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It just dawned on me that, when I was young, game consoles like the NES and SNES, and possibly other devices like VCR's, required the TV to be tuned to channel 4. I asked around, and some remembered it being channel 3.

Why did it matter what channel the TV was tuned to?

Ryan_L
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    On my TV, it was on the AUX channel. It was the button right at the bottom. Easy to tell kids who couldn't remember number sequences or recognize numbers. The 1979 TV (which I still have) had 5 channels + an AUX button. Anyway there was a little screwdriver which you could use to tune each channel individually so it could have been any channel. – cup Apr 05 '20 at 04:48
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    Do you mean why channel 3/4 specifically, or just why you have to tune into a channel at all? – mnem Apr 05 '20 at 05:59
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    Keep in mind that many TVs of that era did not have separate video inputs. Game consoles typically connected via an RF jack where the TV normally would connect to an antenna or a cable box (which also required tuning to channel 3 or 4). – jamesdlin Apr 05 '20 at 09:45
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    Unrelated but this reminded me when we were kids, sometimes we'd have the tv or games on when we were supposed to be asleep. At some point we figured out that if we leave the tv ever so slightly de-tuned (it was a rotating dial) so there was some noise, it worked like a radar. When dad started moving around and headed out of the living room to check up on us in the our room, it did something with the interference that we could recognize, so me and my brother would quickly switch off the tv and jump into bed and pretend to be asleep. – Frank Apr 05 '20 at 13:53
  • @cup AUX (when available) was the best option - a raw composite video signal skipping modulation and demodulation in both devices. – fraxinus Apr 05 '20 at 16:03
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    You understand that the game console essentially generated the same signal as a broadcast tv-station? – Thorbjørn Ravn Andersen Apr 05 '20 at 18:42
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    @ThorbjørnRavnAndersen I didn't, but I'm coming to that realization now. – Ryan_L Apr 05 '20 at 18:48
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    @Frank This sounds like it comes straight out of an espionage movie. "The electromagnetic interference of a human body was powerful enough to cause a recognizable pattern on an out-of-tune television. This allowed the agents to monitor their mark from a different room." – MechMK1 Apr 06 '20 at 09:19
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    @MechMK1 I am not kidding when I say that was dad-detector v2. The original version was that my joystick broke, and we 12/13 year old kids discovered that if you take it apart you could extend the microswitches in the joystick by connecting them with little wire.There was always lots of wire. Crystal radio sets and the like. So when the parents were out we ripped up the carpets and put the microswitches under the threshold with the living room and hall, while plugging in the remains of the joystick to the bbc micro ad converter. Didn't work. Dad spotted that we'd knackered the carpets. – Frank Apr 06 '20 at 09:52
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    @Frank If true, it proves that some kids are definitely ahead of others. I'm one of those who were behind - a lot :D – MechMK1 Apr 06 '20 at 10:37
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    @Frank I agree, generally. Creative skills are just as valid as technical ones. But as you said, there are people who are just "thick". – MechMK1 Apr 06 '20 at 11:02
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    @Frank Cheers, I raise my glass to that! – MechMK1 Apr 06 '20 at 11:16
  • I feel like this highly depends on the country. As far as I know I didn't have this with my NES and SNES because they were connected by SCART I think. Not using the same broadcast signal as mentioned in the comments and answers – Ivo Apr 06 '20 at 12:09
  • @cup your comment suggests that you weren't in the US. Ryan_L's question suggests that he was. – phoog Apr 08 '20 at 06:59

5 Answers5

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TL;DR:

In the US Channel 3 was preferred as it was in the midst of the continious 2/3/4 assignment. By lifting or lowering the carrier by 6 Mhz either channel 2 or 4 could be offered as alternative.

Another-Dave already answered the basic issue of avoiding collisions. The reason why it was channel 2/3/4 is due historic channel assignment (in the US, *1).

The following is all about the classic VHF low-bands (1/2).

Channel assignment during the 70s to 90s for VHF band 1 in the US:

Range      VHF Chn.   TV Chn.  
44..50 MHz   1          (1)    [Not used since the 1950s][4]
50..54 MHz   gap         -     4 MHz gap for [6m band][2]
54..60 MHz   A2          2
60..66 MHz   A3          3
66..72 MHz   A4          4
72..76 MHz   gap         -
76..82 MHz   A5          5
82..88 MHz   A6          6

To start with, the channel assignments have shifted over time (*2). For example the original 6 meter gap was 10 MHz from 56 to 66 MHz, while 84 to 90 MHz was Channel 6. The gap got narrowed and with everything after moved to squeeze in another channel (*3) while widening the band for FM radio at the same time.

