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The Bryston DAC chip we use is a hybrid multi-bit
delta sigma DAC (digital to analogue converter) device. Before the
digital data reaches the delta-sigma modulator, it is run through an
8x over-sampling process and digital interpolation filter. Due to
the inherent over-sampling process of a delta- sigma modulator
architecture, the overall interpolation ratio of the device is 128
times. Over-sampling and up-sampling have some great benefits when
implemented correctly.
There is a significant difference between up-sampling
and oversampling. Over-sampling is when the samples are repeated
(2x, 4x, 8x, etc.) to create a new sampling frequency. The new
samples are then generally run through an interpolation filter to
create a more analog-like waveform.
Up-sampling usually refers to a mathematical process
in which the new sample rate is not a multiple of the old sample
rate, and the numbers have to be calculated in real-time. This is
what is happening when a sample rate converter chip is in an
up-convert mode. So in the BP-26 DA, if a 44.1 kHz signal is
present: we up-sample to 96kHz (with the Sample Rate Converter) and
then the DAC will internally 8x over-sample that signal. The great
benefit of both of these methods is that it relaxes the requirements
of the analog filter after the DAC.
This is important because it allows designers to implement higher
cutoff frequencies with slower roll-offs, which results in a phase
linear circuit in the audio band.
The up-sampling method, when implemented correctly,
also has the side effect of jitter reduction, since the output clock
is usually asynchronous from the input clock. Another thing to note
is that if the over-sampling or up-sampling processes are
implemented poorly, they can actually introduce jitter. This is
usually a result of poor power supply implementation.
So, in the Bryston DAC, each stage in the digital chain (input
receiver > sample rate converter > digital audio converter) is
independently regulated to prevent any interactions and to provide
rock solid power supplies for any up-sampling/over sampling process.
All of this being said the most critical part of the
DAC circuit in the Bryston Preamplifiers and Bryston Integrated
Amplifiers is that the output from the D to A is directly connected
to a pair of Bryston proprietary Class A Discrete Operational
Amplifiers rather than the typical IC chips employed in most other
products. This makes a huge difference in resolution and dynamic
headroom performance.
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