M2Tech Vaughan – a white paper
The Vaughan 384/32 DAC is an innovative product with many distinctive features which are worth
to be explored in detail.
The philosophy
To say that the Vaughan is a no-compromise design would be unfair. A real no-compromise design
would necessarily be ridiculously expensive and would imply certain features which not always
meet the user’s expectations, all in sake of the “absolute sound”. No, the Vaughan is a design in
which the most important aspects (with regards to the sound) have been addressed with a reasonable
amount of effort and budget share. They are: the integrity of the processing chain, the quality of the
supplies, the quality of timings with regards to the jitter and the interfacing capabilities. Of course,
an expensive product must also give users the versatility and ease of use they expect for the price.
We’ll see in the text which follows as various choices impacted on the important aspects listed
above, as well as fulfilling the owners’ expectations.
High-resolution processing chain
Even if it is nice to think of a DAC as a digital cable directly connected from its input to the sigma-
delta modulators of the conversion IC, things are rather different. Converting a digital signal into its
analog counterpart means transferring the digital data from the source (be it a computer, a disc or a
network interface) to the DAC, processing it in terms of volume, bandwidth, noise shaping, etc.,
then increasing its original sampling frequency to a much higher one (by either asynchronous
sampling rate conversion or oversampling) to drive the output sigma-delta modulators which
produce the analog output current to feed the I-V converter prior the analog buffer/low pass filter.
Every computation increases the need for bits to represent the various results without losing
resolution. Using a word length less than the necessary results in truncation of the data, which in
turn results in distortion.
Truncation effects are magnified by successive processing and truncations. It’s advisable to avoid
any truncation prior the final data delivery to the modulators. This is what we do in the Vaughan:
the 32-bit input signal (either filled with 24-bit data from S/PDIF inputs or with 32-bit data from
USB and I
2
S) is sent to a processing core which is responsible for volume control and
oversampling. This processing core uses all the bits which are necessary to carry out all
computations without truncations in between. The output of the processing core is ready to be
delivered to the output modulators, which work with 32 bits. The distortion increase due to this
final truncation is well buried below the thermal noise floor of the modulators, so it’s not audible.
The volume control is worth a specific comment. Many so-called experts claim that post-dithering
noise shaping must be used to avoid the signal-to-noise reduction implied in volume control. But
they refer to the past, when 16 bits was the bit depth available and reducing the listening level by a
mere 18db meant losing 3 bits of resolution. Today, we operate with a system in which the ENOB
(equivalent number of bits) due to the thermal noise is 21 bits, while the available bit depth at the
modulators is 32 bits. This means that it is possible to attenuate the signal down to -68dB before
suffering any distortion due to the truncation. Not only: when listening to a 24-bits signal, no
resolution is lost down to -50dB attenuation. The volume control in the Vaughan doesn’t use any
dithering nor noise shaping, simply because it’s not necessary: at -50dB attenuation, the resolution
is still higher than that allowed by most phono stages.
Oversampling vs. asynchronous sample rate conversion
The Vaughan performs oversampling to increase the input signal’s sampling frequency to the value
to which the output modulators operate. This value can be 705.6kHz or 768kHz depending on the
input sampling frequency. Oversampling is a processing that, provided the sufficient bit-depth is
available, doesn’t adversely affect the sound quality.
(10 pages)
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