Analog video recording principles

[DVtyro]

Originally Posted by old_tv_nut

It's the mathematics of what the TIME waveform of the FM signal looks like, and how it is composed of a combination of pure sine waves that individually have a constant frequency and amplitude (the spectrum).

With the extreme case where the luma consists of a high-frequency alternating stripe pattern having peaks and troughs and a certain average level, the TIME waveform of the FM signal shows the FM carrier frequency changing rapidly and repeatedly from low frequency (wide cycles) to high frequency (narrow cycles) representing the troughs and peaks of the luma waveform. Fourier analysis shows that this TIME waveform of the FM signal can be obtained by adding a sine wave with the (constant) average frequency to sine waves of the sideband frequencies. The plot of how much of the average frequency sine wave is there and how much of the sideband sine wave is there is the frequency domain plot of the signal.

Um... Not sure I understood this. Can you explain in layman terms? Say, I have a certain pattern, it is described by the sideband, right? Then I have the same pattern, but brighter, this means that it should look like the previous pattern, but moved to the right? But the frequencies, numerically, will change, they will all move to the right. How the display device will know that it is the same pattern unless it displays the image in relation to the [ever changing] middle frequency?

Thinking about it now, maybe this can be likened to a more simpler case of analog tape recorder. If tape plays faster than normal (so the average frequency is higher) I still hear music, just all notes are shifted, but the intervals between them remain the same. Not sure though, how video patterns correlate to music tones - is it a good enough analogy?

[old_tv_nut]

Let me see if I can make a few drawings. This will take a while. Unfortunately, I have reached my limit for posting images to this site, so have to host them elswhere and link here.

Meanwhile, when you ask about "patterns," please specify if you mean a pattern in the video image, in the video time signal, or in the FM spectrum.

[DVtyro]

Originally Posted by old_tv_nut

Let me see if I can make a few drawings. This will take a while. Unfortunately, I have reached my limit for posting images to this site, so have to host them elswhere and link here.

Meanwhile, when you ask about "patterns," please specify if you mean a pattern in the video image, in the video time signal, or in the FM spectrum.

Thanks, I appreciate your help! When I say pattern I primarily mean video image.

[Electronic M]

It's the instantaneous time domain changes that determine the pattern on screen. You can have more than 3 different video frequencies in effect on a segment of a line of video and the phase relationship of those video frequencies determine where on the line they add to and or subtract from each other and the nature and position of the pattern they make.
The side bands don't exactly describe the just the signal. If you modulate one sine wave with another regardless of the modulation scheme you get the main sidebands and an infinite series of harmonics at multiples of the carrier. You also get additional harmonics from non-linearities in all non-ideal (read that real world) modulators. (Ever been close to a radio tower and gotten the station on multiple places on the dial? Those are harmonics.) So there can be things in a spectrum analyzer view of a sideband that are more incidental energy than desired ideal signal.

The passband of a signal chain tells you what frequencies will be let through, but not whether in actual video they're intended or unintended to be in the picture. Without the precise phase timing duration and the rate of rise and fall of the different video frequency elements you could get a variety of patterns and images from the same sideband pattern you see from a line or frame duration sideband spectrum sample.

[old_tv_nut]

Does this help?

[DVtyro]

Um, sort of. I think I understood that the whole spectrum would move, but I did not understand how a TV set would know that the frequencies that moved to a different location represent the same image. I suppose, the TV set "measures" it relative to the mid frequency and it can detect this mid frequency automatically, just like a wow & flutter tester can, if you have two sidebands, just measure the full difference from left sideband to right sideband and divide by half. But the right sideband is cut off, so I am not sure how this works.



Anyway, I think I know enough to be able to read the charts and to make some conclusions, like you can have the most detail at the brightness close to the max, and without color it would be even higher.

BTW, I've heard that analog TV signal used to have a "flag" that indicated a program as color or B&W. Was it widely used? If a VCR was connected to HF antenna cable, it was able to receive this flag. Did VCRs act upon it? Did they omit recording chrominance? Is this flag sent over composite/SVideo?

[old_tv_nut]

Originally Posted by DVtyro

Um, sort of. I think I understood that the whole spectrum would move, but I did not understand how a TV set would know that the frequencies that moved to a different location represent the same image.

It is not the TV that interprets the FM signal. The VCR has an FM decoder that reconstitutes the baseband video signal you see in the top row of drawings.

The time domain plot of the FM signal during the pattern shows the carrier frquency during the pattern varying directly according to the instantaneous baseband voltage in the top row diagram.

So, just picking some numbers out of the air thatcould apply to the HI-8 system:

If the baseband video pattern average is mid gray voltage, meaning the sine wave varies equally above and below mid gray, the instantaneous FM time signal might vary in frequency from 6.8 MHz to 7.2 MHz. The instantaneous FM frequency corresponds exactly to the instantaneous video baseband voltage. The FM demodulator converts this instantaneous frequency back to the exactly corresponding video baseband voltage, forming the signal that is sent to the TV. The FM spectrum is only telling you that you can make this varying frequency FM time signal by adding togehter a certain collection of sine waves that each have their own constant unvarying frequency and amplitude.

