Re: [Avid-L2] 29.97 and HD / RANT

Black and White TV was 60i but when they added color they offset the frame rate to help fit in the Color information or Chroma signal. The color subcarriers signal was a bandwidth of 3.58 MHz. Of course it's far more complicated then that. The reason for this was the FCC demanded reverse compatibility so B&W receivers could still playback or demux the B&W or luminance signal within the embedded color signal. 

You also have horizontal scan rate Offset, guard bands between the color, Luminance and audio sub carrier and then guard bands between channels. For the time it was a genius broadcast engineering solution. 

TMcD

On Aug 4, 2020, at 6:26 PM, David Baud <david.baud@gmail.com> wrote:

John,

You might well be right for the specifics details. I am not sure. I just know they were trying to resolve transmission issues between the way the chroma was encoded and the sound modulation. I am sure they were quite a bit of experimentation for a while before the first color broadcast program in the late 1940s?

Someone who would know for sure is Mark Schubin… actually I just checked his YouTube channel SchubinCafe and see he posted A History of Television 3-days ago! We might find the details there...

David Baud

Colorist & Finishing Editor

david at kosmos-productions.com

On Aug 4, 2020, at 3:58 PM, John Moore <bigfish@pacbell.net> wrote:

"For the USA, the TV NTSC color encoded system devised in the late 1930s had to make sure video and audio frequency signals did not interfere with each others when transmitted (ie audio creating a visual pattern). Hence the engineers decided to adjust the video carrier by a factor of 0.1% to avoid intermodulation between video and audio signals."

Perhaps I'm misremembering but the shift to 29.97 was based on the the color subcarrier frequency and it's relationship with the horizontal scan rate.  The subcarrier needed to be an odd multiple of one half the line frequency so that the any influence the color information might have on luminence would be cancelled out by the neighboring line. 

The audio was FM and transmitted at 4.5 MHz above the video carrier.  Video was amplitude modulated and audio FM modulated.  It was when video excursions from high level video made the video information encroach into the audio carrier frequency we heard buzz.

I'm enjoying all this info and if my recollection is inaccurate please refresh my memory.