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In Reply to: RE: Don't know. posted by jmlpartners on June 06, 2009 at 12:31:36
I think again your theory is a little too invasive here - and probably not correct.
Gordon Rankin of Wavelength, whom I believe has a PhD in math, is a huge proponent of USB as a transport interface. Have you read his stuff?
I admittedly have not tried my AN DAC with a SP/DIF transport, but digital with the Mac Mini is soo good I really cannot envision that I am experiencing serious jitter. Admittedly, that's neither rigorous nor conclusive!
I have read Gordon’s rants and I have talked with him. He makes his living selling USB DACs. I’ll say no more.
So, you think a Mac Mini and a USB-fed AN DAC is the ne plus ultra. In most aesthetic pursuits there will be plateaus where, “It doesn’t get any better than this!” and then you experience something better. I think most audiophiles have had that experience at least once in their journey.
I am convinced USB audio was conceived with MP3 quality playback in mind. With USB audio, the PC is in total control: So much so the DAC doesn’t even know what the desired sample rate is. Every millisecond the PC sends the DAC a packet that contains all the samples to be converted during the next millisecond. It’s up to the DAC to adjust the sample clock accordingly. According to the USB audio spec, the PC can change the sample rate at any time. In fact, that happens many times a second when the target sample rate is 44.1K. That sample rate is not evenly divisible in 1ms intervals so some packets will have to have an additional sample to make up for the shortfall in previous packets. That’s where the 1ms granularity comes from. Although the chip makers have tried to minimize the effect with different PLL schemes, it still comes down to a VCO phase locked to the 1ms USB heartbeat using a divisor that changes many times per second. Many people don’t hear it just as many people aren’t bothered by the frame-rate flicker of motion pictures. On top of that, there is no error correction and only rudimentary error detection. If the computed CRC doesn’t match the CRC in the packet header the entire packet is discarded and the DAC is mute for the next millisecond. Given the inherent reliability the digital data transmission, it is a rare event, but it does happen.
I suspect you are experiencing a lot of jitter. Even if you know what jitter is you probably don’t know what is sounds like. You have probably heard from 'knowledgeable people' that jitter is bad, yet most audiophiles, given a choice, prefer more jitter rather than less.
If you’ve spent any time around audio forums where DACs are discussed you have heard about asynchronous reclocking. That’s where the ‘high-jitter’ S/PDIF-derived clock is asynchronously reclocked by a much faster, ‘low-jitter’ clock. The proponents claim significant benefit, which they attribute to reduced jitter. But asynchronous reclocking increases jitter. That’s because the time period of every sample clock is extended to the next edge of the reclocking oscillator.
Some years ago, dCS, the purveyor of ultra high-end digital audio components, introduced the $7K+ Verona Master Clock. Not only was this clock more stable and more accurate than any other, it included a unique feature, dither. That’s right, with the flip of a switch, dither was added to the ultra low jitter clock output and the “golden ears” who reviewed it all preferred the dithered clock. According to dCS, “…it is better to add random noise to the clock, continually forcing [the PLL] out of the dead zone (and allowing the PLL to better do its job) than it is to further improve the stability of the clock…” By “dead zone” they are referring the period when the PLL is locked on frequency and further adjustment of the VCO is not necessary to remain in phase with the master clock.
In the three instances I cited, a PLL is forced to constantly adjust the VCO and hunt for the correct lock. The ‘experts’ say the sample clock should be absolutely stable and without jitter, yet audiophiles seem to prefer, or, at least, not complain, when the sample clock is not stable and contains significant jitter, dither, or other variability. That’s why I put no stock in what ‘experts,’ ‘knowledgeable people,’ and ‘audiophiles’ have to say but only rely on my own knowledge and experience.
One comment: Gordon makes many products besides DACs and it's quite clear hat he chose USB interfaces for his DACs over other methods because he considers it superior. You may disagree, but to insinuate he's not an objective authority on the matter isn't very logical IMO.
(The Wavelength DACs are nice... and my AN Kit 2.1C runs circles around the Cosecant at 60% the price.)
On second thought, the positive qualities you attribute to USB are misapplied because you are not really listening to a pure USB transport. The auxiliary circuit board you use is translating the USB to S/PDIF and that is what is driving the AN digital receiver. In other words, the recovered clock is passing through two PLLs. Perhaps the S/PDIF PLL is ameliorating the damage inflicted by the USB PLL and that’s why you don’t hear what I hear.
