View Full Version : Question about television channels

07-03-2012, 10:23 PM
Does anyone know when the idea of a television channel with a fixed total band width, video carrier and audio carrier at the same relative points within it, etc. was conceived? Very early television didn't seem to have this, but by about 1934 or so it looks like it was an established practice.

07-04-2012, 03:06 PM
Developing established channels was perhaps the first effort at standards... Right about at the same time that it was decided that electronic scanning was going to be the way to go.

Channelization was also one of the first efforts to put political and sociological controls into TV...

07-04-2012, 11:16 PM
Radio already had standardized frequency assignments and bandwidth, so it was a simple idea to extend this to TV.

Getting to the regulated status for radio broadcasting was a messy process that occurred in the 1920's, prompted by the chaos and high interference levels as radio grew. Along with banning a large percentage of the tens of thousands of transmitters already on the air, legislators debated whether broadcast radio should be commercially or government supported, whether there was danger of RCA becoming a politically active monopoly, and also discussion that certain speech should be disallowed, such as obscenity; but also "dangerous" ideas, e.g., discussion of evolution or revolution among other things.

Broadcasting is a case where allowing the most-free free speech (everyone has a transmitter) has a contrary effect that no one can be heard. Whether we should do it differently is definitely a political debate and not for this forum, but it is clear that technical standards and some restriction of use in terms of numbers of transmitters is necessary or it doesn't work at all.

07-07-2012, 11:31 AM
Although radio had standardized frequency assignments and bandwidth, these were also more of a political consideration than technical necessity. While in Europe, the Copenhagen Convention of 1946 (IIRC) set frequencies and allocations, they were doled out almost as arbitrarily as the Havana (or NARBA) Treaty for North America. TV, on the other hand, offered more opportunity to put an “electronic curtain” around your population while not causing ill-will with neighboring countries. While in the analog era, virtually everything in the Western Hemisphere was NTSC/CCIR system M (except the French territories like St. Pierre et Miquelon), you had two countries go PAL (Brazil and Argentina) for color while another attempted SECAM only to go NTSC later (Cuba). In Europe, look at the channelization differences between east and west. Such was the early recognized power of TV! Or, look at channel 13 in the Florida Keys. Problem with TV is that like all VHF and wideband signals, it’s easy to jam. Unfortunately, jamming also makes you look like a totalitarian political pariah. Anyone remember what happened with Radio Nordzee International on 1367 KHz? As for the political muscle of RCA, how many former FCC Commissioners later became members of the RCA Board of Directors? To some in the 1930’s-1950’s, the FCC was not much more than a subsidiary of RCA… Now, in the digital era, besides the RF considerations, you also have the transmission protocols to deal with. How many countries are adopting ATSC vs. ISDB vs. DVB-T?

07-30-2012, 10:02 PM
But even if politicians and the FCC wanted to establish political and social controls on television broadcasting, why did it have to take this particular form? For example, why would it be necessary to have both the audio and the video carrier in the same frequency range, with all the potential problems with beat frequencies? Why not have the video carriers in one part of the broadcast spectrum, and the audio carriers in another? True, the viewer would have to tune them separately, which is maybe a little more effort, but it would remove the need to have a standard for the distance they would have to be separated.

12-30-2014, 05:45 AM
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Electronic M
12-30-2014, 08:01 PM
I think alison3492 is not understanding that this thread is about over the air (recieved thru an antenna) broadcast TV of decades before HD was even being developed.

I think the main reason that the carriers were in the same channel bandwidth was a consumer simplicity requirement. Many people did not buy radios until the TRF sets were gone or equipped with simplified one knob tuners. This was because the average consumer could not wrap their head around using the 2-4 tuning knobs of the average TRF set to tune a station. I suspect the engineers looked at that (then recent event) and thought 'if we tune the sound and video separately in different bands most folks will not be able figure out how to tune their new EXPENSIVE set and will get mad, bad mouth the technology, and this whole thing will be a flop'....As an example of this in action just look at how many consumers called the service shops to fix their color set due to 'no color' symptoms when all that was wrong was that they were too dumb to adjust their sets fine tuning knob....Which caused AFT (Automatic Fine Tuning) to be invented and take over.

I suspect most separate carrier systems (which were mechanical and experimental electronic mostly) were done that way so that they could take 2 out of the box AM audio transmitters, make a test station, and only have to creatively focus on the image systems....Having to experiment with RF design would only have distracted from electronic image system development that was sorely needed in the 20's and 30's...