[mythtv-users] Interlacing [was: Pvr250 issues - any suggestions?]

[Nick Craig-Wood]

On Mon, Apr 19, 2004 at 07:57:21AM -0400, papenfuss at juneau.me.vt.edu wrote:
> > >I find this whole interlacing business very annoying!  Interlace was a
> > >technical trick from the start of television to reduce the bandwidth
> > >requirements - we really don't need it any more!
> > 
> > Yeah legacy sucks :-)
> > 
> 	Actually, unless you want your cable and broadcast TV to have half as 
> many channels, half as many scanlines, or a 30 Hz refresh, it's still 
> necessary... :)  I wouldn't say interlacing is a trick so much as a requirement 
> for analog video to make the bandwidth requirements reasonable.  The clever 
> technology in color television is very impressive... fit 3 channels of video 
> into 1/2 the bandwidth of a 15kHz x 60Hz B&W video signal.  Pretty clever... 
> and the interlacing was a simple factor of 2 reduction that didn't affect human 
> perception much at all.

What interlacing really buys you is double the flicker rate.  24 fps
is quite fast enough as a frame rate (its good enough for the movies),
but at the cinema each frame is shown to you 3 times, giving a flicker
rate of 72 Hz.  People need a refresh rate of > 50 Hz in order to not
percieve the flicker.  For some people even 50 Hz is too low, hence
the cinema being 72 Hz, and 100 Hz TVs etc.

If TV were broadcast at 30fps at 720x480 resolution, but each frame
was shown 3 times (non-interlaces) at 90 Hz, everyone would think TV
was amazing compared to what it looks like now.  And remarkably this
would take exactly the same bandwidth to transmit as current TV
signals.  What it would require (and which wasn't available to the TV
pioneers) is a frame buffer to store the frame and show it 3 times.
This is easy now-a-days though.

-- 
Nick Craig-Wood
ncw1 at axis.demon.co.uk