What has enabled this proliferation of USB-based D/A converters is a series of parts from Burr-Brown
(Texas Instruments) called the PCM270x series. These parts are inexpensive (only a few dollars), very easy
to use, and require no programming skills. They are similar enough to conventional D/A chips that any digital
audio engineer can easily design a product around them.
The problem with all of the chips in the Burr-Brown PCM270x series is that they have high levels of jitter. A
fixed-frequency master audio clock is not employed. Instead a variable-frequency master audio clock is generated
based on the timing of the incoming audio data. The computer sends packets of audio data at one
millisecond intervals across the USB connection. The Burr-Brown chips have internal circuitry that measures
the interval between the audio packets, and then uses this information to generate a new master audio
clock that matches the rate at which the computer sends packets.
This system, called adaptive USB mode, creates several opportunities where jitter will be added to the
D/A converters master clock:
1) Any variable frequency clock will intrinsically have more jitter than an equivalent fixed-frequency clock.
In the adaptive USB mode, the clock in the D/A converter must adapt to match the rate that the
computer sends out audio packets. In the case of the Burr-Brown PCM270x parts, the master audio
clock comes from a VCO and inherently exhibits high levels of jitter.
2) The computer cannot send audio packets at a perfectly fixed rate. Firstly, the computers internal clock
is not designed to have low jitter. In a market segment where costs are literally shaved to the fractions
of a cent, it would add too much the cost of a computer to use a high-performance, low-jitter clock.
Secondly, the inside of a computer is filled with RF interference, making it impossible for even the best
of clocks to maintain their spectral purity.
3) The Burr-Brown USB-enabled D/A converter chips update (change) the frequency of the master audio
clock each time a packet of audio data is received. This happens once a millisecond so there will be a
strong jitter component at 1 kHz, right in the middle of the audio band. Typical measurements of the
PCM270x chips exhibit jitter levels in the thousands of picoseconds, orders of magnitude worse than a
well-designed one-box player.
The question remainsis there any practical method of using ones personal computer to enjoy the
convenience and power of the computer, yet at the same time achieving true audiophile-level performance?
Therein lies a story.
At the same time that Texas Instruments was developing the PCM270x USB-enabled D/A converter chips,
they also developed an obscure part called the TAS1020B Stereo USB Audio Interface chip. This chip
integrates multiple functions into one parta USB transceiver, a microprocessor, a memory controller with
a FIFO buffer, and an I2S interface to allow easy connection to A/D and D/A converter chips.
Obviously this chip is very powerful and offers great potential to the digital audio designer. The difficulty is
that this chip is a blank slatecompletely unusable unless programmed. The TAS1020B offers the potential
for improved performance, but it is no easy task to write the necessary software code for this part. To give
an idea of the complexity of the chip, the datasheet is no less than 110 pages longand it doesnt even tell
the full story. Let us just say that tackling a project with the TAS1020B is not for the faint of heart.
A third-party company has been certified by Texas Instruments to develop software for the TAS1020B.
This company mainly works with manufacturers in the pro audio business, but also supplies their
software solution to several audiophile-oriented companies. Their code allows high-resolution audio data
(up to 96/24) to be transmitted across the USB port. While this approach offers the advantage of allowing
the transfer of high-resolution audio data, it does little to address the jitter problem inherent in the
PCM270x chips.
Their software averages the timing interval of four audio data packets instead of measuring each individual
packet, but the master audio clock still changes frequency 250 times per second. And unlike the analog PLL
used in the PCM270x parts that can continuously change the frequency of the master audio clock, the
TAS1020B has a frequency synthesizer that can only change in discrete steps, exacerbating the jitter
problem. However, as the D/A chip is now separated from the USB receiver, many audio companies use
some sort of jitter reduction circuitry between the two, often in the form of an ASRC chip. But whatever
approach is used, these circuits can only reduce jitter and not eliminate it.
Clearly a new approach was called for if the full sonic potential for computer-based audio was to be
achieved. Enter one J. Gordon Rankin, Chief Scientist and Owner of Wavelength Audio, best known for his
sweet-sounding amplifiers and preamplifiers using directly-heated single-ended triodes. (He also has a
thriving business supplying custom guitar amplifiers with the same design philosophy to a growing list of
ecstatic customers.)
But before Gordon founded Wavelength, he had a past life nearly 180 degrees in the opposite direction
he was the Chief Engineer at the 6th-largest manufacturer of PCs in the world. While employed there, he
built his vacuum-tube based designs as a sideline. And when he won a Product of the Year award from The
Absolute Sound for one of his amplifiers in 1995, he knew it was time to change careers.
Now the right ingredients existed to take advantage of the performance possibilities of the TAS1020B chip
a serious audiophile design engineer who also had extensive knowledge of computers and programming.
Gordons first approach was to use the expanded memory buffer of the TAS1020B to reduce the jitter
while still using the adaptive USB transfer mode.
Recall that the previous adaptive solutions changed the frequency of the master audio clock at least 250
times per second. In contrast, Gordon wrote software code for the TAS1020B that allowed the chip to calculate
the average data transfer rate of 250 audio packets, and then make minute corrections to a low-jitter,
crystal-based external master audio clock only 4 times per second. The jitter was radically reduced and
naturally the sonic benefits were commensurate. Wavelength began to establish a strong reputation for
their line of USB D/A converter boxes.
As with all successful audio designers, Gordon was not content to rest on his laurels. He continued to
pursue new methods to achieve even higher performance from the TAS1020B chip. The key was to use an
obscure method of USB audio data transmission that was hidden in one of the lengthy USB audio standards
documents. The name for this mode is asynchronous USB data transmission.
The asynchronous part of the name should not be confused with Asynchronous Sample Rate Converters.
Instead, it simply means that the master audio clock in the D/A converter box need not be synchronized
with any of the clocks in the computer. This was the breakthrough that finally allowed for the jitter of an
external D/A converter box to be as low as a one-box disc player.
With the asynchronous mode, the master audio clock is in the D/A converter box, where it belongs. A
buffer in the D/A converter box stores the incoming audio data from the computer and the TAS1020B
controller chip tells the computer to send more audio data as the buffer empties during playback.
It sounds simple, but Gordon spent nearly two years developing the software that allowed the TAS1020B
to work in asynchronous mode with both PCs and Macs, using only the native drivers included in their
standard operating systems. This ensures full compatibility with all music playback programs and connecting
the USB D/A converter becomes simple and easy. Naturally, Gordon took advantage of the TAS1020Bs
capabilities to ensure that it will support all sample rates up to and including 96/24.
The radically lowered jitter of the asynchronous mode was a key ingredient in improving the sonic capabilities
of USB-based computer audio. Wavelengths cutting-edge USB D/A converters using Gordons custom
adaptive operation took a giant leap forward in overall musical performance when he upgraded
them to asynchronous mode.
Ayre is proud to be the first licensee of Wavelength Audios new asynchronous USB technology called
Streamlength software. For the first time ever, Ayre can make an external D/A converter box that has the
critical master audio clock where it belongsright next to the D/A chip itself, and without resorting to a
non-standard interface.