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My understanding is that an analog video signal (PAL or NTSC) has a particular form/structure.

That signal can be transmitted over the airwaves at a particular frequency, & multiple analog TV signals can be transmitted at different frequencies.

Composite video is where a single video signal goes down a cable with RCA connectors at each end and is displayed on an analog TV.

Analog televisions do not need to be "tuned in" to the composite cable signal.

So I have a few questions:

What is the real difference between the signal on a composite cable and the RF signal that comes over the air?

I understand that the composite cable signal does not have audio, but apart from that, what is different?

What frequency is the signal transmitted down the composite cable?

Would it be possible to transmit at a different frequency down the composite cable, say for example at a frequency range that an analog TV could pick up?

And if yes, would it be possible to just have that signal transmit into the air, effectively making the composite cable a transmitting antenna?

And could an analog TV sitting nearby then pick up that analog TV signal?

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    \$\begingroup\$ In USA analog NTSC broadcast is no longer legal, since the FCC reallocted the frequency spectrum... however a lot of the technical details you are asking about are mentioned on this question on retrocomputing: retrocomputing.stackexchange.com/questions/14280/… \$\endgroup\$ – MarkU Apr 20 '20 at 6:35
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    \$\begingroup\$ Composite video (CVBS) is actually just a plain 'baseband' signal with a bandwidth of about 6MHz, it does not have an RF carrier frequency. If you mixed the baseband signal with an RF carrier of about 60MHz, you'd have a radio signal that an old analog TV set (in USA) would be able to tune in as channel 3. After the tuner de-modulates the signal from the carrier, what's left is the baseband signal. A TV signal is more complicated than an AM radio audio signal, but many of the concepts are the same -- there is a local oscillator (LO) that tunes the RF signal to baseband and demodulates it. \$\endgroup\$ – MarkU Apr 20 '20 at 6:47
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    \$\begingroup\$ @MarkU OK thanks I think this is a key piece of the puzzle for me - I need to research and understand the concept of baseband signal versus RF signal. So what I hear you saying is that the RF signal is the baseband signal modulated with a carrier signal that is then transmitted at a frequency that a TV can tune in to? Am I getting this right? \$\endgroup\$ – Duke Dougal Apr 20 '20 at 8:11
  • \$\begingroup\$ It's the same difference as between the signal coming out of e.g. the earphone socket on your hifi, and the signal transmitted and received from a local (analogue) radio station. The one is the original (baseband) signal, the other has been modulated at a higher radio frequency, a requirement for getting it to travel easily through free space without any wires (hence "wireless") from transmitter to receiver, whether TV or (audio) radio. You could also think of wired vs. wireless computer networking. \$\endgroup\$ – Reversed Engineer Apr 20 '20 at 15:35
  • \$\begingroup\$ @MarkU for reference "UNDERSTANDING THE FCC REGULATIONS FOR LOW-POWER, NON-LICENSED TRANSMITTERS" transition.fcc.gov/Bureaus/Engineering_Technology/Documents/… I'm guessing this would come under "Home-Built Transmitters that are Not for Sale". \$\endgroup\$ – Duke Dougal Apr 20 '20 at 21:53
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Composite video is widely used for surveillance and auto backup cameras, because it only requires a single coaxial cable to connect. The strongest "frequency" associated with it is about 15.7 kHz (horizontal sweep frequency) in USA-NTSC, and varies from country to country. The frequency bandwidth, however, is circa 4.5 MHz. Vertical sweep frequency (for USA-NTSC) is 60 Hz. More accurate standards applied to analog broadcast (but for a home setup, accuracy is not vital).

Yes, there are boxes that will convert video and stereo sound to broadcast analog TV (usually analog channel 3 or 4), Radio Shack 15-2526 and with a bit of power boost, you can drive an antenna (there are low-power allowances for such 'broadcast'). The 'multiple' available frequencies are... only those two, channel 3 or 4, with most such boxes. Channel 3 spans 60 MHz to 66 MHz, and Channel 4 is 66 MHz to 72 MHz, and usually you cannot transmit adjacent channels (they interfere) so you just get one at a time. The converter output is radio-transmission-ready.

What is the real difference between the signal on a composite cable and the RF signal that comes over the air?

There's no great similarity; the video is modulated in vestigial sideband (like AM modulation, but with narrowed bandwidth) and the audio becomes an FM signal at the high end of the bandpass. Sync is the highest AM power, black is slightly lower, and white is minimum. Stereo sound uses a second subcarrier. More NTSC details here There is no way that the composite (a baseband signal) can go out an antenna to any receiver; it just can't contain any channel 3 character.

There are many currently available TVs that still tune analog channels (necessary for those older video games), and/or accept composite video directly. There are also analog monitors (many computer monitors, and of course many surveillance and backup-camera monitors) that accept composite video.

And could an analog TV sitting nearby then pick up that analog TV signal?

