Duane - N9DG wrote: > --- 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 > Duane, thank you for the expanation, I had no idea about that change. I remembered the figure (or better how to derive it) from where I began to study that matter in the sixties, but as you surely know here in Europe we use the PAL system at 15625 Hz (625 lines, 50 fields). So returnig to the thread, if the spurs are exactly 15734 Hz apart, the source is TV related.
Switching power supplies can make a lot of RFI but ther frequency is not predictable as in the TV, and computer monitors use higher frequencies. As a example, 1024 x 768 monitor at 75 Hz vertical rate runs at 60 Khz, and the lowest today used frequency is 37.9 Khz when the monitor runs at 800 x 600, 60 Hz. I totally agree with you in your opinions about the decisions when stablishing standards. Sometimes making a system totally compatible with the existing (and aging equipment) introduces penaltys in the performance that lasts years and later are patched in some non desirable ways. -- 73 de Ignacio, EB4APL -------------- next part -------------- An HTML attachment was scrubbed... URL: http://mail.flex-radio.biz/pipermail/flexradio_flex-radio.biz/attachments/20070119/1c307bbe/attachment.html _______________________________________________ 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/
