Reverse video to give higher transmission power during black was chosen, because noise is more visible to the eye in the black levels of video. With the whites, noise is less visible to the eye. Therefore the higher transmission power levels in black levels allow for less visible noise due to any RF carrier signal loss. It has nothing to do with sync pulses. The sync occurs during the blanking time at the beginning of each picture frame. The total horizontal blanking is approximately 10.6 to 10.8 usec. The sync pulse approximately 4.2 to 4.8 usec wide. The colour colour burst pulse sits on the back-porch of the blanking. Excellent web page about the structure of a video signal: http://zone.ni.com/devzone/cda/tut/p/id/4750 -- Jerry G. "Green Xenon [Radium]" <glucegen1@excite.com> wrote in message news:46f1c33d$0$32547$4c368faf@roadrunner.com... On Sep 3, 8:27 pm, isw <i...@witzend.com> wrote in http://groups.google.com/group/sci.electronics.basics/msg/6689f0d1e8eff770 : > Prolonged blacks can damage television transmitters, however (video > is inverted for transmission, so black requires full power from the > transmitter). Why is video inverted for transmission?
Why is video inverted for transmission?
Started by ●September 19, 2007
Reply by ●September 20, 20072007-09-20
Reply by ●September 20, 20072007-09-20
"Jerry G." <jerryg50@NOSPAMhotmail.com> writes:> Reverse video to give higher transmission power during black was chosen, > because noise is more visible to the eye in the black levels of video. > With the whites, noise is less visible to the eye. Therefore the higher > transmission power levels in black levels allow for less visible noise > due to any RF carrier signal loss. > > It has nothing to do with sync pulses. The sync occurs during the > blanking time at the beginning of each picture frame. The total > horizontal blanking is approximately 10.6 to 10.8 usec. The sync pulse > approximately 4.2 to 4.8 usec wide. The colour colour burst pulse sits > on the back-porch of the blanking. > > Excellent web page about the structure of a video signal: > http://zone.ni.com/devzone/cda/tut/p/id/4750While we're on the subject of TVs, here's a question that had me going for awhile several years back. In the old days the television screen, when turned off, was a dark greenish color. Why is it (or was it), then, that areas of "black" in a picture look(ed) black and not green? -- % Randy Yates % "My Shangri-la has gone away, fading like %% Fuquay-Varina, NC % the Beatles on 'Hey Jude'" %%% 919-577-9882 % %%%% <yates@ieee.org> % 'Shangri-La', *A New World Record*, ELO http://home.earthlink.net/~yatescr
Reply by ●September 20, 20072007-09-20
On Sep 20, 4:44 pm, "Jerry G." <jerry...@NOSPAMhotmail.com>> > It has nothing to do with sync pulses. The sync occurs during the > blanking time at the beginning of each picture frame.________ Note that sync pulses are transmitted between each horizontal LINE (not frame), and in the vertical interval between interlaced fields. The peak amplitude of the sync and equalizing pulses is transmitted with the maximum ERP allowed for the TV station, which makes for more stable pictures on receivers in weak signal areas. The power ratio of peak-of-sync to reference black video in normal r-f transmission is about 1.68 (NTSC). In fact, the peak output power of an NTSC TV transmitter is determined by measuring the average power dissipated in a matched dummy load when the transmitter is modulated by reference black + sync, and multiplying that value by 1.68. If the peak of sync was transmitted at 100% NEGATIVE amplitude modulation, as it would be without inverting the polarity of the composite video waveform from the studio, then the ERP of the TV station would be zero at the peaks of those pulses, with which TV receivers could have a problem.
