Q&A with Charles Hansen of Ayre Acoustics
Ayre Acoustics makes a complete line of electronics including amplifiers, preamplifiers, integrated amplifiers, disc players, DACs, ADCs and more. Our focus today will be on their digital side and specifically look at issues that affect file-based playback. The idea for this Q&A came about from a series of email exchanges I had with Charlie Hansen, Ayre's Founder and Designer, related to my recent post which contained a list of NOS DACs. You'll see that we touch on this topic and it was my hope that we could come to a better understanding of some of the underlying issues involved in the D/A process in general which might lead to a better understanding of why a list of NOS DACs is about as useful a grouping as four-legged animals when looking for the ideal DAC or pet. I'd like to thank Charlie Hansen for his time and very informative and detailed answers.
Could you give us some history of your involvement in digital component design?
Ayre was founded in 1993. I had a non-compete agreement with Avalon to not make loudspeakers, so we started with electronics. Our first product was a power amp, for no particular reason. We followed that up with a preamp for obvious reasons. We wanted to do a digital product, but the rumors of a new format (eventually named DVD) were flying around, so we decided to wait until that was format available.
"We have always been pushing the envelope, and doing something completely new with every product we build."
When the DVD spec was announced in December of 1996, they had included 96/24 at the last minute, probably at the behest of Pioneer who were huge supporters of high-resolution audio. When I found this out I immediately let Mike Hobson of Classic Records, David Chesky of Chesky Records, Kevin Halverson of Muse Electronics, and Jeff Kalt of Resolution Audio know about it. I knew that we needed a consortium of both hardware and software providers to make it successful. Theta Digital and Mark Levinson Audio (actually their Proceed division) worked on it independently. A year later at the 1998 CES, we all had prototype players and about a half a dozen discs. It was quite the talk of the show that year.
We have always been pushing the envelope, and doing something completely new with every product we build. A list of our digital accomplishments includes:
- World’s first audiophile grade DVD player to provide support for 96/24 audio discs.
- World’s first disc player to provide user-selectable digital filter responses, including “slow roll-off” algorithm with improved transient response.
- World’s only DVD player to provide total galvanic isolation between audio and video circuits.
- World’s first production progressive-scan DVD player.
- World’s first DVD player with 14-bit video DACs.
- Consultant to Analog Devices on bringing 12- and 14-bit video DAC chips to the mass market.
- World’s first audiophile grade CD player to use a computer ROM transport mechanism—joint project with Resolution Audio.
- World’s first CD player to use both “upsampling” and oversampling for a data rate of 1.4112 MHz at 24 bits.
- World’s first audio-only universal stereo player to play all available audio optical disc formats.
- World’s first disc players to provide user-selectable “Minimum Phase” digital filter responses, including both “slow roll-off” algorithm with improved transient response and “apodizing” algorithm for removal of ringing from digital filters used to produce the disc.
- World’s first disc players to implement 16x oversampling in a single-pass path rather than the conventional cascade of 2x filters.
- World’s first solid-state USB D/A converter with asynchronous data transfer control for zero interface-induced jitter.
- World’s first all-format (video and audio) disc player with asynchronous USB audio input.
- World’s first A/D converter with all-discrete, fully-balanced, zero-feedback analog circuitry.
- World’s first A/D converter with moving-average digital low-pass filters to achieve perfect transient response with zero overshoot, pre-ringing, or post-ringing.
In the early days of digital audio, a DAC chip was just a DAC chip. You fed digital data to the input and it would output a current that was proportional to the input data.
"In general, the more bits it has, the better the performance will be."
As time went on, the main demand has been for smaller, cheaper DAC chips with lower power consumption. This is due to the iPod craze. There is still a market for high-performance audio DAC chips, but there are only a handful left. All but one (the Burr-Brown PCM1704) use some form of a delta-sigma design that typically has only one to six bits, and relies on oversampling and noise shaping to attain reasonable performance.
In general, the more bits it has, the better the performance will be. However with a ladder DAC, all of the bits beyond 18 or so are called "marketing bits" as there is no audio-grade ladder that can exceed 18 bits of resolution. For example when Burr-Brown replaced the "20-bit" PCM1702 with the "24-bit" PCM1704, not one single specification changed. The only difference was that you could feed it digital words that were 24 bits long.
A typical modern high-performance DAC chip will include:
a) A data reformatter to change the serial input data into parallel words for conversion. It will also separate out the left and right data, as all modern DAC chips have at least two channels.
b) An equalizer to correct the frequency response of discs that were made with pre-emphasis. This is a relic from the very early days of CD and it is extremely rare to find a disc like this, but this equalizer is required for proper playback of all discs.
c) A digital oversampling filter, most commonly 8x in modern chips.
d) An interpolator to bring the oversampling rate up to the rate of the modulator, which is where the 24 (or whatever) bits are truncated to the 1 to 6 bits of the actual DAC and it's digital feedback loop.
Embarrassingly enough, almost all "interpolators" simply repeat the same data over and over again. For example a common modulator will operate at 64 x Fs (Fs = the sample rate). The 8x oversampling filter gets us half-way there, so then most chips will simply repeat each of those new words 8 times in a row to get 8 x 8 = 64 times oversampling. Not very sophisticated, but remember that cost is the main driver here.
e) The output of the modulator will have between 1 and 6 bits running at (most commonly) 64 times the rate of the original input signal. This is fed to various types of ways to change this into an output voltage or current proportional to the input data. Each different way has its own advantages and disadvantages, and are often proprietary to each chip maker.
f) If it is a current-output DAC chip, you are done. The output is a current and an external current-to-voltage converter must be added by the manufacturer of the final product. 99% of the time, this is done with an op-amp, which is probably the worst way to do it. It looks great on paper, but in the real world, there is no op-amp that can keep up with the high-speed transitions generated by the DAC chip.
g) If it is a voltage-output DAC, it is simpler for the manufacturer of the final product to implement, but it also means that you have turned over one of the most critical elements that contributes to the sound of the unit to the hands of the chip manufacturer. That is not something that I would ever care to do. They don't have audiophiles designing these chips, that is for sure. Instead they will do one of two things:1) Just put an op-amp inside the DAC chip to perform the current-to-voltage conversion. Now you are stuck with whatever op-amp they put in there, with no way to change it or tweak it.
2) Use a switched-capacitor output stage instead of a switched-current source output stage. Switched capacitors are like everything else in life -- a series of trade-offs. In my opinion most of the trade-offs are bad ones when it comes to sound. For one example, just think of how large and expensive an audio grade polystyrene or Teflon capacitor is. Then think of what the capacitor inside an integrated circuit is like, and how it is made, and how it sounds. All I can say is that if they sounded good, high-end audio manufacturers could save a lot of money and space by using those tiny little capacitors as used inside a switched-capacitor DAC chip.