Important for consoles/home computers/VCR and alike in the 70s to 90s is that the FCC usually tried to space regional stations out to reduce interference between them. This didn't always work out in heavy populated areas, but usually either 3 or 2/4 was not assigned to a local (strong) station. Thus a modulator offering a choice to switch between 2/3 or 3/4 could usually solve any channel conflict.

A few devices offered even a three way switch (2/3/4) to allow the use of either channel within the group, but most manufacturers tried to save a penny, thus offering only two choices.

The whole issue about channel numbers on devices gets further complicates when looking at Europe(*4). US channels were 6 MHz wide, starting at 50 MHz, while European started at 40 MHz, occupying 7 MHz each (*5). So US 3 is located similar to European channel 2, while US 4 is European 3 (*6). So a European device marked as Channel 2 would in the US mean one has to turn to channel 3 :)) In addition in Europe UHF channels were often used instead - but that's different story (*7).

Now, in Japan TV did broadcast in the 90 to 108 MHz range (Channel 1..3), that's were FM radio is located in most other places. So Japanese market consoles / computers use frequencies usually not available to European and US TV sets.

And then there is (US) Channel 7 as Alex Hajnal reminded. It lies way above in VHF band III at 174 MHz. While channels 1..6 of band I are of different assignment around the world, 174 MHz is almost everywhere used for TV. In addition it's as well (in most countries) the first channel in this band, independent of spacing/alignment, thus ideal for world wide distribution.

Confusing? Yes, for sure, but in the end, one simply connected the console and turned the knob until some picture came up - if the result was poor, the switch got flipped to see if the other channel came better.

*1 - In Europe and other parts of the world it was similar, but different as the channels were assigned to different frequencies and of different size and arrangement.

*2 - Channel numbers and their logic are weird anyway, as they depend on convention, not physics. One have gone with frequencies, much like with Radio (and, if at all, use logical channels as in parts of Europe).

*3 - That's why they are names An in above table.

*4 - I'll spare the UK 405 line system using 5 MHz channels, as well as the special 1a and 2a channels.

*5 - Well, err , being Europe, there is much unity in doing things different. Beside the English 5 MHz system, there was the old French 14 MHz (awesome picture with 800 lines) as well as the new French 8 MHz system. The later also used in Eastern Europe and China but of course all with different frequencies and various gaps. For example the 6 meter gap is (mostly) observed in most parts of the world, except China and Ireland.

*6 - US 2 is located half way between European 1 and 2.

*7 - Most notably here European channel 36, which happens to be channel 34 in the US, thus despite the different channel spacing, the same modulator hardware could be used for devices designated for the US and Europe and thus essentially the whole world except Australia and China.

