Randy Yates wrote:> "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/4750 > > While 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?When your TV is turned off, you can see (but generally ignore) specular reflections from the picture tube. Those don't get darker when the set is on, but like the green color, they go completely unnoticed. Jerry -- Engineering is the art of making what you want from things you can get. ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
Why is video inverted for transmission?
Started by ●September 19, 2007
Reply by ●September 21, 20072007-09-21
Reply by ●September 21, 20072007-09-21
"Tam/WB2TT" <t-tammaru@c0mca$t.net> wrote in message news:gfydnc169OJfnm7bnZ2dnUVZ_r6rnZ2d@comcast.com...> Supposedly, the average scene is more light than dark, and transmitting > black actually saves power.Not so. The average video/movie scene, ignoring oddities such as static frames of text as might appear at the end of a commercial or some such, generally has an average overall luminance well under 50% of that of a full white screen. Most often, in fact, you're down around 25% or under. Bob M.
Reply by ●September 21, 20072007-09-21
"Randy Yates" <yates@ieee.org> wrote in message news:m37imlt34o.fsf_-_@ieee.org...> 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?What you see as "black" at any given moment really just means "an area that's a whole lot darker than its surroundings." With the TV off, you see the screen "as it is," with no bright light sources in your field of view to make it look darker. With the TV on, those parts of the screen which aren't making light are the darkest thing by far in your field of view, and look "black." Vision in general doesn't work in terms of absolutes, but instead deals with differences and change; it's all relative. By the way, the screen itself (in a color CRT) is still a greenish-gray, if you could see it directly. In modern color tubes, though, you're typically looking at the phosphor screen through a dark glass faceplate, which is made so to enhance the perceived contrast. (Up really close, the phosphor screen is stripes or rectangular dots of the red, green, and blue phosphors - each of which in the "off" state typically has a pale tinge of that color - with each individual dot surrounded by a black material, again put there as a contrast-enhancement measure.) Bob M.
Reply by ●September 21, 20072007-09-21
On Thu, 20 Sep 2007 17:44:01 -0400, "Jerry G." <jerryg50@NOSPAMhotmail.com> 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.So do you think the noise gets smaller during the higher parts of the signal (the blacks)? Let me assure you it doesn't. The amplitude of the noise in the black area is identical whether it is placed at the top of the modulation or the bottom. Noise adds linearly to the signal, it doesn't multiply. It makes not a jot of difference to the noise whether black it at the top, or whit is. The polarity of the signal is all about making every frame identical in height so that each component of the signal can be identified unambiguously. d -- Pearce Consulting http://www.pearce.uk.com
Reply by ●September 21, 20072007-09-21
Jerry Avins wrote: <snip>> 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. > > JerryI don't think that people who haven't been there can comprehend how much heat a large number of vacuum tubes can generate. The Nike systems had separate trailer-mounted air conditioners that were ducted to the van with the computer. Even with four feet of snow on the ground, the failure of the air conditioner meant failure of the computer. Incidentally, I'm surprised that the Harvard computer had so many 6SN7s--an octal-base tube. In the Nike computer there were only a few octals, mostly MILSPEC equivalents of 6L6s; the rest were 7- and 9-pin mini tubes. So much for the electronic misnamed "good old days". Allen
Reply by ●September 21, 20072007-09-21
On 9/21/07 6:12 AM, in article 46f3c34b$0$17087$4c368faf@roadrunner.com, "Allen" <allen@nothere.net> wrote:> Jerry Avins wrote: > <snip> >> 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 > I don't think that people who haven't been there can comprehend how much > heat a large number of vacuum tubes can generate. The Nike systems had > separate trailer-mounted air conditioners that were ducted to the van > with the computer. Even with four feet of snow on the ground, the > failure of the air conditioner meant failure of the computer. > Incidentally, I'm surprised that the Harvard computer had so many > 6SN7s--an octal-base tube.I'm curious about why you are surprised. The 6SN7 predated the 12AX7. Both were good tubes. And the octal tubes seated better (more solid) in their sockets than did the 7 and 9 pin min tubes.> In the Nike computer there were only a few > octals, mostly MILSPEC equivalents of 6L6s; the rest were 7- and 9-pin > mini tubes. So much for the electronic misnamed "good old days".In what manner were they NOT the good old days? Because you didn't like the tubes? I think they were were the good old days, and I was there.> Allen >
