Hmmmmm....Takes me back to my satellite news uplink days......In our case, NTSC meant "Never Twice Same Color"
----- Original Message ----- From: "Duane - N9DG" <[EMAIL PROTECTED]> To: "Ignacio Cembreros" <[EMAIL PROTECTED]>; <[email protected]> Sent: Thursday, January 18, 2007 8:51 PM Subject: Re: [Flexradio] OT: NTSC horizontal sweep frequency - WAS: Spur > --- Ignacio Cembreros <[EMAIL PROTECTED]> wrote: > >> A comb like spectrum with lines every 16 kHz ? The cause >> can be a TV, >> TV projector or the like. It seems to me harmonics of the >> line scan >> frequency. You can measure precisely those frequencies an >> if the >> difference is exactly 15750 Hz (I assume you are in the >> USA) you better >> start to look for the offending "reverse TVI" source. > > Actually since the advent of NTSC color in the 1950's the > horizontal sweep frequency is 15734 Hz (NTSC black and white > TV was indeed 15750 Hz). That slight shift in frequency in > turn gave rise for the need of a "dropframe" video timecode > standard (often expressed as 29.97). Supporting dropframe > video timecode has caused countless engineering headaches in > video equipment design and user base for over 5 decades now. > These dropframe headaches even persist into the current DTV > specifications. > > The whole reason for that slight shift in first place was to > move a harmonic spur for the color burst pulse away from the > audio subcarrier. The other choice would have been to move > the audio subcarrier a slight amount instead to keep the > color burst spurs out of the FM audio subcarrier's passband. > At that time shifting the horizontal sweep frequency was > deemed to be the better design choice since existing B&W TV's > were tolerant of the slightly lower horizontal sweep > frequency. In retrospect that was a really *bad* decision. A > better solution IMHO would have been for the NTSC group to > have moved the audio subcarrier frequency instead. That > probably would have required the existing B&W TV owners to > all have a service visit by their TV repairman, but by doing > so we could have avoided 5 decades of engineering headaches > all directly related NTSC dropframe. > > The irony is that the number of B&W TV's that would have been > made obsolete or needed to have slight modifications done to > them was still quite small. And I'd bet not a single one of > those B&W TV's in existence when NTSC color came about are > even being used today. But the legacy of maintaining support > for them looks like it will live on forever :-(. > > As recently as a two years ago I was involved with finding > video editing software bug that was dropframe timecode > calculation in nature. That was not the first time. And I > know that the company I work for isn't the only one who has > to fight those issues either. So I can only wonder how many > countless thousands of extra hardware/software engineering > effort man-hours that the NTSC dropframe choice has caused. > > Just goes to show that sometimes causing a little extra pain > for the existing installed user base may save a lot of other > headaches for the designers, - and ultimately that same end > user base in the long run. So when faced with decisions like > NTSC's 15750 vs. 15734 - choose wisely, - later generations > will thank you :-) > > Duane > N9DG > > > > > ____________________________________________________________________________________ > Get your own web address. > Have a HUGE year through Yahoo! Small Business. > http://smallbusiness.yahoo.com/domains/?p=BESTDEAL > > _______________________________________________ > FlexRadio mailing list > [email protected] > http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz > Archive Link: http://www.mail-archive.com/flexradio%40flex-radio.biz/ > FlexRadio Homepage: http://www.flex-radio.com/ > > FlexRadio Knowledge Base: http://kb.flex-radio.com/ _______________________________________________ FlexRadio mailing list [email protected] http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz Archive Link: http://www.mail-archive.com/flexradio%40flex-radio.biz/ FlexRadio Homepage: http://www.flex-radio.com/ FlexRadio Knowledge Base: http://kb.flex-radio.com/
