Originally Posted by mgoodman
Guys - there is tons of jitter on the USB cable! It's not even funny - If you were to take the samples arriving from the USB cable and feed them straight to the DAC chip, it would sound mushy and dull with extremely low level of detail (read: high jitter).
Adaptive or asynchronous - what comes in from the USB cable needs to be cleaned up. Period. If you know how to clean it up, it doesn't matter which approach you use. If you don't, your equipment will not sound transparent. That's the end of the technical argument.
The rest is marketing from companies trying to position themselves in a better light through consumer deception disguised as technical expertise. Deception never works for a long time - eventually facts take over.
I strongly agree with the first and third paragraphs that Mr. Goodman wrote.
However the second paragraph is simply incorrect (somewhat ironic given the content of the third paragraph). The truth is that, if all else is equal, it is absolutely impossible to build a variable-frequency clock (as used in an adaptive mode USB DAC) with as low a jitter level as a fixed-frequency clock (as used in an asynchronous mode USB DAC).
Generally speaking, the more money you throw at a technology, the higher the performance you can achieve. And it is possible to get high levels of performance from variable-frequency clocks (generally using a Phase-Locked-Loop or PLL). But not as high as you can get with fixed frequency clocks. That is a fact. Mr. Goodman is peddling deception.
Sure, it is possible to make a really good variable-frequency clock that has lower jitter than a fixed-frequency clock. But only by spending a great deal more money or if the fixed-frequency clock is an extremely poor design. Engineers have worked for decades to reduce the jitter of variable-frequency clocks. High-performance ones tend to be complex and expensive. In the meantime, fixed-frequency clocks are simple to design and build. Even the simplest, most inexpensive fixed frequency clock will have lower jitter than an excellent variable-frequency clock as long as you feed it with a nice quiet power supply.
As Mr. Goodman pointed out in the first paragraph, the incoming USB signal has high levels of jitter. A typical adaptive clocking scheme is to use a PLL, which will filter out some of the jitter. More sophisticated (and complex and expensive) systems will use two PLLs to achieve higher levels of jitter filtering. But with asynchronous, the incoming jitter on the USB line is completely immaterial. The only thing that matters is the quality of the fixed-frequency clock, which as noted previously is practically a trivial thing to design.
I would be happy to compare phase-noise plots of our fixed-frequency clock with Mr. Goodman's variable-frequency clock any time. (A phase noise plot is the most sensitive way to measure the jitter of a high-frequency clock.)