In recent versions of CSOS Triode and JackOfAll have added upsampling capabilities to Squeezelite (at my request). I wanted to start a thread here for discussing sonic differences people hear between no upsampling and different upsampling parameters.
I'm also going to give some history of my involvement with DAC interpolation filters and why I wanted to do this in the first place. First off this is NOT about bit perfectness, upsampling changes bits! It is supposed to! There is a real measurable difference in the waveforms, this is about how you think those changes affect the sound. First a quick refresher on what this is all about. When a DAC outputs a signal there are "aliases" above the half Nyquist frequency, these aliases can inter-modulate with the audio signals to produce spurious signals in the audio band which ARE audible. The reconstruction filter is there to get rid of these aliases. With redbook source this is difficult, the half Nyquist freequency is just barely above the audio band. In early days this was done with anolog filters, but they really messed up the sound. Then chip makers came up with using digital filters to do this. These are called "interpolation" filters because they add samples in between the original samples. Every DAC chip manufacturer has used a different approach to these filters. The metric of filter quality has been "how much does it attenuate the aliases". At first they could get -60db, then it was -90db, then -120db, these days there is big contest between DAC manufacturers to see how far down below -130 they can get. But do you really need to get down that far? The further down you go the more computations need to take place. It takes more silicon and faster clock speeds to do it. Early on in the process the chip manufacturers realized that implementing the filters using straight forward simple filters such as the infamous "windowed sinc" (I'm NOT going to go into details on that here, look it up if you want the details) would take far too much silicon real estate and drive up the costs of their chips. So some massive investigations started looking into how to get these extremely high alias attenuations without using a pile of silicon. The result has been some very complex digital filters where multiple filter types are cascaded together. These do their job, they attenuate the aliases but they seem to have some side affects. I first noticed this when I started building my own DACs many years ago, NOS DACs were coming into vouge so I decided to make my own. I was startled. They sounded way better in many ways than the traditional ones with the builtin digital filters, BUT they had the aliases which caused other problems, I like to call this "dirty sound". So there was a choice, clean, flat univolved sound, or dirty, alive and exciting sound. Many people that liked the the NOS sound were convinced that digital filters were evil and should be expunged from all things audiophile. This then started one of my infamous journeys of exploration. I'm not going to go into all the details here, but the upshot is that it's NOT digital filers per se that are bad, it is those complex digital filters used in the DAC chips that seem to be the culprit. If the filter function is implemented by a simple filter (such as the infamous windowed sinc) you can get the best of both worlds. Note that I really don't have any idea exactly what it is about these complex digital filters that humans don't like. I have built a DAC with DAC chips that don't include digital filters (1704s) and have used an FPGA to implement simple and complex filters that give roughly the same filter response, and the simple filter sound vastly better. And I really do mean vastly, it is NOT something barely distinguishable. Many people are not going to believe this since they have spent their entire digital audio life listening to music through these complex filters. And NOS DACs don't really count since you are trading off one set of sonic issues for another. Even the DAC chips with several filter curves don't really help to hear this since all their filters are done with the complex filters. So what does upsampling have to do with all this? There are quite a few programs out there that can implement the interpolation filter using simple filters, you can either run these real time or upsample the source files so you don't need to run anything at play time. If you have a NOS DAC you can use these to really hear what a simple filter can do, you don't have to choose between the above listed issues. But even if you don't have a NOS DAC, properly done external upsampling can make a big difference. Many DACs use simpler filters (or NONE!) for the higher sample rates so using external upsampling can frequently give you better sound than filters in the chip. Note this is NOT about the asynchronous sample rate converter (ASRC) chips used in some boxes, these do their job using multiple complex filters that are even worse sounding than the ones in DAC chips. Above I talked about different filter curves, they do make a difference, but the BIG difference is just getting rid of the complex filters, once you do THAT then you can start playing with the other parameters. Particularly if you are doing this with a DAC chip that already has a digital filter (a non NOS DAC) the parameters that give the best sound are going to be different than those for a different DAC. Since you are overlaying the upsampling filter on top of the existing filter the "best" one is going to be different depending on what that original filter is. SoX is a program that does it's upsampling using simple filters AND it supports many different parameters. SB systems have been including SoX for quite some time so it seemed like a good choice for doing upsampling in CSOS. Not only is it good at it, but it has a library that could be embedded directly into Squeezelite which is what Triode has done. JackOfAll has implemented a web interface for specifying these parameters which makes it easy to explore different parameters. In the next post I'll cover what some of these parameters are and what I have found sound good to me with my DACs (which of course may not be the best for YOUR DAC). John S. ------------------------------------------------------------------------ JohnSwenson's Profile: http://forums.slimdevices.com/member.php?userid=5974 View this thread: http://forums.slimdevices.com/showthread.php?t=99088 _______________________________________________ audiophiles mailing list audiophiles@lists.slimdevices.com http://lists.slimdevices.com/mailman/listinfo/audiophiles
