[opendtv] Re: Precision

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

_________________________________________________________________
Like the way Microsoft Office Outlook works? You?ll love Windows Live Hotmail. http://imagine-windowslive.com/hotmail/?locale=en-us&ocid=TXT_TAGHM_migration_HM_mini_outlook_0507 



----------------------------------------------------------------------
You can UNSUBSCRIBE from the OpenDTV list in two ways:
- Using the UNSUBSCRIBE command in your user configuration settings at FreeLists.org 
- By sending a message to: opendtv-request@xxxxxxxxxxxxx with the word 
unsubscribe in the subject line.

Other related posts: