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Positive Feedback ISSUE 41
january/february 2009


Ten Questions about Computer Audio with Andreas Koch of Playback Designs


Computer Audio has become the new rage in audio and for good reason: one has an easy and instant access to all their music as well the ability to search out countless other titles via the internet.  The issue is simply where to start, though the answer is quite obvious: get a computer, rip and store the files, and then play them back to some DAC. Of course being audiophiles …err the nuts we are… the questions start to pile up rather quickly.

Mac or PC, and then once you got that settled, there is all the minutiae related to just setting-up that computer’s OS and configuration. Then comes how best to rip and how best to save the files, and then to where? Okay, so now that I have my files, how best to play them back and how best to get the files out of the computer and to what DAC? Yadda, yadda, yadda… each question leads to further questions to clarify the previous that then lead to other questions that suggests another question and …a downward spiral down into the rabbit hole we go.

So I went to the 2009 CES and found not only a wealth of information, but a wealth of confusion or at the very least, a wealth of disagreement among those that are either in the recording/software side, the hardware-side, and/or the "expert" sides of computer audio. Now CES is not the best place to get all the answers… time is an issue as is finding all the people to ask, so I came up with 10 ‘key" questions (these are my 10, you may have others or perhaps might not find these of any benefit to you, but I chose them because they are of interest to me and besides they reflect the most common or important areas that seem to pop-up whenever one talks about computer-based audio, so go pound silicon if they don’t work for you.) and emailed them to 12 people in the industry to answer. Their responses are here...


Playback Designs

1. Let's start with interfaces; the obvious choices are USB, Firewire, Optical, and S/PDIF. What is your opinion on any of these interfaces? What if any, are the advantages or disadvantages of one over the others in terms of resolution, jitter, etc.?

With a properly designed DAC, as described below, none of these cables have a bearing, because all that the DAC expects from them is a truthful transmission of bits. Since all of them can do that I don't expect any sonic differences. However, there are differences in length, cost, universal application.

2. With regards to software there are also strong opinions as to some being vastly superior (or for that matter, inferior) to others; people clearly hear differences in how files are being played back and therefore prefer one over the others. There is also a growing opinion that Pro software is the only way to go and that using iTunes, WMP, MAX, or other free software playback programs (FooBar, JRiver, MAX, etc.) is not the way to go. That is, these are sonically and musically inferior to the Pro software because the Pro software (say for example Amarra, Izotpe, etc.) is simply "better" at playing back music files. What is your opinion on what is going on here? That is, why would any of these programs be superior—or for that matter, inferior—to another with respect to say a .wav file in any resolution: 16/44.1, 24/96, or 24/192? Is it a matter of timing and jitter? Issues with the operating software and processing? The fact that some software runs "cleaner" than others—that there is nothing running in the background to muck things up? Or as some suggest that the "math" is simply better in some software than in others?

Again, all a properly designed DAC expects is the truthful shipment of bits. It is true that some applications try to do digital volume control, dither, and truncation to 16 bits or even extension from 16 to 24 bits. The trick is to set up the PC so that no bits get lost or messed with, and that can quite be confusing. I am convinced that this is the reason why some applications sound better than others. The creators of pro software might understand that issue a little better, because they often generate their applications with deeper roots (i.e. custom drivers and APIs). But if these applications are all set up to generate the same exact bits at their digital outputs they are all going to sound the same.

3. Let's move on to ripping. As with the above, there are proponents that claim only certain software, and optical drives for that matter, can "accurately" rip a CD. That they can clearly hear differences between rips via different means; even though the rips are bit for bit perfect. Any thoughts on what is going on here? Is there an advantage to using specific ripping software or drives over another? Say iTunes, WMP, Max or whatever when compared to say EAC?

Ripping is basically a recording process with analog characteristics. The media plays a role, dirt on the media, the air between media and record head, the laser and the recording speed. Sometimes these influences can easily be so large that single bits are changed. Error correction code can catch such cases up to a certain limit, but after that we lose the truthfulness of our original bits. That is when we will start hearing effects even with our "2-dimensional" DAC. It is not always very easy to determine if a disc has been ripped with all bits correctly in place. Let's assume for instance, that the CD-R obtained a bit error during ripping in an area of the disc where the reflectivity is right on the edge of the spec (as is very often the case). Because of variances in components and execution we may get a bit for bit correct output from our CD-ROM in the PC that us running our bit comparison application, but not from a different CD drive in an audio player. In fact that combination of bit error during ripping and poor reflectivity may cause the audio player to interpolate audio. This clearly was not a bit for bit correct copy, even though our software said it was. We really don't have any scientific way of telling whether we are even comparing apples with apples when we talk about different drives, media and applications.

4. File formats. Any reason why a .wav, AIFF, or FLAC file is better than say Apple Lossless? Again people suggest a strong preference for one over the others, so something must be going on here?

A properly designed DAC makes that all irrelevant as long as we don't lose any bits.

5. There is also a movement towards Pro DACs. Naturally there are DACs of varying quality and performance, but is there any reason why a PRO DAC would be better than a DAC made by a manufacturer from the audio community? Say ones of comparable quality and build?

In fact, I would think that many Pro DACs sound worse, because they need to have many more features to be functional in studios and they are often also designed with much more limited budgets (nobody wants to pay for music and it all goes back to the studios who don't have any revenue anymore). As is always the case in audio, the "KISS Principle" (Keep it simple stupid) is in effect, "less is more" and if you can spend a little more money on things that do matter even only by a small amount then you end up with a better product. Stay with audiophile products and pay the premium, they are generally better sounding.

6. Along those same lines, what makes one DAC a better choice for computer-based audio than another? Jitter reduction, chip sets, power supply, etc?