No, BUT some other-than-television pickup might be managed; it's called Van Eck phreaking.

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    \$\begingroup\$ Um, at least for PAL, composite is literally downmixed broadcast signal. There's no greater similarity! \$\endgroup\$ – Marcus Müller Apr 20 '20 at 7:46
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The composite video is the baseband signal. The RF signal is basically the same composite video baseband signal, which is just amplitude modulated to higher frequency with the RF carrier signal. The tuner in a TV demodulates the RF and the result is the same composite video baseband signal. To send composite video to a TV antenna input, you use a simple modulator box, they were commonly used with home computers and gaming consoles 30-40 years ago. But transmitting it wirelessly via antennas, it's possible but requires a transmitter that transmits at the TV channel frequencies, which may not be allowed depending on your local regulations.

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  • \$\begingroup\$ Isn't the audio signal next to the video signals on the spectrum? Why does the audio signal require different cables? Bandwidth? \$\endgroup\$ – HH- Apologize to Carole Baskin Apr 20 '20 at 15:31
  • \$\begingroup\$ @horsehair Quality. Ever noticed how component video (three separate video signals for the brightness and colors) is substantially better quality than composite? Modulating several analog signals onto one wire will degrade the signals somewhat (though this particular case is worse than most due to the need to make it compatible with B&W tvs messing it up). \$\endgroup\$ – Hearth Apr 20 '20 at 16:46
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    \$\begingroup\$ @Hearth what's causing that degradation though? Some characteristic of the transmission line - is it bandwidth? Because free space is also a transmission medium and doesn't have that issue I assume \$\endgroup\$ – HH- Apologize to Carole Baskin Apr 20 '20 at 17:28
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    \$\begingroup\$ @horsehair I'm pretty sure a large part of it is just in the modulation/demodulation process itself. \$\endgroup\$ – Hearth Apr 20 '20 at 17:31
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    \$\begingroup\$ The composite signal really doesn't have enough bandwidth for luma and chroma channels. In order to make them fit, luma has to be band limited to significantly below what could otherwise be supported. If you don't do this, sharp patterns in the luma will turn into color fringing. \$\endgroup\$ – R.. GitHub STOP HELPING ICE Apr 20 '20 at 21:27
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Legacy television used RF carriers in the VHF and UHF bands. A standard NTSC TV channel occupied 6MHz:

enter image description here

(from here: https://www.physicsforums.com/threads/generating-tv-spectrum-in-ltspice-xvii.920362/)

Video and audio were sent separately, but still within the 6MHz channel:

  • Composite video: AM carrier as single (vestigial)-sideband at about 4MHz bandwidth.
  • Sound: FM carrier, 75KHz bandwidth, 4.5Mhz offset from video channel base.

And the TV channel map:

enter image description here

(from here: https://otadtv.com/frequency/index.html)

When ATSC replaced NTSC, the 6MHz-wide VHF/UHF channels were reallocated to ATSC digital ones, but like NTSC are still single (vestigial)-sideband and 6MHz. This helped with the conversion process, as over-the-air and cable RF hardware and infrastructure could be reused with the new digital signal.

ATSC channel spectrum, for comparison:

enter image description here

You'll notice from the spectrum that great pains are taken to use the entire 6MHz channel, with no gaps. To do this, ATSC uses a modulation scheme called 8-VSB, sending a digital feed using an MPEG-2 Transport Stream multiplex that carries compressed video, compressed audio and other kinds of data streams. Compression and efficient coding - that's what it takes to get 19.4 Mbit/s down that 6MHz bandwidth so you can watch sportsball in glorious 1920i or 720p 16:9 HD. (It's hard to believe that HDTV has been around for more than 20 years now, but here we are.)

Anyway, you were asking about old-school analog TV. It’s still possible to modulate analog TV onto an RF channel. This was popular with early video games that sent video over channel 3 or 4, and some camcorders had this ability as well. The reason for this is TVs didn’t have A/V inputs until fairly late (mid-80s or so), they only had antenna 'clip-leads' or a cable coax input, both of which required RF.

That said, it’s difficult now to find a recent TV set that can receive analog over-the-air TV, though most still accept composite video.

If you find an old VCR at a yard sale or in grandma’s attic, that device can receive legacy TV and output composite + audio, and also take composite + audio and send it out as RF on channel 3 or 4.

There were also boxes called 'TV extenders' that allowed multiple viewers from the same cable box or VCR. 'TV Rabbit' was an early, popular one.

