Specifications

4
More About the Digital Filter
One of our primary goals at MSB is to provide the music lover with the most accurate musical experience possible. During years of
careful design and improvement of our custom discrete DACs, which form the heart of your Power DAC, we realized that the Power
DACs sound quality was no longer limited by them. We soon narrowed the problem to the Digital Filter which was feeding our DACs.
While the excellent Burr-Brown (Now owned by Texas Instruments)
DF1704 Digital Filter had served us well in the past, it had became
the bottleneck once we started using our new Second Generation
DAC modules. After a thorough search of all the available off the
shelf and custom DSP based Digital Filters we realized that little
improvement could be had from any of them. With no other option
in sight we decided to build our own solution.
Converting the ones and zeros of Digital Audio into music is an
enormously delicate and critical process. Each individual sample
that makes up the audio stream must be converted into the high
resolution, continuous analog voltage that can be transformed into
the sound that you hear. Any misstep can corrupt the nal result
ending with audio that does not sound anything like the original
recording. Errors in translation can make a harsh, veiled, muddy,
and/or tonally colored result. Minimizing each potential problem
allows the original recording to shine through.
Audio reproduction starts when the DAC receives the binary coded
information from the source. The rst step requires recovering the
audio samples, which represent the nal output voltages, and the timing, which tells the DAC when to output those voltages. Next the
sample rate is raised and the data is digitally ltered. While it is possible to feed the DAC with the original audio samples thereby avoid-
ing the use of a digital lter skipping this step has many unintended consequences. After being digitally ltered the digital stream is feed
to the DAC. The DAC receives the digital audio samples and converts them into a continuous analog voltage. The best DACs, such as
our Second Generation DAC Modules or Signature DAC Modules, instantly convert the data into a precise continuous voltage waveform
with timing determined by the DACs conversion clock.
The digital lter is necessary because mirrored image frequencies created during the conversion process must be removed. If the DAC
did not have a digital lter, an analog lter with an aggressive response must remove these image frequencies. These brick wall analog
lters seriously damage the signal by corrupting the original phase of the sound and cannot fully remove the high frequency images. This
results in harsh or rolled off high frequencies and poor soundstage focus.
Traditional digital lter designs consist of cascaded FIR (Finite Impulse Response) lters, each of which raise the sample rate by two.
The intermediate data between the lters is usually stored at less than 40 bit resolution. Since the next lter works with previously com-
puted data the resolution decreases with each lter pass. This limits higher quality digital lters to a low oversampling rate (usually 8x)
before the output starts to deteriorate. The loss in resolution is typically not apparent when using the best conventional digital lters with
standard DAC chips, but in combination with our high resolution 24 bit Second Generation, or Signature DACs the problem is very ap-
parent. The sound becomes muddy, veiled and un-involving when using any off the shelf digital lter. To counter this problem the MSB
16x Digital Filter does it’s ltering in one lter stage that raises the sampling rate by 16.
FIR lters operate by multiplying each sample in the data by a set of lter coefcients and then summing the result. Most digital lters
round the result of each addition before the adding next sample. This repeated roundoff results in a similar problem to the cascaded 2x
lter approach, muddy sound. MSBs digital lter uses bit perfect accumulation in an 80 bit accumulator completely eliminating these
debilitating roundoff errors. Only as the last step do we carefully convert the audio to the 24 bits our DACs require. The high sampling
rate of the output allows us to include advanced ultrasonic dither and noise shaping techniques in this step to achieve greater than 24 bit
effective resolution.
Through extensive listening tests we have found that the choice of lter coefcients has a great impact on the tone of the music. We have
found that steep, phase perfect “Brick Wall” lters tend to sound the most neutral but are also the most difcult to implement without
problems. Improvements we have made in our digital lter, with its single stage design and 80 bit computation, allow us to use very
steep lters with no compromises.
Custom Filter Options
We recognize that different people often have very different taste in their music. For those who do not like the standard choice of lter,
or require a different lter response, we have provided a very easy method for customizing your MSB 16x Digital Filter. By playing
a standard CD containing a custom lter algorithm you can temporarily change the lter inside your 16x Digital Filter. This CD lter
loading allows you to load almost any FIR lter quickly and easily. You may also upgrade your MSB 16x Digital Filter with the latest
custom software just by playing a special Firmware Upgrade CD.