Raffzahn
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    In Canada, channel 3 was often used - probably because it meant less likelihood of interference from a US station. So by allowing the user to choose from 3 or 4, your device could work with most of the North American market without hardware changes. – user3486184 Apr 05 '20 at 04:23
  • (Reference is U.S.) (1) 6-meter band is 50-54 MHz, putting it just above the original channel one. (2) I don't recall ever seeing a TV that didn't have channel 3. (3) If your city didn't have channel 3, then the next city over did. So if NYC had 2, 4, &7, then Philadelphia got 3 and 6. But this works into the original answer: There may have been a strong local station on 2 or 3, but never on both at once. – gbarry Apr 05 '20 at 04:28
  • I agree @gbarry, I’m from Philly, born in the 60’s and our local VHF channels were always 3, 6, 10, and 12 (NBC, ABC, CBS, PBS respectively). I used channel 3 for my Atari 2600 and Atari 400. – mannaggia Apr 05 '20 at 11:33
  • @gbarry (1) the band gap between TV channel 2 and 4 was originally 10 MHz, from 56 to 66, until it got narrowed to 4 MHz and the following channels shifted (now called A2..A4) There is no TV channel 1. (2) There were TV with digital control that automatically skipped this band when searching. (3) that part got rewritten - I based my numbers on the original assignment. – Raffzahn Apr 05 '20 at 11:36
  • @mannaggia NYC (~150km from Philly) used 2 (CBS), 4, 5 (Fox), 7, 9 (indie), 11, and 13 (PBS). – Alex Hajnal Apr 06 '20 at 01:27
  • @Raffzahn I've seen a few devices that used channel 7 as one of the choices (either 3 + 7 or 4 + 7). Any ideas why that was done? – Alex Hajnal Apr 06 '20 at 02:18
  • @AlexHajnal Channel 7 is a pretty obvious choice for world wide usage. It's in VHF band IIL at 174 MHz, which is used in next to all countries worldwide for TV, even those that don't use band II (were Ch.3 is). In fact, it's in most cases the first channel of this band, thus easy to find as well. – Raffzahn Apr 06 '20 at 08:22
  • In the Netherlands devices like NES and SNES were connected with SCART if I remember correct. You needed to switch to an AV channel for that. – Ivo Apr 06 '20 at 12:11
  • Many RF modulator circuits used in computers and video games are really horrible. Generally, they use a tuned coil to set one of the channels, and a switch which shifts the frequency by something around 6-7MHz, and rely upon the user to adjust the fine tuning on the set if needed to achieve good results. I suspect the reason many units have one channel that works much better than the other is that the tuning coil was set for the one that works better, and the other channel just ended up wherever. – supercat Apr 06 '20 at 17:32
  • In the UK, games consoles and video recorders usually output on channel 36. – TripeHound Apr 09 '20 at 19:19
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Why tune the TV to a channel at all?

  • Early consumer television sets were built for one purpose: to watch broadcast television. The only input was for an antenna. The idea of using your television set for any other purpose simply wasn't yet conceived, so there were no connectors for any other kind of signal.
  • The first additional uses for household television sets were the first generation video games (e.g. pong) of the late 1970s. The Apple I/II of this era also used a television set. Because the television's only input was for an antenna, these devices needed a modulator which took the video signal and raised it in frequency, emulating the signal of a real TV station. It worked, although the process of modulating the signal and then demodulating it again inside the TV decreased the quality of the display. (It didn't matter much, as these early systems were low resolution.)
  • Consumer-grade VCRs did not become popular until the early 1980s. They followed the conventions already established by home computers and video games. Sure, there were professional video recorders before this time, but they cost tens of thousands of dollars and directly interfaced to other video equipment; they didn't modulate their signal to a TV set.
  • Cable TV converter boxes didn't appear until the 1980s, and also followed the conventions started by home computers and video games.
  • As consumers starting adopting TVs for more uses, the TV manufacturers started adding input connections better suited for these uses. Composite video and later S-video inputs were added to television sets. The picture quality improved, at a time when the resolution of these devices were increasing. Of course, these new TV sets provided either a switch setting or a dedicated switch to choose these new inputs. You no longer had to "tune to a channel".

Why have two choices of channels?

  • Back in the 1940s when the first television channels were being assigned, TV sets did a poor job of demodulating the signals. In particular, the signals on adjacent channel numbers often interfered with each other. The FCC's solution was to skip channel numbers when assigning stations in the same local area.
  • Adjacent numbers did often exist between adjacent broadcast areas. If you were near a city, you got strong signals from your city's stations, and you didn't care if they interfered with the signals from another city. People living in rural areas between two cities might get both cities' channels if they were lucky, or interference if they were unlucky.
  • Modulators output their signal to only one channel at a time. Had they fixed it to a single channel, then there would be a certain portion of the country which had a broadcast station on that same frequency.
  • The nominal explanation was that the broadcast station's signal would degrade or completely block a modulator's signal on the same frequency. However, my own experience in the 1970s was that it worked in the other direction: the signal from a modulator on channel 4 would interfere with watching broadcast channel 4 in a nearby room. Legally, this wasn't supposed to happen, yet it did.
  • A switch on a modulator that could choose between two channels allowed the modulator to be set to the channel that was not in use in your area. Using a device and watching TV would then not interfere with each other.
  • Because interference was only an issue between adjacent channels, there was no need for the modulator to support more than 2 channels.

Why channels 3+4 or 2+3?

