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USB audio, TAS1020, digital filters, jitter etc.

Posted by John Swenson (A) on November 15, 2004 at 21:08:06

I just found this forum here and have been reading everything, I finally have a little free time so I'm going to make my BIG post on the subject.

First off I've been playing around with my own DAC implementations for quite sometime and have a pretty good feel for whats going on, and even have some plausible theories for what I hear. About 6 months ago I started looking into the USB audio interface issues and how they fit into what I have learned so far.

What I have found agrees with what Thomas has been saying. I've come up with two main points for getting the best sound. Of course this is not ALL that is required, you still have to do all the normal stuff like extremely good power supplies, very low jitter clocks etc, that stuff doesn't go away. If a certain implementation doesn't have these it doesn't mean its going to automatically sound awfull, just that its not achiveing its full potential.

The main precepts are:

If you want the best USB audio you have to go with the asynchronous mode.

Anything that includes S/PDIF in the chain is going to be compromised.

Unforunately there are VERY few implementations that do this, and I'm not quite sure why.

The reasons for these two have already been stated here recently so I won't go into a lot of detail, just the quick over view.

Any implementation using synchrounous or even adaptive mode HAS to have a clock generated by some form of PLL, this is always going to be worse than running the DAC directly off a really good clock. As audiengr has found using a really good clock to feed the PLL circuit can help significantly, but it can never be as good as no PLL at all.

The perils of S/PDIF are well understood, but its not just the "cable" and connector thats a problem. Even feeding a good S/PDIF signal to a receiver on the same chip STILL has a PLL in the path. Even with a reclocker after the receiver its not at its best. A while back I built a DAC with a Tent clock driving the DAC with a reclocker and FIFO and STILL had audible effects from the S/PDIF. As long as the PLL is on the same board or box the correlated jitter from the PLL manages to work its way to the DAC, usually as noise in the power supply (that especially includes the ground). Its incredibly difficult to get rid of this stuff.

So what that means is we need a system that uses async mode and directly outputs I2S signals to a DAC that is clocked by an ultra low jitter clock. The TAS1020 chip does exactly that. The problem with this chip is that its not simple to use. At its heart is a 8052 MCU, at powerup it reads in the code for its operation from an external EEPROM. Almost all the implementations using the 1020 use the synchronous mode, my guess is that is what is in the "sample" code distributed by TI, so most implementations just do simple tweaks to the code or maybe even use it as is.

In order to use async mode and I2S some programming is going to have to be done. The datasheet from TI is woefully absent in any real useful information on programming this chip. I presume they have something they send to "developers" otherwise nobody could ever get this thing to work at all. I haven't bought the evaluation board, but maybe it comes with the details and thats how you are supposed to get them. Does anybody know where the details on programming this chip are located?

Gordon, If you've managed to read this far, I'd be very interested to know how you achieved this with your design. I certainly don't want you to post any code you've written, thats your IP for sure, but if you wouldn't mind I'd love to know what chip you used and where you got the detailed information on programming it. You don't have to answer if you don't want to.

On a completely different subject I'd like to make a few observations on digital filters.

I've been looking into the subject of digital filers and CD playback for a long time. For a living I work on very complex custom chips and have worked with some experts in DSP matters. From looking at what other chip makers and high end companies have done we came to the conclusion that nobody has yet done a CD digital filter correctly. Every implementation we looked at was a significant compromise. Different implementations have different compromises which do effect the sound.

It turns out that in order to implement the digital filter correctly takes a huge amount of resources (which is why all designs were compromises). It turns out that it CAN be done right in the size chips that I work with, but it would cost a fair amount of money. The chips would be too expensive for someone like TI to make, the market for the expensive chips would be way too small and the high end companies that would be willing to spend the per piece price can't afford the engineering and setup charges such a chip would take. The approach that some high end companies have taken, using DSP processors, doesn't give nearly enough horsepower either.

I think the above is a primary reason why NONOS has caught on so much. Its not that a digital filter is bad, its that all implementation you can get out there don't do it right. Getting rid of the filter altogether sounds better. I also don't think that the oversampling per say is really a big issue, but that almost all oversampling DACs include digital filters so whenever you hear an oversampling DAC you are also hearing the compromised digital filter.

Some of the recent very good oversampling DACs have a mode that turns off the digital filter, I propose that we should make some implemetations using these DACs with the filter turned off before we throw out the oversampling DACs altogether. Unfortunately the filterless modes are not easy to implement! For some reason when you put the DAC in filterless mode you can't use the regular I2S input stream, you need to use a different protocol and pins on the DAC!!!

Given all the above, a really good thing to try would be a USB DAC using the 1020 in async mode, programmed so it provides this other interface and feed it to one of these new DACs in filterless mode, then use a transformer output stage which does a good job of the remaining filtering necessary. I think this would be a really exceptional device.

Thats probably way more than enough for one message so I'll quit now.

John S.

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