Reply by ●September 20, 20072007-09-20
"Jerry G." <jerryg50@NOSPAMhotmail.com> wrote in message news:13f5qjlao5ki629@corp.supernews.com...> Reverse video to give higher transmission power during black was chosen, > because noise is more visible to the eye in the black levels of video. > With the whites, noise is less visible to the eye. Therefore the higher > transmission power levels in black levels allow for less visible noise > due to any RF carrier signal loss. >There is a rather logical reason for this. For maximum stability, you want the most power to occur during the sync pulses. That means the sync pulses have to be either blacker than black, or whiter than white. But, you want the screen to be black during sync pulses/retrace. Remember, we are talking 1940s technology here; and no color.> It has nothing to do with sync pulses. The sync occurs during the > blanking time at the beginning of each picture frame. The total > horizontal blanking is approximately 10.6 to 10.8 usec. The sync pulse > approximately 4.2 to 4.8 usec wide. The colour colour burst pulse sits > on the back-porch of the blanking. > > Excellent web page about the structure of a video signal: > http://zone.ni.com/devzone/cda/tut/p/id/4750 > > > -- > > Jerry G. > > > "Green Xenon [Radium]" <glucegen1@excite.com> wrote in message > news:46f1c33d$0$32547$4c368faf@roadrunner.com... > On Sep 3, 8:27 pm, isw <i...@witzend.com> wrote in > http://groups.google.com/group/sci.electronics.basics/msg/6689f0d1e8eff770 > : > > > Prolonged blacks can damage television transmitters, however (video > > is inverted for transmission, so black requires full power from the > > transmitter). >Supposedly, the average scene is more light than dark, and transmitting black actually saves power. Tam> Why is video inverted for transmission? > >
Reply by ●September 20, 20072007-09-20
Tam/WB2TT wrote: (snip)> Supposedly, the average scene is more light than dark, > and transmitting black actually saves power.Maybe even more after gamma correction. -- glen
Reply by ●September 20, 20072007-09-20
Frank Raffaeli wrote:> On Sep 19, 7:58 pm, "Green Xenon [Radium]" <gluceg...@excite.com> > wrote: > >>On Sep 3, 8:27 pm, isw <i...@witzend.com> wrote inhttp://groups.google.com/group/sci.electronics.basics/msg/6689f0d1e8e...: >> >> > Prolonged blacks can damage television transmitters, however (video >> > is inverted for transmission, so black requires full power from the >> > transmitter). >> >>Why is video inverted for transmission? > > > There are exceptions. System L, still used in France, for example, has > peak white at max RF power. The analog audio sub-carrier is amplitude > modulated. > > There is a digitally modulated sub-carrier for stereo audio (NICAM) >Well, That's france, what else would you expect! :) -- "I'm never wrong, once i thought i was, but was mistaken" Real Programmers Do things like this. http://webpages.charter.net/jamie_5
Reply by ●September 20, 20072007-09-20
Jerry G. wrote:> Reverse video to give higher transmission power during black was chosen, > because noise is more visible to the eye in the black levels of video. > With the whites, noise is less visible to the eye. Therefore the higher > transmission power levels in black levels allow for less visible noise > due to any RF carrier signal loss. > > It has nothing to do with sync pulses. The sync occurs during the > blanking time at the beginning of each picture frame. The total > horizontal blanking is approximately 10.6 to 10.8 usec. The sync pulse > approximately 4.2 to 4.8 usec wide. The colour colour burst pulse sits > on the back-porch of the blanking. > > Excellent web page about the structure of a video signal: > http://zone.ni.com/devzone/cda/tut/p/id/4750 > >What has not been considered so far in this thread is the TYPE of noise. 1. White noise is roughly symmetrical about zero and, regardless of the video modulation polarity, will appear as a mixture of white spots and black spots. 2. Impulse noise (from electrical appliances and so on) produces a video signal which is asymmetrical, being relatively unlimited in the direction corresponding to more carrier and of course limited to zero in the direction corresponding to less carrier. So, impulse noise can produce either intense white spots or very black spots, depending of the vide modulation polarity. Negative video modulation has no effect on perceived white noise, but does make impulse noise less obtrusive, so was chosen for this reason. Regards, John
Reply by ●September 21, 20072007-09-21
