[opendtv] Re: Digital vs. Analog Quality

  • From: Craig Birkmaier <craig@xxxxxxxxx>
  • To: opendtv@xxxxxxxxxxxxx
  • Date: Tue, 8 Jul 2008 05:04:09 -0400

At 1:00 PM -0400 7/7/08, Cliff Benham wrote:
Craig Birkmaier wrote:
It is true that the luminance bandwidth can exceed the
4.2 MHz bandpass of an NTSC encoder, but the process of creating the I & Q components for baseband video is the same process used in an NTSC encoder - you must reduce the Q component (mostly blue) to only 0.5 MHz bandpass and the I component to 1.5 MHz.

While the FCC rules for broadcasting analog NTSC specify I&Q as the color components, I think for most digital OTA broadcasts wideband R-Y&B-Y are used which, when decoded into NTSC in the $60 boxes results in images with much more color detail.

Correct. Digital encoding does not have the limitations of NTSC with respect to the bandwidth of the color difference signals, although they are typically sampled at half the bandwidth of the luminance signal (actually 4:2:0 for emission). The worst case for digital broadcast is when the source is interlaced - the vertical color subsampling to achieve 4:2:0 does not work well with interlace.

But the thread I was responding to suggested that baseband NTSC could be far better than transmitted NTSC. True for the luminance, but not true for the color difference signals.


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