It was quite some time ago when USB audio devices appeared and I was intrigued. One of the first USB DACs with audiophile pretensions was the Stereo-Link. I have one. Technically, it does a lot of things right. Separate AC power supply, RCA I/O instead of a mini phone plug, and galvanic separation of the PC and audio grounds. Unfortunately, it sounds like a USB DAC. At about the same time Wavelength introduced their first USB DAC and Gordon was making extraordinary claims about it. I contacted him for clarification and he apparently mistook me for an audiophile. What he told me was total BS. After I followed up with technical challenges to his most outrageous claims he clammed up saying his methods were proprietary.
DACs are my passion. I have been working with computers and audio since 1973: Before PCs, before CDs, and before it was even called digital audio. I think I know a little more about the subject than the average audiophile. As I see it, Gordon saw an opportunity to be get into the immerging, ‘audiophile approved,’ digital audio market and took it. As a method for transporting audio samples from one device to another USB offers no technical benefit. In its current form, as you well know, USB doesn’t go above 48K without proprietary chips and drivers and, as I’ve described, it has poor clock stability and data integrity. USB’s only selling point is plug’n’play simplicity/portability and that apparently trumps all other considerations for members of the iPod generation.
Have you listened to an AN DAC fed by a USB transport?
Why would I want to do that? What exactly would I learn that I don’t already know? I have listened to quite a few USB DACs. In fact, from where I am sitting in my office I can see three, ‘audiophile approved’ USB DACs. I don’t like the way USB puts its mark on the recovered music. In that regard, it’s worse then the S/PDIF. If you don’t hear the effects of USB or it doesn’t bother you then enjoy your USB-based digital audio system. Why do you want my approval?
I have an Audio Note 1.2 DAC kit in my audio system. I like it because it doesn’t offend. Aside from audio shows and dealer showrooms, the only other AN DAC I’ve properly auditioned is the 3.1x. After reading rave reviews and glowing owner testimonials regarding the 3.1x I thought it would be a nice addition to my growing DAC collection. I contacted my local AN dealer to arrange a demo. Eventually the demo was arranged but the AN distributor, who supplied the equipment, insisted I listen to the DAC with the AN CDT and the hideously expensive, AN digital cable. In other words, I had to immerse myself the total Audio Note Digital Audio Experience. What a joke. If your AN DAC sounds anything like the 3.1x, you can have it.
Finally, as long as you cling to USB as the savior of all digital audio then you have to accept its limitations. If you want a plastic disk transport then you have to accept its limitations. If you are willing to use the dread, dare I mention its name, S/PDIF from a PC sound card, then there is no reason you can’t supply your DAC with 192K/24. My AN 1.2 DAC is fitted with a CS8415A that actually plays music with 192K/24 input even though the chip is only rated to 96K. Truncating to 18 bits is no big deal because the linearity of most R2R-type DACs falls off after 18 bits. (Purportedly heard from a Burr-Brown engineer – “Question: What the difference between the PCM1702 and the PCM1704? Answer: Four marketing bits.” In case you don’t get the joke, the PCM1702 is a 20-bit DAC and the PCM1704 is a 24-bit DAC.)
Hi JML. I get the impression that your modified AN DAC1.2 sounds better than the factory AN DAC3. If it may not be too presumptuous, may I ask whether there was there anything in particular you did? modified? aside from the higher grade receiver chip. TIA
It's not that my 1.2 sounded better; it's that the 3.1x I heard exhibited a quality I cannot abide. After playing for only 30 seconds it went back in its shipping box never to heard from again. That was very unfortunate because I was expecting great things from AN in that price range. It was especially unfortunate for the dealer who acquired all the gear and drove 140 miles to my place. We spent the afternoon comparing transports, digital cables, and LP vs CD. I could not hear a difference between the $3K AN digital cable and my $0.15 wire. The same with the CDT. On the other hand, LPs were a clear winner.
BTW, I hear the same defect in other DACs that omit a proper reconstruction filter.
I vaguely recall that the AN DAC1 used a low pass LC from the DAC Chip to the grid of the output tube, while the AN DAC3, cascaded transformers. Could that matter?
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