Dithering the 24 bit data to 16 bits of course... That a signal which is
correctly dithered to 24 bit is undithered with respect to 16 bits be
obvious.

"Karl Uppiano" wrote in message
news:5ELFb.456764$275.1339926@attbi_s53...
"Bruno Putzeys" wrote in message
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I like to put it as "a correctly dithered converter has an infinite
number
of bits in resolution"
If you take a perfectly linear 16-bit converter, you can feed it a 24
bit
signal and through dithering reproduce it faithfully.
If the 24 bit signal is correctly dithered, the system resolution is
even
higher.

I'm not sure if I completely agree. Or maybe I don't completely
understand.
It's all a bit fuzzy (pun partially intended) when you start talking about
dither. The behavior is much more like analog in a lot of ways. Dither
does
allow the reproduction (preservation might be a better word) of signals
below the implied resolution of an un-dithered converter. But there is a
danger: If you feed a 24-bit signal -- dithered for 24 bits -- into a
16-bit
D/A converter, you will end up with a raw, un-dithered signal. Any time
you
re-quantize a digital signal it also needs to be re-dithered for the new
quantization format. If you go from 16 bits to 24 bits, it's OK, you'll
have
the original 16-bit dithered signal. You won't get 24-bit performance, but
it won't be any worse than a properly dithered 16-bit recording.

Noise is a different affair. If it's decorrelated it's noise. Any
current
24bit converter has noise above 20 bits.

"Marketing bits" have always been with us. For various reasons, most
converters don't deliver performance equal to the number of bits claimed.
A
lot of the time, it's due to layout problems on the circuit board allowing
digital signals to be induced into critical analog circuitry. Mind you,
the
noise is down more than 90dB or so, which is *excellent* by analog
standards. If it's correlated noise though, it's much more audible than
random noise, because the energy is concentrated at a single frequency or
group of frequencies, so the amplitude is higher. Noise is usually
measured
as a weighted average, rendering overly optimistic numbers in cases like
this.

About ten years ago, I was at a design seminar put on by Crystal
Semiconductor, promoting one of their new 24-bit sigma-delta converters
(for
laboratory data acquisition, not audio). In that application, they were
concerned about DC offsets, which, at 24-bit resolution, could be caused
simply by the thermocouple effects of a simple solder joint!