Allen wrote:> Arny Krueger wrote: >> "Unclaimed Mysteries" >> <theletter_k_andthenumeral_4_doh@unclaimedmysteries.net> >> wrote in message news:13f3v3cdjqesj66@corp.supernews.com >>> Richard Crowley wrote: >>>> "Charles" wrote ... >>>> >>>>>> Why is video inverted for transmission? >>>>> Noise in the black portion shows up more than in the >>>>> white. Black is transmitted at higher power, more >>>>> received signal. less noise. >>>> Yes, that is my recollection of the original explanation >>>> by the engineers & scientists who devised the system. >>>> >>>> We tend to forget how primitive things were back then >>>> (>50 years ago) by modern standards. I can't believe >>>> what they did with vacuum tubes ("valves") in those days. >> >> Especially if your name was Muntz. ;-) >> >>> A LOT of maintenance. >> >> Agreed. I once maintained equipment with about 400 "high reliability" >> tubes in it. MTBF: less than a day. >> > I can top that. I helped to maintain a 1500-tube (all but three of which > were dual or triple purpose) analog computer back a little over 50 years > ago. It was part of a Nike Surface-to-air missile system. The standard > first try repair was to kick it in the area where we suspected the > problem lay, which was effective more often than not. One day, though, a > general was making an announced inspection tour; on that morning, of > course, the computer failed. We applied the standard fix, but this time > the fixer kicked too hard and caved in one of the doors. The general > came in, looked around and asked "Did that fix it?" We told that it had, > and he said "good!" and walked out. We hadn't even told him what had > happened. And, incidentally, we called the van it was in the "pizza > oven". You can't believe how much heat 1500 tubes put out unless you've > been there. Viva la Solid State.There was no heating system in Harvard University's computer building in the 1950s. Plenty of air conditioning for use in warm weather, but in winter, they took air in through vents in the roof -- it was cleaner up there -- and blew it out through gratings in the sidewalk. It was a welcome relief when walking down the street in sub-zero weather to stand in the flow of 60-degree air that emerged. The tubes in the computer were special red (high reliability with red Bakelite bases) mostly 6SN7 dual triodes. Jerry -- Engineering is the art of making what you want from things you can get. ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
Reply by ●September 21, 20072007-09-21
Charles wrote:> On Wed, 19 Sep 2007 17:58:23 -0700, "Green Xenon [Radium]" > <glucegen1@excite.com> wrote: > >> On Sep 3, 8:27 pm, isw <i...@witzend.com> wrote in >> http://groups.google.com/group/sci.electronics.basics/msg/6689f0d1e8eff770 : >> >>> Prolonged blacks can damage television transmitters, however (video >>> is inverted for transmission, so black requires full power from the >>> transmitter). >> Why is video inverted for transmission? > > > Noise in the black portion shows up more than in the white. Black is > transmitted at higher power, more received signal. less noise.No. Noise is noise and is just as visible whatever the level. What matters is signal-to-noise ratio, especially for synchronizing. Black level is naturally used for blanking, and sync is "blacker than black" so it can't be seen. That part of a line wants the most power. Jerry -- Engineering is the art of making what you want from things you can get. ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
Reply by ●September 21, 20072007-09-21
Jerry G. wrote:> Reverse video to give higher transmission power during black was chosen, > because noise is more visible to the eye in the black levels of video. > With the whites, noise is less visible to the eye. Therefore the higher > transmission power levels in black levels allow for less visible noise > due to any RF carrier signal loss. > > It has nothing to do with sync pulses. The sync occurs during the > blanking time at the beginning of each picture frame. The total > horizontal blanking is approximately 10.6 to 10.8 usec. The sync pulse > approximately 4.2 to 4.8 usec wide. The colour colour burst pulse sits > on the back-porch of the blanking.It makes sense to use black for blanking. If you use whiter than white for sync, how would you blank that?> Excellent web page about the structure of a video signal: > http://zone.ni.com/devzone/cda/tut/p/id/4750Jerry -- Engineering is the art of making what you want from things you can get. ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