  • There never was a channel 1 on U.S. analog broadcast television.
  • Early consumer-grade modulators of the 1970s were made with cheap components that could not support higher frequencies. Because channels 2, 3, and 4 had the lowest frequencies, those were the channels used by these modulators.
  • Some early TV sets were bad at receiving channel 2, and it earned the reputation of a channel to avoid. In particular, early broadcasters didn't want that channel.
  • A rare few TV sets also had trouble with channel 3, but it never earned the stigma of channel 2. Some sources claim that channel 4 was more common than channel 3, but in reality both channels were widely assigned. Channel 4 would often be found in the biggest (i.e. wealthiest) markets and channel 3 in mid-range markets, with exceptions.
  • There were many exceptions to the preceding two points, because desirable numbers already assigned in nearby markets forced the adoption of another number. Also, as time progressed, TV sets got better at receiving these lower channel numbers, and the number of stations grew, so more broadcasters were assigned channels 3 or 2.
  • By the time modulators appeared in the 1970s, channel 2 still had its (then-obsolete) stigma of a channel to avoid. Thus, the combination of 3+4 was more common than 2+3.

Combining all of the above issues, a modulator with a switch that could choose between channels 3 and 4 was the best solution.

DrSheldon
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    It wasn't just the 1940s where TVs had trouble. I had a television built in 1983 that, if tuned to channel 3, would show a low-quality copy of channel 4's broadcast. Oddly, it wouldn't do this with any of the other pairs of adjacent channels. – Mark Apr 05 '20 at 21:48
  • The Odyssey brand video games console was available for use with home television sets before the invention of the PONG brand arcade machine (early 1970s). – supercat Apr 06 '20 at 02:03
  • @supercat: I was giving one example; there were many systems of that era. – DrSheldon Apr 06 '20 at 02:09
  • I remember the channel 3 thing, but I grew up in the ’90s and my first system was a SNES that used composite cables. If it was no longer using the antenna, why’d I have to worry about the channel? – KRyan Apr 06 '20 at 15:57
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    I'm not sure if this is related or not but I had one TV in particular, if I tuned directly to channel 3, I would get the game's signal in black and white; to get a full color signal I would have to tune to 3, then 4, then back to 3; I remember this happening with the N64 at least (note that the TV was much older than the N64). Do you think this was happening because channel 2 was bleeding into channel 3 by any chance? Channels 2 and 4 were broadcast channels at the time. – jrh Apr 06 '20 at 19:34
  • Interestingly, the first TV I ever encountered with a composite input actually used a channel number to view the input. It was an RCA TV, and the channel number was 91. – Jeremy Apr 07 '20 at 16:04
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Because those are the two frequencies that the RF modulator in the game console can produce. "Tuning to a channel" means "choosing the frequency band to receive", so clearly you need to receive on the same frequency band that the transmitter is sending. (Warning, non-electronic-engineer loose wording above).

Why was there a choice of two rather than one fixed? That I do not know.

dave
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    I believe the choice was to help avoid interference from broadcast signals; it was unlikely in North America at least to have broadcast TV on both of those channels. https://en.wikipedia.org/wiki/RF_modulator#Channels – Joe Apr 05 '20 at 01:44
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    Some devices offered channels 2 or 3 and other offered channels 3 or 4. If one channel had interference, you could switch to the other. – Tim Locke Apr 05 '20 at 01:51
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    A switch would let you have two devices connected to the TV and powered on. For example the computer, and a VCR - which would let you record your game-play – CSM Apr 05 '20 at 13:58
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The bandwidth of a typical composite video signal is about 6 MHz.

The video hardware in in early game consoles (and early home computers) generates its composite video signal in what's called baseband format: the frequency range is 0 Hz to 6 MHz. Only one baseband signal can be sent down a cable or broadcast through the air at any one time.¹

So that we can have multiple TV channels (among other reasons), the broadcast system takes the baseband signals for different programmes and modulates them on to different higher-frequency carrier signals. This basically "moves them up" in the electromagnetic spectrum range: for channel 2 the 0-6 MHz signal is moved up to the 54-60 MHz range, and for channel 3 the 0-6 MHz signal is moved up to the 60-66 MHz range.²

TVs have a tuner that picks a particular range and then demodulates that signal, producing the original 0-6 MHz baseband signal used for that particular channel. Modern TVs and of course computer monitors will often have a direct input for a baseband composite signal so this modulation-demodulation process isn't necessary, but older TVs from the '70s and '80s usually didn't offer this.