Noisy environments like PC's, USB etc. need a "2-dimensional" DAC. Standard "1-dimensional" DACs allow way too much clock jitter to get through which destroys the sonics.

As we all know, audio is represented in a y/x-axis system: the y-axis for amplitude and the x-axis for time. Mostly because of analog audio's sensitivity problems in the y-axis, digital audio was introduced. But digital audio not only quantizes the y-axis, it does so as well on the x-axis. Sounds like we got more than we wanted—true and too bad. A typical state-of-the-art DAC converts between quantization levels in the digital y-axis and the analog y-axis and is completely transparent and open as to what happens on the x-axis (time domain). Sounds like we forgot the quantization on the x-axis.

This oversight forced us to treat digital audio signals as if they were analog: use special cables, use all kinds of mechanical devices for our CD players, power conditioners for digital audio etc. Looks like we just shifted the original problem from the y-axis to the x-axis, but the issues are still the same. Instead of interference or crosstalk we now call it clock jitter.

Almost all DACs available today deal with the y-axis only and rely on external devices for the x-axis, such as complicated master/slave clock arrangements or external sync clock generators. At best these devices are band-aids on a wide open wound deep inside the DAC. They help, but do not resolve the problem at the source. We need a 2-dimensional DAC that not only works on the y-axis, but also on the x-axis. With this we can separate the digital world completely from the analog one and render any digital cable, transmission format, storage media and application completely irrelevant to the final sonic performance. The only analog problem that we still have then is the separation of the power supplies for digital and analog.

The DAC inside the Playback Designs product line does exactly that: clock jitter from incoming digital audio signals can be described as an analog signal that gets mixed together with a quantized digital signal (our ideal and constant sample rate clock). So before any processing can happen we need to bring these two components into the same domain: The Playback Designs system quantizes the clock jitter into a digital signal, where it then can be subtracted from the original sample rate while the latter is converted to analog at the same time. Of the course, the DAC also works independently in the y-axis by using a set of unique algorithms in a completely discrete architecture (not even a single Op-Amp is used).

Tests have shown that the DAC inside the Playback Designs product line can be fed by any digital source including PC, inexpensive Discman, DVD player, high-end CD player and none of them seem to make a difference on the sonic performance of the analog output signal.

7. What do you see as being the most important factor in getting the best sound in computer-based audio? That is what should the consumer address with the greatest concern when setting up a computer-based audio system?

External "2-dimensional" DACs with proper power supply separation that eliminates "jitter".

8. Along with that, what do you see as being the most important factor in NOT getting the best sound in computer-based audio? That is, what can have the greatest potential to adversely affect the sound in computer-based audio?

A DAC that does not address the jitter at its root.

9. Some suggest that they computer must be audio dedicated. That is it must be "built" or configured for the specific purpose of only playing music and that any and all non-audio related programs and such must be eliminated. Your feelings on this? Is it important or not, and why so?

With a "2-dimensional" DAC you can use any PC and it will not make any difference.

10. Where do you see the greatest impact to come in computer-based audio for the future?

From the design of better DACs… like that from Playback Designs!

Andreas Koch got his start with Studer ReVox in Switzerland in 1982 where he built world's first fully asynchronous digital audio sample rate converter. In 1984 he designed one of the first filter banks for digital audio where some of the same ideas were used later in audio compression algorithms such as MP3, AC-3 and others. In 1985 he worked for Dolby Labs in San Francisco where he built all the digital signal processing of the AC-1 encoder and decoder (delta modulator). In 1987 he returned to Studer ReVox and managed the development of a professional digital audio tape recorder (48-channel DASH format on 1/2 inch tape). He later was involved in market and technology research for hard disk (PC) recording in professional applications.

In 1990 he transferred to Studer Editech in Menlo Park, CA, to manage a group of engineers designing the ultimate hard disc recorder for professional post production applications. He launched "Dyaxis", a user interface, in 1992. In 1993 he was with Sony in Florida where he oversaw product development for professional audio products and launched various mixing consoles. In 1997 he moved to Sony in San Francisco and managed the development for the world’s first 8-channel DSD recording / editing / mixing machine, Sonoma. It is still used today. During the period of 1998 to 2000, Andreas designed all digital parts of A/D and D/A converters that helped establish DSD as a superior sounding audio format in SACD. Around 2003 he expanded the Sonoma to 32-channels of DSD on a single PC and participated in all standardization committees for SACD in conjunction with Philips.

In 2003 Andreas joined EMM labs where he designed all the digital audio products for EMM Labs; professional and audiophile. At EMM Andreas also designed various revolutionary algorithms for sample rate conversion (upsampling), as can only be expected from one of the original inventors of SRC. He also developed discrete D/A converter and unique architecture for clock management from digital audio transmission inputs.

In 2008, Andreas launched Playback Designs which is an integrated SACD/CD player with a variety of digital inputs that incorporates all the experience, knowledge, and algorithms gathered and developed over the last 25 years, right from the beginning of digital audio.

Use the links below to read other responses to these questions

Larry Moore and Eric Hider of Ultra Fi Audio Designs

Andreas Koch of Playback Designs

Tony Lauck

Steve Nugent of Empirical Audio

Gordon Rankin of Wavelength Audio

Jon Reichbach of Sonic Studio/Amarra

Vinnie Rossi of Red Wine Audio

John Stronczer of Bel Canto Designs

Daniel Weiss of Weiss Digital Audio

Vincent Sanders and John Hughes of VRS Audio Solutions

Kent Poon of Design w Sound

Charles Hansen of Ayre Acoustics

Pete Davey of Positive Feedback Online