[wado1942]

The text, in my understanding, makes no reference to quantization noise (i.e. quantization distortion) whatsoever.

There generally isn't any dither in a 24-bit signal. The principle works simply like this... The converters will always exibit a certain amount of noise/distortion. By doubling the sample rate, the distortion is spreak over double the bandwidth, which is partially above the human hearing range. So it simply SOUNDS like there's less distortion. When dither comes into the mix, you can actually shape the noise so that there's virtually nothing in the audible band so this rule doesn't exactly apply to dither.

I remember reading in one of Dan Lavry's white papers that many converters in fact actually exhibit less usable dynamic range at higher sample rates. Can anyone confirm this?

I can. I've tested several ADCs at different rates for my book. It all depends on the clock they use. A good ADC uses a fixed crystal clock to drive the sampling so it always runs at the same rate. To get different output sample rates, it has an internal SRC to change the sample rate to whatever you want. These kinds of converters exibit best performance at their highest output sample rates. A lot of ADCs use voltage controlled oscillators similar to the ones used in analogue synthesizers to create the clocks. Those converters use fixed DSP for all filtering etc and the actual clock itself if varied to get different sample rates. The oscillators have a certain amount of instability and the harder you push them, the more obvious it becomes. If you double the sample rate, the jitter, which in actuallity is constant, has double the influence on the signal. I can't get into the physics of it very well because I'm just learing about it myself. But one of the converters I tested yelded a 91dB S/N ratio at 24/96 but 94dB at 24/48. At 22.05K, you guessed it, 98dB. Another ADC I tested yielded 120dB S/N ratio regardless of the sample rate but SOUNDED cleaner at 96KHz, suggesting it used a fixed clock.