Video Home System (VHS) bandwidth

Question

Standard play VHS video cassette recorders offered only about 3 MHz luminance bandwidth by recording the signal on an FM carrier. What is that carrier frequency? Is it possible to confirm the carrier frequency or even the FM deviation by analyzing archived VHS recordings?

Later edit: Since answers are describing the chroma handling in VHS recording I point out that it is only the luminance that I seek information about. Specifically, how is a European Teletext waveform distorted? Teletext is a non-return-to-zero digital stream at 6.9375 Mb/s added to luminance on spare lines in the vertical blanking interval. Recovering Teletext from archived VHS recordings would require detecting the phase of the clock run-in stream at half bitrate i.e. 3.46875 MHz which is below the noise AFAIK.

Answer 1

For video the carrier is around 3.4 --> 4.4MHz (For NTSC)

for sound its 1.3 to 1.7MHz

http://www.repairfaq.org/REPAIR/F_vcrfaqd.html#VCRFAQD_001

https://books.google.co.uk/books?id=NXEIVQrhKP4C&pg=PA295&lpg=PA295&dq=VHS+video+carrier&source=bl&ots=KKHuRBUTXs&sig=nieUO1jEaV17-_syjPm7Pr-jzwU&hl=en&sa=X&ved=0CDgQ6AEwCTgKahUKEwjpzP3Q9Y3GAhUqWdsKHU2CACE#v=onepage&q=VHS%20video%20carrier&f=false

Answer 2

I went and looked up the early papers from the JVC engineers. Unfortunately they don't seem to be available outside of IEEExplore. Presumably this information can be found in textbooks on magnetic recording.

For recording the luminance, the video signal is low-pass filtered, given pre-emphasis, then FM modulated with carrier "3.4-4.4 MHz". I'm not clear on why this is a range.

The chrominance is band-pass filtered, boosted by 6 dB, mixed down from 3.58 MHz to 629 kHz, and low-pass filtered. Thus it sits 'under' the luma in the combined signal.

There is a further wrinkle to the chroma that is pretty interesting. The azimuthal recording technique effectively reduces crosstalk from adjacent tracks in the case of mistracking, but only for the luma frequencies. The low-frequency crosstalk for the chroma can be much worse, so additional steps are required. The phase of the chroma signal gets advanced by 90 degrees every horizontal period. During playback, the phase is restored, and the signal added to a delayed copy of itself. (This is for NTSC, the process is slightly different for PAL.)

I don't see any reason you wouldn't be able to see this if you analyzed the signal recorded on a tape. You'd have tap into it before demodulation though.

1: http://dx.doi.org/10.1109/TMAG.1978.1059827

2: http://dx.doi.org/10.1109/TCE.1978.267051

Answer 3

Note: When I wrote this answer, I assumed, like other signals, that the composite video signal was recorded directly to tape with only the odd quirk of being striped diagonally instead of a continuous timebase. It turns out that that's not true. The color information is actually decoded and re-encoded differently for tape, then decoded again and re-encoded back to the composite standard for playback. So I learned something new this week. See the other answers for more detail.

Original answer is as follows:

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VHS video as I understand it is simply a composite (RCA yellow) video signal recorded on magnetic tape, along with a separate analog soundtrack. This signal is divided into 1-frame increments that are stored diagonally across the tape, hence the angled, spinning head if you've ever looked inside a VCR. It's not RF modulated except as defined within the signal itself for color information. Googling "composite video standard" without quotes gives lots of results about how the signal works. Here's a good one from google images:

For black-and-white video, there is no RF modulation at all. Just an instantaneous DC level that gets amplified and sent to the electron gun at the back of a CRT (tube) display. For color, and to maintain B&W-compatibility, the difference from pure white is encoded in two mixed AM waves of the same frequency and different phase compared to the color burst that appears between each line of the picture. This frequency is 3.579545MHz, but it only applies for color video. B&W displays just don't have the bandwidth of their own to respond to that, so they only react to the average level, which is precisely the luminance signal that they always used.

Also to note are the negative-going H and V sync pulses; the only difference is that that V. sync covers an entire line while the H. sync is only a blip between the previous line and the color burst.

In summary, a color composite video signal requires 3.579545MHz, so any equipment that supports it must have at least that much also, but don't be surprised if it's only barely, like maybe 3.7MHz or so.

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As a side-note, analog TV broadcast was simply that composite signal on another AM radio wave. That's 2 layers of AM for color. Audio was FM on a different frequency, just like analog radio broadcast, so the TV required two tuners to receive one channel. This is probably why analog TV tuners for computers often include FM radio but not AM radio - the hardware is already there without modification.