[opendtv] Re: Precision
- From: "Albert Manfredi" <bert22306@xxxxxxxxxxx>
- To: opendtv@xxxxxxxxxxxxx
- Date: Sun, 27 May 2007 16:42:12 -0400
Al Limberg wrote:
"Aliasing" is used to describe what happens in digital video
when it is sampled at a rate less than twice its highest
frequency. The undersampling of the high frequencies
creates spurious low-frequency components. The
phemenon is sometimes referred to as "frequency-spectrum
folding".
In general, in any sampled waveform.
On a spectrum analyzer, a waveform sampled at regular intervals in the time
domain looks like a series of identical frequency spectra in the frequency
domain, recurring at n*sampling frequency, for any integer n from - infinity
to + infinity. (That's why the D/A converter needs a final low-pass filter.
Need to get rid of all those other spectra, which would otherwise become
high frequency noise.)
If you fiddle with the initial low-pass filter in the A/D converter, or with
the sampling frequency, you can see the train of identical frequency spectra
come closer together or move further apart. If you tune the system so that
the sampling frequency is less than twice the max signal frequency, the
train of identical frequency spectra will begin to overlap over one another.
I think that's what "aliasing" means. You see an alias of the frequency
spectrum intruding into the baseband signal frequency spectrum you're
interested in. Frequency spectrum folding describes the same phenomenon.
Oversampling in the A/D or the D/A conversion process is a trick to move the
frequency spectra further apart than they otherwise would have been. Its
purpose is to to make the initial and/or final low-pass filters less
critical. Don't need to be as steep to remove the other spectra, so they
have less of an opportunity to distort the original signal.
Bert
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