[opendtv] Re: Precision

'aliasing' before digital simply meant that when combining two video signals
(analog audio aliasing?) and getting a "alias" video image where the two
meet. Just look at newscasts from the 1970's -- 'halos' around the heads of
less-than-angelic anchors.

Character generators were also 'good' for aliasing.

It also led to audio ringing, because at least a few dots of the alias could
be counted on to be outside NTSC specs -- they are unintentional -- and, if
were as much or more so white than permitted in NTSC, they would result in
buzzing in the audio.  

So, to constrain aliasing to the digital domain is to ignore much of what
has been learned about aliasing and how to deal with it.  (There were many
analog solutions, but none as cost-efficient or effective as we now have
with digital.

I can't remember when I last saw aliasing in a contemporaneous program. Must
have been the late 1970's.

John Willkie

-----Mensaje original-----
De: opendtv-bounce@xxxxxxxxxxxxx [mailto:opendtv-bounce@xxxxxxxxxxxxx] En
nombre de Albert Manfredi
Enviado el: Sunday, May 27, 2007 1:42 PM
Para: opendtv@xxxxxxxxxxxxx
Asunto: [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_migratio
n_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.


 
 
----------------------------------------------------------------------
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: