Klas Strandberg <> wrote:
...
> Even if all of them were described as professional converters,
some of them
> were audibly "scratchy". Others just sounded "unpleasant",
compared to the
> analog.
>
> What about the A/D converters used in MD's and DAT's today?? And
worst,
> perhaps - what about A/D converters used with PC sound cards?? Are
they all
> perfect?
Hi Klas,
A/D and D/A converters are of course not perfect.
One important aspect is the integrated anti-aliasing filter. As you
probably know, if there are very high-frequency components in the
sound to be recorded, the signals will be mirrored around the
nyquist frequency (half the sampling frequency). For example, if
there were a strong signal at 24 kHz and the signal were sampled at
44.1 kHz, that 24 kHz signal would appear in the recorded soundfile
at 20.1 kHz. Thanks to the anti-aliasing filter, this unwanted
signal will be attenuated to some extend, but sometimes it is not
removed completely (because 24 kHz is too close to the cutoff
frequency of the anti-aliasing filter. Usually, this would be no
problem at all, because we can not hear that irregular signal at
20.1 kHz.
However, when playing that digital recording back through a
loudspeaker, that high-frequency signal may intermodulate with other
audible signals present in the recording. The resulting
intermodulation products may occur in the audible range between 1
and 5 kHz. This effect is called Aliasing Intermodulation Distortion
(AID). That specific effect could be reduced by applying a digital
low-pass filter (with a cut-off frequency of about 18 kHz) prior to
replay.
There are other effects that may degenerate a digital recording.
Among them is digital noise entering the analog section of the pre-
amplifier and the A/D converter. That digital noise may either
originate from other digital components of the recorder (or from the
computer where the soundcard is installed in) or even from the sharp
transients at the digital output lines of the A/D converter itself.
The manufacturer has to make sure, that these effects will be
minimized.
Best regards,
Raimund
________________________________________________________________________
________________________________________________________________________
Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
|