MPEDANCE MATCHING TRANSFORMERS
The best way to understand the Bryston BDA-1 External DAC is to follow the flow of a signal from when it first arrives at the
BDA-1 in digital form to when it leaves to drive an external analog input. The digital signal first arrives at the BDA-1 via either
the SPDIF COAX, OPTICAL OR AES EBU inputs or the USB interface input. These are the standard digital outputs from a CD
Drive, Sound Card, Computer, Music Server etc. There are 6 digital inputs, which are easily selected using a front panel
switch. This digital signal contains data at over 1 million bits per second that requires a bandwidth of 5 to 10 million hertz
(cycles per second). At these high frequencies, it is very important to maintain the quality of the signal by having the
correct termination at the digital inputs. The BDA-1 provides for this termination in the best possible manner using devices
called impedance matching transformers. Impedance matching transformers provide the optimal interface to the incoming
source under all sorts of signal conditions. Lesser quality terminations will degrade the signal, causing increased jitter.
UPSAMPLING
After the input stage, the signal goes to the SYNCHRONOUS upsampling circuit (sample rate converter). This circuit
converts the digital signal from one sample rate and bit depth to another. In the BDA-1, the sample rate is increased from
the input sample frequency (32K, 48K, or 96K upsamples to 192K and 44.1K or 88.2K upsamples to 176.4K). The 16 bits of
depth (the CD standard) is increased to 24 bits. The added 8 bits are filled with placeholder information. This upsampling
process provides a digital signal for later conversion to analog by the Crystal 4398 DAC chip. The upsampling process
doesnt add any new, but does put the data in a form which can better be translated by the DAC as described below.
The advantage of this synchronous upsampling process is improved processing of the upsampled signal by the DAC chip,
which was designed for higher sample rates and bit depths. There is also a noise shaping process implemented where
noise within the audible spectrum is shifted up to frequencies above audible limits. An added advantage of this
upsampling process is that a totally new clock signal is applied, which results in significant jitter reduction.