First on the term "apodizing": different people have radically different meanings for the term, so it's very difficult to figure out what any body means.
If I remember correctly the original use for the term in regards to audio digital filtering was about a non-brickwall filter applied to higher sample rate signals. Somewhere along the line it got confused with minimum phase filters (I think one box had both apodizing and minimum phase so the press assumed they were the same thing). I have done a fairly extensive set of listening tests both by myself and with others concerning digital filters, both hardware implementations and software implementations. I don't have time to go over all the details right now. The upshot is that the digital filters built into almost all DAC chips are flawed in one way or another. Most of the "digital filters sound bad" sentiment is actually not about pre-ringing etc, the culprit seems to be that the hardware implementations take shortcuts in their implementations in order to both produce good numbers in the spec sheet and meet cost requirements of the chips. Pretty much everybody is assuming these filters are implementing a traditional Sinc function, but in reality they are not. The designers have come up with interesting implementations that that give good numbers but don't take as much chip resources. These shortcuts are what I think are the real issue, not pre-ringing etc. Some results that tend to back this up are: Take a really good NOS DAC feeding something like a 1704, with async USB front end, very low jitter clocks. Listen to it with a DF1704, NOS, digital filter implemented in an FPGA with just basic Sinc response, and software upsampling to 192 again with just basic Sinc. With the DF1704 it sounds flat and uninvolving. Played NOS it sounds way more alive, interesting and musical, BUT it sounds "dirty", you can hear the aliasing going on. With both the FPGA filter and the software filter it keeps the aliveness etc., BUT the "dirtiness" goes away, it's by far the best sounding. This is NOT some just barely noticeable affect, it's actually quite startling when you first hear it. Several DAC chips that contain internal digital filters allow you to bypass them and use your own external filter. I have done the same test with several of these with the same result, internal filter sounds flat, NOS sounds much better but dirty, FPGA or software Sinc sounds wonderful. There is a group of people that are extoling the vitues of software upsampling, but unfortunately most of these people are then feeding this stream into chips that have builtin digital filters implemented with these shortcuts. This severly limits the efectiveness of the software digital filters. Yes some chips support different varients of filters, but they are all implemented with these shortcuts so it makes the inherant differences very difficult to discern. There ARE differences between filters, minimum phase, Sinc etc, but these differences are tiny compared to just getting rid of the hardware filters in the first place. There ARE a few DAC boxes out there that do use FPGA filters instead of the internal filters, but they are few and far between, and usually pretty expensive. John S. ------------------------------------------------------------------------ JohnSwenson's Profile: http://forums.slimdevices.com/member.php?userid=5974 View this thread: http://forums.slimdevices.com/showthread.php?t=96098 _______________________________________________ audiophiles mailing list [email protected] http://lists.slimdevices.com/mailman/listinfo/audiophiles
