspective counter culture Put that way I feel I can put less emphasis on finding the least amount of jitter, and more on building the dac properly.
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