VAC DAC Mk II - Manual (Page 4)

About the VAC DAC
The culmination of three years' development, the VAC DAC one-bit converter is designed to provide
sonic performance comparable with the best analogue LP recordings. With the VAC DAC, a good
transport, and proper cables, many well transferred CDs will actually rival or surpass their LP
counterparts.
In stark contrast to current trends in digital design the VAC contains no microprocessor, no
EPROMs, and no Programmable Logic Arrays. Instead only passive logic (a more expensive
process) is employed, resulting in minimal processing all of which is under the sole control of the
music input signal.
Programmable Logic Arrays and/or EPROM storage techniques are avoided because they are
inherently prone to timing errors and call for the use of a high power microprocessor which produces
wideband noise at odd and varying frequencies quite unrelated to the music. Essentially these devices
are to digital technology what P-mount cartridges were to LPs, less expensive and more convenient
to manufacture but inherently flawed. Nevertheless, VAC has ensured that upgrades may be
accomplished if the future produces any significant sonic improvements.
Only passive logic is used: no processing is performed unless the music itself calls for it. Roughly
speaking this is analogous to hard wiring an analogue stage, keeping the signal path simple and
uncorrupted. The proprietary filtering algorithm completes the accurate reproduction of the music.
The Finite Impulse Response filter pushes the limits of frequency response to the highest possible
heights, to beyond 24 kHz. Very high amplitude resolution is assured by the 128 tap 24 bit capable
design. Jitter has been reduced to the trivial order of picoseconds.
D-A conversion is accomplished using two separate dual channel dacs in a technique we call Direct
Coupled Cross Reference Differential, a form of push-pull that results in a natural cancellation of
noise products and digital processing artifacts. Internal dither signals are handled by a separate bit,
thus avoiding the limitations of previous generations. Stereo processing occupies 34 bits (16 (audio)
+ 1 (dither) x 2 (differential) = 34 bits per channel internal). Separate premium one-bit (Bitstream)
differential dac systems are provided for each channel for the greatest accuracy. In addition, special
techniques maintain tight and total control over the timing of both data and clock signals to the dacs.
The one-bit (Bitstream) system provides performance inherently superior to conventional ladder
dacs. The use of a single reference voltage rather than sixteen (or more) results in an internal
consistency and stability that guarantees superior sound today and tomorrow without the need for
complex recalibration and adjustment. Of course, one-bit requires an all new design, and thus can
not be adapted into the existing ladder dac designs used by most other manufacturers.
Three digital inputs are provided, supporting standard SPDIF input, professional AES-EBU input
(XLR connector optional), and optical. Sampling rates of 32 kHz, 44.1 kHz, and 48 kHz ensure
compatibility with all existing and projected digital transmission and storage systems for music. The
optical input is implemented with a proprietary receiver compatable with the AT&T ODL50
standard. A digital output provides compatability with CD-R and R-DAT recording systems.