There were later units that used 2.4GHz, for security, baby monitor and other legacy CCTV uses. Seems like you can still get those: https://www.amazon.com/AV220-3-Extender-Wireless-Transmitter-Receiver/dp/B07HRK2XNH

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  • \$\begingroup\$ Just to make this part clear, early video games and VCR's did not transmit over the air on channel 2 or 3. They were coupled directly to the television set with a cable, and the signal went to the receiver directly via the cable without ever going over the air. The power level coming out of a VCR or video game is likely not enough for transmission. \$\endgroup\$ – mkeith Apr 20 '20 at 7:19
  • \$\begingroup\$ @mkeith well, anything that moves charges in a cable is enough for transmission, the question is whether that transmission is strong enough for any given receiver in any given distance to pick up. I'd honestly assume that if you build a decent enough transmit antenna, this would work, for a couple of wavelengths distance between transmitter and receiver. \$\endgroup\$ – Marcus Müller Apr 20 '20 at 7:49
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    \$\begingroup\$ Remember 'VCR Rabbit'? And those video extenders that used 2.4GHz? Both did exactly that: rebroadcast composite signal. \$\endgroup\$ – hacktastical Apr 20 '20 at 17:55
  • \$\begingroup\$ Yeah I should have said not enough for reception over any great distance without an amplifier. But comments can't be edited. I have no doubt that Maxwell's equations work just as well at uW as they do at mW. \$\endgroup\$ – mkeith Apr 20 '20 at 19:41
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Analog televisions do not need to be "tuned in" to the composite cable signal.

Huh, you need to select "composite input" on your TV, don't you? That means it sets up its internal receiver chain so that it can deal with the signal.

And it's really what happens. Essentially, selecting the composite input configures your TV to tune to a really low frequency, which it receives through that RCA connector.

So I have a few questions:

And we might have a few answers!

What is the real difference between the signal on a composite cable and the RF signal that comes over the air?

The carrier frequency. In fact, in early 1990s lingo, when I started playing around on a TV, the technical term used in the manual was "FBAS" signal (and that's German for Farb-Bild-Austast-Synchron-Signal, Color Video Blanking Synchronization, CVBS in English). And it's identical to what is in picture information on your favourite analgue TV station at their carrier frequency, but at a carrier frequency that's effectively putting the whole signal symmetrically around 0Hz – Baseband.

I understand that the composite cable signal does not have audio, but apart from that, what is different?

Nothing, really. It's more or less PAL, SECAM or NTSC (depending on which analog TV standard your composite video source produces), mixed to baseband.

What frequency is the signal transmitted down the composite cable?

Well, strictly speaking, the carrier would be at 1.25 MHz, I think, with the signal then extending from somewhere above 0 Hz to ~ 5.25 MHz, depending on the video bandwidth.

Would it be possible to transmit at a different frequency down the composite cable, say for example at a frequency range that an analog TV could pick up?

It's already doing that.

Also, no. Composite video cables are a nightmare. They are better (if even) audio cables. The fact that you shove through megahertzes of bandwidth through an RCA connector is already an indication of how terribly designed that whole consumer standard is:

Low cost above literally everything.

And if yes, would it be possible to just have that signal transmit into the air, effectively making the composite cable a transmitting antenna?

Modulator boxes. Was very common before every TV hat a composite input.

And could an analog TV sitting nearby then pick up that analog TV signal?

Be very careful about transmitting in a band that your analog TV can pick up – that's illegal. These frequencies are either still in use for analog TV or have been repurposed for digital TV or other applications (this depends on your country's legislation), and you are not entitled by law to use that spectrum.

So, yes, modulator + amplifier + antenna = transmitter, but also = hefty fines or jail if someone else also picks up your signal and reports you to your national spectrum authority (FCC, BNetzA, OFCom, …).

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  • \$\begingroup\$ So are modulator boxes now illegal? I mean the ones that transmit signal to RF that can be picked up over the air from a TV with rabbit ears. \$\endgroup\$ – Duke Dougal Apr 20 '20 at 8:16
  • \$\begingroup\$ @DukeDougal these were, to my knowledge, never legal in Germany, but might have been legal for example in the US if their transmit power was low enough to be very restricted in how far they are detectable. I don't think there's a single place on earth where it's legal to just "transmit" on broadcast TV spectrum with enough power for your neighbors' neighbors to pick up without having acquired a license for that part of the spectrum. I don't know whether laws revolving that have changed – also, I don't even know which laws apply to you, so "illegal" is a very hard term to define. \$\endgroup\$ – Marcus Müller Apr 20 '20 at 8:26
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Composite video is a baseband signal. The frequency range goes from DC to somewhere around 6MHz. As as baseband signal it is not suitable to be directly transmitted over the air.

The basic video signal is split up into frames and lines by sync pulses. Before the start of each line a reference signal known as the colorburst is transmitted. This is used to synchronise the QAM demodulator.

Within each line, the baseband signal carries the luminance, while the two chrominance channels are QAM modulated and superimposed. This is far from a perfect system and crosstalk between the luminance signal and the two chrominance signals is a fact of life with composite video.

For radio transmission the video signal was modulated onto a carrier signal of much higher frequency, somewhere in the hundreds of megahertz, to save bandwidth a vestigal side band modulation scheme is used rather than regular amplitude modulation. The audio signals are modulated onto seperate RF carriers.

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