If you're talking about the U.S. standard, it's vestigial-sideband AM (the upper sideband is the "main" one, carrying the luminance information) for the Y signal, quadrature modulation (dual sideband AM, but the sidebands are carrying two independent signals) for the chroma, and FM for the audio. Most PAL-encoded systems are similar, but if there over-the-air SECAM typically uses FM for the chroma components.
One quirk in the AM transmission of the "black and white" information is that it's sent inverted - i.e., the "black" end of the signal actually results in the deepest modulation. This is done so that the synchronization pulses (which are "blacker than black" in the original signal) are the strongest part of the transmitted signal - which means that the signal can get very noisy, but still not lose synchronization.
Only if the noise were sufficiently high level so as to drive a white area all the way to black; that's kinda the point. The assumption is that in normal operation, the video signal average power is considerably greater than the noise power. High-amplitude noise transients, since they will cause deviation of the video (luminance) amplitude in both direction as seen by the receiver, result in both errors that are "whiter" and "blacker" than the intended level, resulting in the typical "snow" pattern seen "over" the image. Early video systems DID use positive modulation, mostly because it just seemed the natural thing to do - but it has sufficient problems that negative modulation quickly became the norm. See
(on that last page, negative modulation is referenced in the "Problems with NTSC" section, leading up to the issues that adding color into this signal created.)
A chief advantage of negative modulation, as noted before, is that it makes the sync information the most noise-immune portion of the transmitted signal.
Note that all these really means is that the "black" levels (and sync, which is "blacker than black") are conveyed by the highest instantaneous-power portions of the transmitted signal - the signal may still commonly be viewed as "white positive" when you look at it within the receiver. Oddly enough, though, the signal must again be inverted (white-negative) if we're talking about a conventionally-driven CRT, since "white" on a CRT is what you get when you drive the cathode more negative.
[...] Others sound homeworky but this particular question is a good one. "Black" only appears to be "Black" because our brains make it that way. Put a static picture on screen and cut out a cardboard 'mask' that only allows a "Black" part of the picture to be viewed. On no account allow any light from the lighter areas to show through. Examine closely the patch of Black. You'll find it's that grey green colour of a switched off telly. The illusion of a real Black is very, very convincing john
OK - so did you read the referenced pages, in particular the Michael Robin article on the Broadcast Engineering site? And so NOW do you remember the NTSC standard's requirement for negative modulation? Remember where the infamous "inter- carrier buzz" comes from? From that article:
"White level (100 IRE) reduces the carrier to 12.5 percent. The carrier cancels at 120 IRE. Saturated yellow and cyan colors can produce video signal levels of 130.8 IRE, resulting in carrier cancellation and inter-carrier "buzz." Because saturated yellow and cyan colors do not normally exist in nature, camera-generated video signals would not create problems. However, synthetically generated signals (100 percent color bars or character generators) could create problems."
Michael R. also was co-author of the highly-recommended "Digital Television Fundamentals," a text I wouldn't be without in my daily work (and maybe you can guess what *I* do for a living, too, now). You can get even more about the requirements for negative modulation (in all systems except for the thrice-damned French SECAM) in section 1.7.1 of that book (page 28, at least in the 1998 first edition).
Intercarrier buzz can also be caused by poor IF alignment, and some bozo setting the AGC too high. No decent engineer or transmitter operator runs the video at more than a full 100 units. BTW, the first station I worked at was a military B&W plant that had no color equipment, yet I managed to transmit our station ID in color. Also, try running a military TV station with only one working video waveform monitor, and three barely usable monochrome video monitors because everything is depot level maintenance, and the depot tells you they will send an engineer who is authorized to repair the equipment, "In a couple years.". I broke the regulations and rebuilt every piece of equipment at the site, and was threatened with 20 years in prison because I didn't wait for them to get around to us.
I am now 100% disabled, so it is highly unlikely I'll ever work on another 5 MW EIRP transmitter site, studios in multiple cities and build mobile production vans. It was fun, while it lasted. Damn, I never did get a chance to ride the elevator to the top of our 1749 foot tower. :( that was the second TV station, which had a 130 KW Comark transmitter at the new transmitter site in Orange City, Florida, and an old RCA TTU-25B at the original site in Lisbon FLorida. Three STL systems to get from the new studio to he new transmitter site, the original CARS equipment to change the station from a cable access station to a real TV station, and another new STL system between the two studio sites.
