JohnSwenson;699117 Wrote: > My objection to ASRC is that the current implementations out there don't > seem to sound good to me. I don't have an objection to the concept, but > I haven't heard an actual implementation that I like. My guess is that > it's the issue I have with almost all digital filters in commercial > audio chips in general: they seem to have compromised implementations, > my guess is in order to cut costs in the chips. > > So far I have not heard a delta-sigma DAC chip that can beat a 1704, > BUT if done properly they can come quite close. The big issue is to > bypass the internal digital filter and instead use a properly > implemented digital filter. > > For example I have taken a 1794 (or was it the 1792, I don't remember > right now) and fed it from an FPGA, I could either send it I2S and use > its own digital filter or put my own digital filter in the FPGA and > bypass the internal filter. Note this filter was a straight forward > SINC brickwall filter, nothing fancy. The FPGA filter sounded much > better. I did this listening with a bunch of people under blind > conditions and everyone prefered the FPGA filter. So even though both > filters were supposed to be doing the same thing something was > different. > > Two obvious possibilities for the difference: the internal filter is > somehow not doing what it should, or the extra processing of the > internal filter is generating more noise on the internal PS and ground > traces causing noise which is messing up the sound. > > I tested the second possibility by putting the DAC chip in external > mode and using a DF1704 digital filter instead of the FPGA filter, it > sounded almost identical to the internal filter. This tends to rule out > the extra processing going on in the DAC chip as the culprit. This > leaves the implementation of the digital filter itself as a prime > suspect. > > I am guessing that the designers of these chips cut corners in the > implementation of the digital filters. An implementation of the proper > filter for 44.1 sample rate takes a fair amount of horsepower, which > can lead to a chip costing more than the management prefers. Thus the > designers are under preasure to figure out how to cut corners to > decrease cost, but still meet the specifications. I know from > experience that this is quite common in the industry, I have personally > worked on quite a few DSP chips (mostly for image processing) and there > has always been preasure from management to figure out how to make > things "good enough" to keep costs down. > > Another telltale sign is the graphs that the manufacturers publish for > their filter functions: they don't look anything like what a simple > SINC filter should look like. They are doing something else, I'm not > exactly sure what, I'm not good enough at DSP theory to reverse > engineer the hardware from the graphs, but I do know its not a simple > SINC. > > When I do a simple SINC in an FPGA it sounds much better, but does take > a lot of resources, so my guess is that the designers of these chips are > getting fancy, using something other than a simple SINC in order to get > the spec sheet numbers they are after and still fall within the cost > parameters the management wants, and somehow this does not sound as > good. > > And no I have not done this with every DAC chip on the planet, but I > have done it with several. In order to do my test it has to be a chip > that allows you to turn off the internal filter and use an external > one. Many DAC chips do not allow this. But I have done enough of this > and heard the difference between my simple filter and what is inside > the chips to get a good feeling for difference in sound. I have > listened to quite a few of the chips that don't let you disable the > internal filter and I hear a similar sound to what I was getting from > the chips where I could use an external filter. > > So my conclusion is that this compromise is pretty pervasive in the > industry. > > I guess all this is a very long winded way of saying that yes a I think > you can get very good results out of a delta sigma DAC chip, you just > need one that lets you use an external digital filter. > > And BTW I think this is also a major reason for the NOS movement. > Going back to old DAC chips without digital filters seems to let > parts of the music "through" which normally somehow get messed up by > the internal digital filters. Of course then you have aliasing all over > the place. For some people its worth the tradeoff. > > Of course the proper solution is to use proper digital filters so you > can have the best of both. > > John S. Thanks John
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