2D DAC Technology

2 Dimensional DAC Technology and Computer Audio

2 Dimensional DACs

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 2 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 a PC, an inexpensive Discman, a DVD player, or high-end CD transport and none of them seem to make a difference on the sonic performance of the analog output signal. Ultimately this means that as long as you are sending our DAC truthful complete bits the source does not make a difference. We believe if you own a home computer, you already own a music server that cannot be sonically bettered!

Why do we want a computer for audio playback?

Simply speaking, ease of use.

Having the ability to easily store music, create playlists and select music by genre makes computer playback the most enjoyable method of playback today. You no longer have to have a room filled with hundreds or even thousands of CDs. Everything is controlled easily without having to get out of your listening chair.

The most impressive part of computer audio is having the capability of playing music at a much higher resolution than a CD can provide. With the Playback Designs products, you can actually listen to files that are up to 128 times the resolution of a CD.

The download bottleneck

File sizes for a 3 minute song and download times (assuming 5Mb/sec internet connection):

  • Redbook (16/44.1kHz)    32MB @ 1 min.
  • 24/88.2kHz                    95MB @ 2.6 min.
  • 24/96kHz                       103MB @ 2.8 min.
  • 24/176.4kHz                  190MB @ 5 min.
  • DXD (24/352.8kHz)        380MB @ 10 min.
  • DSD                              127MB @ 3.4 min.

As we can see DSD is not only very efficient for downloads, but at the same time has the highest resolution available. DSD should easily become the dominant format for high resolution downloads.

Bit accuracy

Within the computer itself, you have the software player application, the computer’s operating system and the computer driver software. All of this controls what goes out to an external DAC. Quite often one or more of these elements within the computer offers digital volume control and / or processing which degrades sonic performance.

It is imperative that you turn off all computer audio processing and set all computer volume controls to exactly 0db (i.e. wide open). This will help ensure that all the bits are intact by the time your DAC recieves them.

Also, your computer has internal sounds and audible alerts. These need to be turned off as well. If not, during playback, you may hear these sounds and / or audible alerts through your loudspealers.

Have you dropped a bit lately?

Windows is not a real time operating system and often causes audio dropouts. If you are using a PC, we recommend you download the latency checker application to configure your PC for dropout-free performance. You can find this programming by visiting http://www.thesycon.de/deu/latency_check.shtml

We have not experienced these same problems on the newer MACs.

The manifold (numerous) joys of an external DAC

Generally speaking, an external DAC should provide much better sound than having one built into your computer or music server. Manufacturers have greater flexibility in designing a power supply for an external chassis. Space is not as much of an issue and there can be better separation between the power supply and critical audio circuitry.

The PC is a very noisy environment which can have a real negative effect on the DAC’s performance. Separating the two is critical. One primary benefit is the ability to have control over clock generation. In this case the DAC can be the clock master or clock generator which eliminates a lot of jitter problems normally associated with computer audio.

Link to external DAC

AES/EBU

  • Pro: cable length, synchronous
  • Con: limited sample rate, no DSD, requires second link for master clock and in most cases an additional sound card

S/PDIF

  • Pro: synchronous
  • Con: short cable length, limited sample rate, no DSD, requires second link for master clock and in most cases an additional sound card

TosLink – please do not use this

HDMI

  • Pro: synchronous, DSD
  • Con: limited sample rate, limited cable length, expensive

USB

  • Pro: standard on all computers, some support by standard operating systems, open ended (can support DSD or any future format), not limited by sample rate, no additional link for master clock necessary
  • Con: limited cable length, non-synchronous

Firewire

  • Very similar to USB, but less and less popular with computer manufacturers

“Asynchronous” USB link

We believe the USB connection is currently the best connection to use between a computer and a DAC. USB connections are standard on all computers and have extremely wide bandwidth for all current digital formats (PCM and DSD) as well as future possibilities. It also requires no additional link to handle clocking.

There are a number of different ways to send audio from a computer to a DAC via a USB link, but only one that really works well.

Adaptive USB Mode is the first we will discuss. It only sends data one way and relies on the computer’s clock to control the timing of the data sent. It is not desired.

Asynchronous USB Mode is the desired method. It allows two way communication between the computer and the DAC. It is important that the clock resides with the DAC and that the DAC does not rely on the inferior unstable computer clock which cannot maintain consistent timing of the data sent. In short, Asynchronous USB Mode allows the DAC to be the clock master or generator and the computer to be the clock slave.