[opendtv] Re: Digital vs. Analog Quality

  • From: Craig Birkmaier <craig@xxxxxxxxx>
  • To: opendtv@xxxxxxxxxxxxx
  • Date: Mon, 7 Jul 2008 08:32:40 -0400

At 4:12 PM -0500 7/6/08, Doug McDonald wrote:
That may be true with RF outputy. But judging visual quality of DTV with a box
using S-video or even baseband NTSC is not silly. Baseband NTSC can be
enormously better than broadcast NTSC, because the bandwidth, both luma and
color, can be much larger than broadcast. It can and should look as good
as the very very best ordinary (not HD) DVDs. And, at least
here where I live, it sometimes does.

Doug is correct that baseband composite video is usually better than NTSC - I prefer not to call composite baseband NTSC, because NTSC encoding reduces the frequency content that was available in the composite baseband source.

But he is incorrect that the color bandwidth can be larger than broadcast NTSC. 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. It is possible to keep the subcarrier separate using Y/C processing (S-video), but for a basband signal you must combine the luminance and subcarrier signals to get them onto one wire.

Fortunately, the quality of a digitally compressed video stream CAN BE significantly better than even the best baseband composite video.

The reason is simple: digital compression operates upon COMPONENT VIDEO, not composite video. The simple act of turning analog component video into composite video SIGNIFICANTLY reduces the frequency content of the color difference signals, and by combining them into a color subcarrier, makes it much more difficult to recover the color difference signals to recreate the color components for display.

Digital compression can ALSO torture the color difference components, especially if the source is interlaced.

But good progressive source (720 x 480 @ 24P) properly encoded for DVD distribution will provide much higher quality at the analog component or HDMI outputs of the player than the same source after it has gone through the S-video and composite video encoding in the DVD player.

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