Up & Over Sampling... Continued

I agree with Trelja! Craig has kept it all together, adding knowledge and understanding through expert testimony along the way. Great job, Craig! And thanks for a copy of the Madrigal paper!! I love the title: "Upconversion and the Emperor's New Clothes".

Hi Greysquirrel, I hear you loud and clear about the analog output section. I guess I was espousing my thoughts about what I was interpreting from Halverson's e-mail. I definately agree that there is much more than number crunching going on, hence the reason you have great players, lousy players and all points between. I guess I was just curious about these upsampling devices that exist out there and if they would bring a good D/A up to par with a great D/A costing much more $$. I certainly acknowledge the performance will only be achieved when/if all of the devices mate well together. -Tony

Trelja-- thanks for the kind words-- just after the truth. And thanks to Greysquirrel for getting Jeff Kalt's thoughts on this thorny issue-- it was an excellent contribution and really started the "hunt". And it would actually be nice to have that post on this thread. It was really Jeff Kalt, Kevin Halverson, and Madrigal's positon paper that provided the "red meat" for us on this issue, so the biggest thanks goes to them for sharing their knowledge. Unscrutable-- whatever it was, I hope it was pleasurable rather than painful:). Cheers. Craig.

Thanks Craig. I'm at work now, but I'll post the Jeff Kalt e-mail when I get off tommorrow morning. Haven't seen the Madrigal paper posted on their site yet. I think it's a great paper to read for anyone interested. Hold tight. Jordan

Here is the e-mail I sent to Jeff Kalt (Resolution Audio)on 7/28/00 concerning upsampling/oversampling and his reply follows: "I've heard good things about the sound of your CD55 CD player. Most people have attributed the sound of this player to the upsampling you employ. I am confused though. Perhaps you can clear something up for me? I asked this question to several other audio enthusiasts, but haven't really gotten a consensus: what are the differences between upsampling and oversampling? Can you explain the difference or are there differences? My understanding is that upsampling and oversampling are basically the same. By upsampling/oversampling the digital filtering can be more aggressive (outside the audio range), leaving only a gentle analog filter before output. Is this basically correct? Upsampling/Oversampling can't actually create information; the process can only allow more accurate retrieval of what is contained in the 16/44.1 signal. Right? As for oversampling, all delta/sigma (1 bit) type DACS must use oversampling? Are the new 24/96 DACS mostly delta/sigma types or are they ladder DACS? The recent attention to upsampling has me wondering if my understanding of the process is correct? Did mfg's just get better at implementing oversampling techniques to get better sound and needed new marketing jargon to draw interest? I appreciate any help you can provide in explaining this somewhat confusing topic. Thank you, Jordan" Jeff Kalts Reply: "Indeed, there is no technical difference between upsampling and oversampling. The only difference I can discern is in the marketing. Indeed, digital filters can be very aggressive above the audio band without the adverse effects that analog brick-wall filters have. This is possible because of FIR (finite-impulse response) filters, which have constant group delay (zero phase effect vs. frequency). There is no physical realization of an FIR filter in analog. Using FIR digital filters allows the analog filter to be relaxed significantly, because the first "images" are located at much higher frequencies. In our cd55, we use a passive third order filter which is down only 0.2 dB at 20 kHz, yet the rejection of the images at 700 kHz is about 60 dB. And indeed, the digital filters do not create information that may have existed before the mic feed was converted to digital. Some external "upsamplers" may by their nature apply some other filter/eq, but this is independent of the a/d - d/a process. You are also correct regarding the delta-sigma dacs. These dacs are rated for maximum input rate, currently as high as 192 kHz. These converters all run at the output at much higher rates -- typically 12 MHz or thereabouts. The better ones from Analog Devices use extra filter stages when the input rate is lower. Essentially, the dacs run, say, 256x at 44.1 or 48 kHz, 128x at 88.2 or 96, and 64x at 176.4 or 192. This puts the noise modulator heart of the converter at the same frequency regardless of input. The best multi-bits, including the PCM1704, run upwards of 800 kHz, which allows 16x at 44.1 (and 8x at 96, and 4x at 192 input rates). In sum, your perception of "market jargon to draw interest" is dead-on. In addition to preying on the consumer base which generally does not have engineering degrees (and some manufacturers as well), these products offer the opportunity to sneak in digital eqs which will absolutely sound different. Better? That's a different story. Finally, we have just started talking to a dealer in Indiana. If all goes well, I'll pass along the info in a couple of days." Regards, Jeff Kalt Resolution Audio [email protected]