Back before infomertials, a lot of broadcast channels would, when they had nothing else to run, broadcast a color bar screen with a high pitched tone for hours and hours until the shows started up in the morning.
I was going through the stations the other night and we get some chigago broadcast station here on cable (wgn, I think), and they still play the color bars and tone.
If there’s nothing for a broadcaster to show, wouldn’t it be smarter just to turn off the antenna and stop broadcasting to save power?
I probably don’t have this 100% correct, as radio/TV transmission is not an area of expertise, but what I always heard was that damage to the transmission equipment was most likely to occur when shutting down or turning on - so actually it was more cost-effective to broadcast the test pattern (your “colored bars”) than to turn the equipment on and off, since the on/off cycle would be more likely to cause damage than continual operation.
Running a test signal/pattern for tuning and maintaining the broadcast system is generally considered a wise thing (considering the difficulty of checking the performance of a system that is not broadcasting). I’d also imagine that, due to emergency broadcast signals and the like, there is some requirement in to be broadcasting…
Any time you have dissimilar materials, they expand and contract at different rates. Old transmitters were vacuum tube based, so they had more in common with a light bulb than a transistor radio, but the concept is still the same. In a light bulb, there is a large inrush of current when voltage is first applied, which tends to cause a bit of a mechanical shock to the filament. Then everything starts heating up, and since the filament and the electrical contacts are all made out of different materials, they all expand at different rates, putting further mechanical stresses on the bulb. Once it all heats up, the resistance of the filament is increased so it draws less current, and it gets into a fairly steady state where there aren’t so many mechanical effects trying to kill it. Even in a steady state the filament will eventually burn out, but the bulb is most likely to go poof when you first turn on the switch. The same thing is true of vacuum tubes, and for a lot of the same reasons.
Semiconductors (like what’s in transistors and computer chips) aren’t immune to this sort of thing either. Their most likely point of failure is in the itty bitty wires that connect the silicon to the outside leads of the chip, which tend to seperate and lift up off of their pads after a lot of repeated thermal cycling (getting hot and cooling off).
As far as the transmitter is concerned pretty much any picture will do. I was under the impression that the color bar was used so that technicians could adjust the equipment on the off hours if desired. The pattern would always be broadcast so no one had to make sure that it was on, and anyone who wanted to use it for whatever piece of equipment was in the system could rely on it being present, including the end user (TV owner) who could adjust their own set. Similarly, a pretty much pure sine wave was broadcast on the audio channel so that equipment for it could be adjusted as well.
The television standard in North America and Japan, NTSC, is not so jokingly refered to in the industry as Never Twice the Same Color due to the dificulties of getting accurate image reproduction between sets.
Yep, sonny. I reckon I remember back when the first television sets - that’s what we called them then - showed up in homes. We all used to go over to Carol Weinfield’s house to watch. Then, we got one of our own. In those days, the set had to warm up. Sometimes, it would take several minutes before the picture came into clear view. (Funny, these new-fangled computer dealies seem to do much the same thing) And, at that time, we didn’t have remote controls, and we didn’t have AFT, which meant that to change the channels, you had to turn a big knob on the front of the set. And behind this knob was a secondary collar that was the fine tuning. So if you wanted to watch a program on channel two, for example, you cranked the knob to 2, and then, while watching and listening to what showed up, you fiddled with the fine tuning until the picture and sound were the best quality. All this is to say that at least one of the original uses of test patterns was to allow people to get their sets all warmed up and tuned in before the actual program began. And, by the way, before there were televisions, there were things called radios and books, but that’s a different tale. xo CC
Hah, shows how old my TV is. It has a digital channel display (like a clock-radio), but it takes about a minute for the picture to come into complete view. I think it’s almost as old as I am! (I just turned 16)
