Why the audio subcarrier is never transmitted with baseband analogue video signal?

Question

In the era of analogue video, when you needed to connect a local video source (STB, VCR, game console, DVD player, etc.), the most ubiquitous way to do so, next to using an RF modulator, would be to use a composite video connection. But it only carried a video signal - for audio you needed a second RCA cable. And as the audio is often in stereo, it became common to see triple RCA cables with colour-coded white, red and yellow plugs on both ends.

There are of course combination cables like SCART or D-Terminal, but let's put that aside for now.

Now the question is - why not just carry the audio signal, modulated in the same manner as in a broadcast channel, within the baseband video cable? It should not significantly affect the quality of either video (as the audio subcarrier is outside the video bandwidth) or audio (as the audio carrier frequency of, depending on region, 4.5-6.5 MHz - and its bandwidth - are high enough to encode all audible frequencies). Stereo could also be carried the usual broadcast way (MTS, A2 or NICAM).

I don't know what is the practical bandwidth for a typical composite video cable, but in PAL regions, the baseband signal already carries the 4.43 MHz chroma carrier - around the same frequency that NTSC (and PAL-M/N) audio is on (4.5 MHz), so I guess that suffices as a proof of feasibility at least in NTSC regions.

Analogue video devices used to commonly include an RF modulator anyway - so all the logic necessary for modulating that FM (or AM in France) audio signal was usually already there - so it shouldn't significantly affect the price of the device.

Such arrangement would allow to connect a baseband video source with a single cable - which seems like a pretty good convenience and selling point - connecting a bunch of identical plugs can be error-prone, especially with the less technology-savvy customers.

In addition, composite video cables can get much thinner, longer, more robust and cheaper than e.g. the afore-mentioned SCART cables - I can see many people willing to accept the sacrifice in quality (compared to S-Video or RGB offered by SCART - and many people wouldn't even see the difference) for the greater flexibility in cable arrangement gained this way.

So... why was it never done that way?

Answer 1

Because the video is baseband. That means that it is NOT modulated onto a VHF or UHF carrier. You are asking to take the audio modulated onto a HF carrier and combine it with the baseband video signal. And then to demodulate the audio back into baseband at the other end of the cable. That has the deadly combination of costing more, plus being lower quality. How is that "better"?

Remember that most consumer "entertainment electronics" started out as audio which already had the Left and Right channel RCA connector interconnection scheme well established. So it was easier to just baseband video on a separate RCA connector/cable.

Furthermore, audio and video signals very often travel by different paths and get treated differently by separate equipment both at the creation end of the chain as well as at the consumer en of the chain. So combining audio and video together is often more of a problem than a convenience. Here in the digital 21st century we must use ADDITIONAL gear to "break-out" audio from video, and then "re-combine" it, which frequently causes loss of A/V sync because it typically takes longer to process the much higher bandwidth video data than for the audio bitstream.

Answer 2

On analog TV, Video is Amplitude Modulation Audio is Frequency Modulation, at least on a broadcast signal. AM can be directly demodulated and converted into a wide band analog signal, usually 4.5 Mhz wide, meaning that it is just that. The broadcast FM signal has to be converted from FM to AM to make it audible... There is too much loss at low frequencies. More importantly, it would take too much circuitry, making VCRs and Boxes much more expensive..

You would have to convert from FM broadcast to AM audio (which all already do), then re-modulate the composite AM audio to a lower band FM signal that fits above 4.5 Mhz.....and then on the receiving end, decode it and convert it back to AM for amplification, etc.

You could. I have done this on specialized security camera systems where I only had a coax and needed audio.

On some systems, one has to also consider that there is composite stereo involved. Demodulating and re-modulating and demodulating requires proper emphasis curves, etc.

The simple answer is that prior to the 1990's many VCRs and even some Laser Discs had "RF OUT" connections that modulated a virtual TV broadcast signal so that a VCR could be connected via one simple connector, making it consumer friendly technology, that your grandparents could handle. However, almost all of the time, the VCRs, Set Top Boxes, etc, failed to pass composite STEREO TV via the coaxial connection.

Thus, there are tons of VHS tapes etc, recorded off of Cable, on stereo VCRs that are two channel Mono. It was a poor implementation of these systems that resulted in having the RCA cables, SCART plugs, etc.

There is less loss in going via audio and video cables versus modulated composite on a TV channel.

VHS recorders as well as Betamax record the audio as an FM signal on the videotape, either at a different angle, or as a subcarrier below the video.

In all reality, the Betamax system does just that, it places a composite FM signal with the video. But TV, such as NTSC was technology of the 40's and early 50's and it was not possible to change over all the TV sets in the world to accommodate a more bandwidth efficient system like consumer Betamax used.

The one advantage of ATSC as a TV signal is that the audio packets go with the video, although, technically, even those are separated packets in a stream.