16 x Digital Filter
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16X Digital Filter Upgrade for all Platinum DAC / CD Player Products
About the Digital Filter
For the first time ever, we have created our own Custom Digital Filter for the Platinum DACs. Previous versions of the Platinum DAC used the Burr-Brown DF1704, which at the time was the best Digital Filter on the market. Our new Digital Filter is built to our specifications and standards, and is designed to get the most out of the Platinum DAC modules.
This new Digital Filter is available as an user installable upgrade for the Platinum DAC, Platinum DAC Plus, Platinum DAC II, as well as the Reference CD Station II. This filter is standard equipment in our Platinum DAC III and Reference CD Station III.
The performance of this new Digital Filter is amazing. Immediately you'll notice the lack of fuzziness around voices and instruments of all frequencies. This new filter also dramatically increases the resolution and dynamics of the machine. This filter is also completely customizable; By playing a special .wav file on a CD new filter coefficients, either temporary or permanent, can be loaded into our DSP.
The Digital Filter is a 16x oversampling, single stage filter with 32bit input resolution, 80bit computation, and 36bit coefficients.
About the Clock / Jitter Control
Jitter control devices (and inputs on most DACs) normally re-clock the input signal in attempt to lessen the jitter of that incoming signal. The Platinum CD III does no such re-clocking. We actually pay no attention to the clock on the input signal (or the input from the internal drive) . All internal clocks are generated by an extremely accurate +/- 2.5ppm temperature compensated clock. Since the input clock is no longer related to the clock of the Platinum, an intelligent ½ second buffer is used to maintain data synchronization.
The MSB 16x Digital Filter is an ultra high-resolution digital front end for your Platinum DAC or CD Player. The combination of a high-speed onboard computer and carefully optimized software and hardware result in the best possible music quality from your Platinum. During the development of this peerless digital audio system MSB has accumulated a wide body of unique knowledge allowing us to design the ultimate 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 Platinum DAC, we realized that the Platinums 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 final 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 first step requires recovering the audio samples, which represent the final 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 filtered. While it is possible to feed the DAC with the original audio samples thereby avoiding the use of a digital filter skipping this step has many unintended consequences. After being digitally filtered 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 filter is necessary because mirrored image frequencies created during the conversion process must be removed. If the DAC did not have a digital filter, an analog filter with an aggressive response must remove these image frequencies. These brick wall analog filters 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 filter designs consist of cascaded FIR (Finite Impulse Response) filters, each of which raise the sample rate by two. The intermediate data between the filters is usually stored at less than 40bit resolution. Since the next filter works with previously computed data the resolution decreases with each filter pass. This limits higher quality digital filters 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 filters with standard DAC chips, but in combination with our high resolution 24bit Second Generation, or Signature DACs the problem is very apparent. The sound becomes muddy, veiled and un-involving when using any off the shelf digital filter. To counter this problem the MSB 16x Digital Filter does it's filtering in one filter stage that raises the sampling rate by 16.
FIR filters operate by multiplying each sample in the data by a set of filter coefficients and then summing the result. Most digital filters round the result of each addition before the adding next sample. This repeated roundoff results in a similar problem to the cascaded 2x filter approach, muddy sound. MSBs digital filter uses bit perfect accumulation in an 80bit accumulator completely eliminating these debilitating round-off errors. Only as the last step do we carefully convert the audio to the 24bits 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 24bit effective resolution.
Through extensive listening tests we have found that the choice of filter coefficients has a great impact on the tone of the music. We have found that steep, phase perfect "Brick Wall" filters tend to sound the most neutral but are also the most difficult to implement without problems. Improvements we have made in our digital filter, with its single stage design and 80bit computation, allow us to use very steep filters with no compromises.
We recognize however that different people often have very different taste in their music. For those who do not like the standard choice of filter, or require a different filter response, we have provided a very easy method for customizing your MSB 16x Digital Filter. By playing a standard CD containing a custom filter algorithm you can temporarily change the filter inside your 16x Digital Filter. This CD filter loading allows you to load almost any FIR filter 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.
The timing of the DACs conversion clock also has a large impact on sound quality. Any periodic frequency change or "jitter" causes a loss of focus in the reproduced audio. The high power and large memory space of the SHARC DSP we use allows us to offer a unique Ultra Clock upgrade to your MSB 16x Digital Filter. The Ultra Clock upgrade adds two ultra precision, ultra low jitter TCXO oscillators and custom clock handling circuitry to your board. This custom circuitry allows us to do much more than simply reclock the data, as other jitter reduction boxes do, but completely replace the incoming clock!
Most jitter reduction schemes rely on Phase Locked Loops or Asynchronous Sample Rate Converters. The Phase Locked Loop is able to reduce jitter to a significant degree but not eliminate it because the output clock must be related to the input clock. The Asynchronous Sample Rate Converter, which completely replaces the data and clock, seems ideal until you realize that the complex computation involved harms the data by permanently imbedding a portion of the clock jitter into the data. Sample Rate conversion is also detrimental to the sound due to the complex and sub optimal digital filters involved, which must change their filter response to track minute changes in the input to output clock ratio.
The Ultra Clock upgrade sidesteps both of these problems by storing the incoming data in an extremely large buffer (1/2 second of audio.) The onboard clock then reads out the data without jitter. The onboard clocks are totally independent from the input clock so that no jitter is transferred to the output. True absolute digital silences such as a track skips are selectively shortened or lengthened by the DSP to keep the buffer synchronized. This process is carried out seamlessly by the DSP to assure that all compatible sample rates have their clocks properly replaced. Compatible sample rates are 44.1Khz, 96Khz, 176.4Khz, 192Khz, and 384Khz. Other sample rates automatically use the recovered clock from the transport without user intervention. The 48Khz sample rate does not support clock replacement due to the large latency incurred by the buffer, which would cause audio from a DVD movie to be unsynchronized.
Custom Software and Temporary Filters:
A custom digital filter will allow you to tailor the sound of your Platinum. Any form of custom FIR filter may be loaded into the MSB 16x Digital Filter. You may select a custom filter from our library, you may have MSB design a filter to your specifications or you may send MSB a list of Fir Filter coefficients that we can format into a CD Loadable file for you. You cannot permanently damage your 16x Digital Filter Board by loading a Temporary Custom Filter, it is only active until you toggle the Filter Option Button, Power down the Platinum or play an audio track containing the Special Cancellation Code. Contact MSB Technology for details on custom filters.