Later, I dismantled and moved the RCA TTU 25B transmitter to Destin Florida and rebuilt it to put it on Ch 58. They didn't have any video test equipment while I was testing the low level stages of the transmitter, so I used my Commodore SX64 as a video test generator, and my HP 191 Video waveform monitor to set the levels.
I doubt that I'll get my hands on one of the new digital plants. The only station where I live is very paranoid, and won't answer e-mails about their plant, or let anyone talk to their engineering dept. I'm beginning to think that they use UPS and an itinerant engineer when he passes through town. :(
Service to my country? Been there, Done that, and I\'ve got my DD214 to
Yeah, but the operative word here, of course, is "decent"; one of the local independents in this area quite often gets intercarrier buzz (the kind I meant originally, resulting from carrier cancellation) each and every time they run one of their zillions of "paid programming" infomercials - which of course features lots of nice, character-gen.- created text in full glorious color, and every time some doofus picks cyan for the "CALL NOW!" banner, TVs across the Rocky Mountains are set a-buzzin'.
Couldn't happen to a nicer bunch. Serves them all right. Broadcast media, infomercial producers, people who watch infomercials, jerks who set up "airwave pollution" with high power spam, they all deserve to get screwed.
You seem to be missing the point here - the only person that reallyt "gets screwed" in this scenario is the poor viewer, whose television suddenly starts emitting a very nasty ~60 Hz buzz whenever this sort of stuff appears on the screen.
Surely you've seen this happen; at the end of a commercial, or even during some "local" programming (where those nice folks at the local station - usually, this would happen at one of the smaller operations where the engineering staff might not be paying as close attention to their video levels, AGC, etc. - slap up a title or some such produced by an in-studio "character generator" box), you seem some bold text appear on the screen, and suddenly you're getting this really loud and annoying buzz in the audio?
What's happening has to do with a potential problem that turned up when the NTSC color encoding system was added to the original "black and white" broadcast standard. As noted earler, the broadcast form of TV video employs negative modulation - increasing the video level toward white results in a reduction of carrier amplitude, not an increase. It was already standard that the maximum "white" level (100 IRE) would result in a carrier amplitude of 12.5% (relative to the unmodulated video carrier). But the color system puts addition components "on top of" the video signal prior to modulation, and can as defined produce peak video signal levels slightly in excess of 130 IRE. A level of about 120 IRE results in complete cancellation of the carrier, so clearly anything greater is to be avoided. Levels in excess of 120 would happen with fully-saturated yellow or cyan colors - fortunately, such colors are very rare in camera-generated video, but could happen with "generated" video as might come out of a color character generator or similar device. If the studio or transmitter engineer isn't watching for this (or has already taken care of it by placing limits into the system), such video DOES cause momentary carrier cancellation and the resulting buzz at the receiver.
You must have some real bozos in your area. I can't recall the last time I saw and overload or heard intercarrier buzz, except when the cable TV company lets one of their monkeys into the headend and the picture looks like they've removed a terminator at the modulator. :(
Service to my country? Been there, Done that, and I\'ve got my DD214 to
You're missing or evading my point. So I will spell it out. I HATE BROADCAST TV, AND THE NEWSPAPERS THAT SPAWNED IT. They are idiot "authoritarians". I like talking back on the I-net and hearing from the minorities, that taken together, are actually the majority.
That's just wonderful, and I'm very happy for you. What relevance that has to the topic at hand, though, I can't possibly imagine.
By the way, the newspapers didn't spawn broadcast TV; it came directly from broadcast radio, which grew independently of (and to some degree, in direct competition with) the print media. Further, there's nothing at all that ensures that a given concept is "correct" simply because it's the opinion of the majority. The broadcast and print media would be "authoritarian" only if they provided a single point of view, and if that point of view were enforceable. At least the last time I looked, there was an extremely diverse set of opinions available via the broadcast media.
There's no doubt the net has made it possible for a much larger number of people to get their opinions before others - however, the reality of the situation is that very, very few of those opinions are actually going to be heard by very many people (or, for that matter, deserve to be). How is someone like Matthew Drudge all that different from his broadcast media counterparts (of course, Drudge also now IS on the broadcast media)? How are the millions of "ordinary people" putting their thoughts down in their blogs (where they will, in the overwhelming majority of cases, remain virtually unread) all that different from those writing in to the letters column of the local paper?