So consoles and computers connected to these older TVs by using sending their baseband signal though an RF modulator, which which is basically a tiny little TV broadcast station doing that same conversion described above. You'd then plug the output of the RF modulation to the antenna input of the TV, it would demodulate the signal back down, and then you can have a picture.

When modulating the signal you of course have to choose which one of the many channels you're going to use. Typically North American RF modulators offered you a choice of using channel 3 or 4.

Why those particular channel numbers, and not others? It had to do with what broadcast frequencies were commonly used in the area, among other considerations. From Wikipedia:

A channel 3/4 output was a common output selection for consumer audiovisual devices sold in North America that were intended to be connected to a TV using a radio frequency (RF) signal. This channel option was provided because it was rare to have broadcast channels 3 and 4 used in the same market, or even just channel 3 itself. The choice allowed the user to select the unused channel in their area so that the connected device would be able to provide video and audio on an RF feed to the television without excessive interference from a broadcast signal.

Other countries had the RF output for video equipment on different groups of frequencies. For example, equipment sold in Europe, South Africa and Hong Kong used UHF channels 30–39 for this purpose. Equipment sold in Japan used channel 1 or 2 (Channel 13–16 is for cable converters). With other channels being used for RF modulation function in other regions, channel 3/4 output is a misnomer for those regions.

¹Actually, this isn't strictly the case, especially for radio, but that gets into some very complex physics and electrical engineering stuff that we don't need to answer this question.

²I'm giving here the channel frequency assignments for North American television of the time; the frequencies will be somewhat different in other parts of the world. You can see more North American channel assignments here, and the chart here compares channel assignemnts for various parts of the world.

Community
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cjs
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    A "typical composite video signal" isn't 6 MHz. For one, even with NTSC and in the US it's 5.25 MHz (5.75 including sound). 6 MHz is the channel width assigned for TV when carrying a composite signal. But more important, 'typical' in other parts of the world ranges from 5 MHz (UK) to 14 MHz (France). – Raffzahn Apr 05 '20 at 11:57
  • @Raffzahn Those are exactly the kind of unimportant details that would make the answer harder to understand for someone who doesn't have your level of technical background and isn't looking to to spend the time developing that level of knowledge – cjs Apr 05 '20 at 13:12
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    I see you do not intend to take well supported hint, so let's put it as bland as possible: You need replace the 'typical' in your first statement by 'US' or US/NTSC' as it is specific to only the US (and Japan). This is not about 'advanced' knowledge, but your answer simply being wrong otherwise. – Raffzahn Apr 05 '20 at 13:26
  • @Raffzahn What are these typical consoles or home computers of the era in question that were generating video signals using bandwidth of significantly greater than about 6 MHz? – cjs Apr 05 '20 at 15:23
  • Wrong question, as that would invalidate your statement even further, by mistaking bandwidth used for bandwidth available. A composite signal in itself is not bandwidth restricted in either direction. No US console/home computer really used the bandwidth claimed for the video part either. For examples, take any Atari 2600 or TI 99/4 sold in Europe, as the sound carrier offset(!) (4.5 MHz for NTSC) is at 5.5 MHz for PAL (DE, UK, etc.) and 6.5 MHz for SECAM (France, Russia), both clearly beyond 6 MHz. Any console for these systems (PAL/SECAM) does need a 7 or 8 MHz channel. – Raffzahn Apr 05 '20 at 15:45
  • @Raffzahn If you think that the exact bandwidth available and used are important to understand to understand baseband vs. broadband signals and RF modulator channels 3 vs. 4 for these systems, you should feel free to add that all to your answer. I that that level of detail is not only unnecessary for this question but makes the answer worse by distracting with irrelevant detail. I'm sorry you can't respect other's opinions on that, but you'll just have to live with people posting here answers you don't like. – cjs Apr 06 '20 at 04:11
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If I may offer a concise answer:

Because while today's TVs have a multitude of direct input options - RGB on phono jacks, SCART, HDMI and others - there used to be only an aerial socket. So devices like VCRs and video games had to pretend to be a TV program, broadcast on a specific channel. Obviously it was a good idea to choose a channel that was 'spare', not being used in your area for a TV transmission. This would be different in different territories. For you it seems to have been Channel 4.

You could 'tune' the device's output channel, usually by adjusting a small control with a screwdriver. Maybe by chosing between a limited number of preset channels. Tune the tv to the empty channel, attach the device to the aerial socket, twiddle the control until the picture came up. It worked.

Laurence
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