Hi,
Some stuff you might like to look over. These were originally written by the designer of the Schiit DACs - Mike Moffat
What is this
magic filter
An SOF (Schiit only feature) The Schiit Footlong Mega Burrito Supersauce Digital Filter:
It is a digital filter/sample rate converter designed to convert all audio to 352.8 or 396KHz sample rates so that it may drive our DACs. You get it from us; it is our filter. It keeps all original samples; those samples contain rudimentary frequency and phase information which can be optimized not only in the time domain but in the frequency domain.
So
what does it do ?
The below are the claims of the Digital Filter/Interpolator/Sample Rate Converter in the Yggy:
1. The filter is absolutely proprietary.
2. The development tools and coefficient calculator to derive the above filters are also proprietary.
3. The math involved in developing the filter and calculating has a closed form solution. It is not an approximation, as all other filters I have studied (most, if not all of them). Therefore, all of the original samples are output. This could be referred to fairly as bit perfect; what comes in goes out.
4. Oversimplified, however essentially correct: The filter is also time domain optimized which means the phase info in the original samples are averaged in the time domain with the filter generated interpolated samples to for corrected minimum phase shift as a function of frequency from DC to the percentage of nyquist - in our case .968. Time domain is well defined at DC - the playback device behaves as a window fan at DC - it either blows (in phase) or sucks (out). It is our time domain optimization that gives the uncanny sonic hologram that only Thetas and Yggys do. (It also allows the filter to disappear. Has to be heard to understand.) Since lower frequency wavelengths are measured in tens of feet, placement in image gets increasingly wrong as a function of decreasing frequency in non time domain optimized recordings - these keep the listener's ability to hear the venue - not to mention the sum of all of the phase errors in the microphones, mixing boards, eq, etc on the record side. An absolute phase switch is of little to no value in a non time domain optimized, stochastic time domain replay system. It makes a huge difference with an Yggy
5. This is combined with a frequency domain optimization which does not otherwise affect the phase optimization. The 0.968 of nyquist also gives us a small advantage that none of the off-the shelf FIR filters (0.907) provide: frequency response out to 21.344KHz, 42.688KHz, 85.3776KHz, and 170.5772KHz bandwidth for native 1,2,4, and 8x 44.1KHz SR multiple recordings - the 48KHz table is 23.232, 46.464, 92.868, and 185.856KHz respectively for 1,2,4, and 8x. This was the portion of the filter that had the divide by zero problem which John Lediaev worked out in 1983, to combine with #4 above AND retain the original samples.
This is what the competition offers:
5. Frequency domain optimization FIR filters with Parks-McClellan optimization. The development tools for these types of filters can be downloaded for a price range of free to $300 on the internet. Parks-McClellan is the goto filter optimization for audio design. These filters are derived with no closed form math; only successive approximation. The original samples are lost. The output is approximated. An educated guess. This optimization is ubiquitous in the front end of delta sigma dacs as well as standalone digital filters. While there is no inherent phase shift within Parks-McClellan filters, there is no optimization of phase either. The listener is left with what remains from the mixing boards, transducers, brick-wall filters, etc which can and usually do destroy proper phase/position information. Finally, it is processor efficient and economical to implement. Read cheap.
Any avoidance of the Parks-McClellan pablum requires a lot of original DSP work. Am I a prophet who received the tablets from God or some other high-end audio drivel. Hell, no. I was the producer and director of this project and worked with Dave Kerstetter (hardware-software), John Lediaev (Math), Tom Lippiat (DSP Code), Warren Goldman (Coefficient Generator and development tools) for a total of 15 or so man years. These folks either taught math at The University of Iowa, Computer Science at Carnegie-Mellon University, worked at think tanks like the Rand Corporation you get the idea. We did this for no money - What we all had in common was that we loved audio. All other audio pros were interested in Parks-McClellan and pointed and laughed at us. That's the way it happened.
It was worth it, every hour, day, and year. So go for it if you want. For what it is, it is not a lot of money.
ciao
gr