Reply by ●September 21, 20072007-09-21
Don Pearce wrote:> On Thu, 20 Sep 2007 17:44:01 -0400, "Jerry G." > <jerryg50@NOSPAMhotmail.com> 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. > > So do you think the noise gets smaller during the higher parts of the > signal (the blacks)? Let me assure you it doesn't. The amplitude of > the noise in the black area is identical whether it is placed at the > top of the modulation or the bottom. Noise adds linearly to the > signal, it doesn't multiply. It makes not a jot of difference to the > noise whether black it at the top, or whit is. > > The polarity of the signal is all about making every frame identical > in height so that each component of the signal can be identified > unambiguously.It's easy to see even fairly dim stars against a dark nighttime sky. Seeing black specks against a bright daytime sky is much harder. The contrast ratio may be the same, but size matters. Jerry -- Engineering is the art of making what you want from things you can get. ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
Reply by ●September 21, 20072007-09-21
On Fri, 21 Sep 2007 14:30:37 -0400, Jerry Avins <jya@ieee.org> wrote:>Don Pearce wrote: >> On Thu, 20 Sep 2007 17:44:01 -0400, "Jerry G." >> <jerryg50@NOSPAMhotmail.com> 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. >> >> So do you think the noise gets smaller during the higher parts of the >> signal (the blacks)? Let me assure you it doesn't. The amplitude of >> the noise in the black area is identical whether it is placed at the >> top of the modulation or the bottom. Noise adds linearly to the >> signal, it doesn't multiply. It makes not a jot of difference to the >> noise whether black it at the top, or whit is. >> >> The polarity of the signal is all about making every frame identical >> in height so that each component of the signal can be identified >> unambiguously. > >It's easy to see even fairly dim stars against a dark nighttime sky. >Seeing black specks against a bright daytime sky is much harder. The >contrast ratio may be the same, but size matters. >What has that to do with anything? Look, it doesn't matter if the black is at the top or the bottom of the signal, it will have exactly the same amount of noise on it. Likewise the white. d -- Pearce Consulting http://www.pearce.uk.com
Reply by ●September 21, 20072007-09-21
Allen wrote:> Jerry Avins wrote: > <snip> >> 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 > I don't think that people who haven't been there can comprehend how much > heat a large number of vacuum tubes can generate. The Nike systems had > separate trailer-mounted air conditioners that were ducted to the van > with the computer. Even with four feet of snow on the ground, the > failure of the air conditioner meant failure of the computer. > Incidentally, I'm surprised that the Harvard computer had so many > 6SN7s--an octal-base tube. In the Nike computer there were only a few > octals, mostly MILSPEC equivalents of 6L6s; the rest were 7- and 9-pin > mini tubes. So much for the electronic misnamed "good old days".Do you really mean 6L6? That is a beam power tetrode, practically the same as a 5881, and KT66s could be used for a little more allowed dissipation. IIRC, the classic Williamson amplifier used 6L6s in the output stage. Jerry -- Engineering is the art of making what you want from things you can get. ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
Reply by ●September 21, 20072007-09-21
Jerry Avins wrote: <snip>> > Do you really mean 6L6? That is a beam power tetrode, practically the > same as a 5881, and KT66s could be used for a little more allowed > dissipation. IIRC, the classic Williamson amplifier used 6L6s in the > output stage. > > JerryYes, I meant 6L6. I don't recall what their use was, but they were definitely, hidden under a MILSPEC number. I was in a strange situation, incidentally. I was drafted, but our techs had to enlist for three years, of which they spent 54 weeks in training. After they completed that period of a year and two weeks, they were specifically NOT authorized to make a soldered connection, although our TO&E included soldering irons and solder. I wound up being assigned to the supply van, which I loved because I was not on the KP or guard rosters. I had been building electronic equipment off and on for ten years, so if soldering was needed the techs had a choice: get me to solder, or take the part to an ordnance depot several miles away and wait while a part-time high school student fixed it. Ah, the army and its mysterious ways. Allen