This is the genius of Fourier, that he showed this is possible - to decompose a varying waveform into a sum of unvarying waveforms. In this case, exact replication of the FM signal requires a center frequency and two sidebands. (As mentioned before, the tape system loses the upper sideband, so the recovered M signal then has amplitude modulation in addition to frequency modulation - but the FM decoder ignores the amplitude modulation.)

Now suppose you have in the baseband video signal the same pattern with the same peak-to-peak amplitude, but its positive and negative peak voltages are both shifted lower by the same amount. All instantaneous FM time signal frequencies are now shifted lower by the same amount compared to the mid-gray case (for example, 6.5 to 6.9 MHz), because the video baseband voltages that control the frequency have been shifted by a particular amount. The FM demodulator sees these shifted frequencies and produces a baseband video with shifted voltages that match the original. The spectrum plot just shows that all the frequencies of the first case are replaced with shifted frequencies in the second case, and the amount of shift is the same for all frequencies.

[old_tv_nut]

"Anyway, I think I know enough to be able to read the charts and to make some conclusions, like you can have the most detail at the brightness close to the max, and without color it would be even higher."

Correct.

[old_tv_nut]

"BTW, I've heard that analog TV signal used to have a "flag" that indicated a program as color or B&W. Was it widely used? "

https://en.wikipedia.org/wiki/Colorburst

Color burst MUST be present in an analog color signal or the color cannot be recovered. An analog TV would detect the presence of burst to turn on the color circuits, thus preventing colored noise in monochrome programs.

Eventually, sloppy practice (especially in analog cable systems) left the color burst on for all programs.

I believe VCRs also detected the burst so that color could be turned off for monochrome programs, but I don't know for sure or if it varied in different products.

[old_tv_nut]

"Is this flag sent over composite/SVideo?

Yes, both, as it is necessary for the analog color circuits to work.

In VCRs it is carried in the color-under signal just like the color subcarrier for the image content, since it is just a sample of the reference chrominance subcarrier. It is used in the VCR's special fast acting correction circuitry to take out variations due to mechanical tolerances and provide a stable chrominance signal for output to the TV.

[Electronic M]

Back in the tube era they would change the horizontal and vertical scanning rates slightly for color TV. I believe one of the things that started to reduce that practice was VCR timecode being used in broadcast automation....The time code was based on frame count and it probably would be hard to handle it accurately every time if it changed between color and monochrome.

I suspect color network logo watermarks in the programming contributed to the constant burst practice, since if the CEO tells you the watermark has to be in color and not change no matter what the program is then the burst has to stay on.
IIRC in the 70s didn't PBS do a thing where they made their bursts ultra precise so NIST could use them as a lab calibration reference?..If that was the case they may have had to keep burst on during monochrome shows to make that work, and may have been doing constant burst before cable.

[ppppenguin]

It is possible to regenerate colour from a NTSC or PAL signal that lacks its colour burst. It was called chromalock and was not reliable. Worse in PAL because it has no information aboujt the polarity of the PAL Switch which had to be set manually.

Why do this at all? Because sometimes the burst would be removed to prevent colour being received. I forget why this was sometimes done.

NTSC and Timecode. AAAARRRRGGGHHH!!! I'm looking at the old NTSC papers in the IRE journal from 1954 which explain the reasons. The H and V frequencies were reduced by 0.1% to minimise patterning that could arise between the sound carrier and colour subcarrier. This avoided retuning the sound circuits on monochrome sets. It seemed harmless at the time. Some years later timecode was invented. The lack of a sensible relationship between frame rate and time of day has caused massive amounts of hassle.

The relationship between subcarrier and H had to be fixed (at an odd multiple of H/2) to minimise visibility of subcarrier on monochrome sets.

PAL is more complex (quarter line offset + half frame offset) which doesn't have a problem against the sound carrier. Hence we have no timecode problems in 50Hz countries.

[Electronic M]

Originally Posted by ppppenguin

Why do this at all? Because sometimes the burst would be removed to prevent colour being received. I forget why this was sometimes done.

ISTR reading this was done in Israel in the 70s because they thought color TV was an unfair luxury....All it did was create a market for systems that would try to achieve color sync without the color burst.

[old_tv_nut]

Originally Posted by Electronic M

Back in the tube era they would change the horizontal and vertical scanning rates slightly for color TV. I believe one of the things that started to reduce that practice was VCR timecode being used in broadcast automation....The time code was based on frame count and it probably would be hard to handle it accurately every time if it changed between color and monochrome.

I suspect color network logo watermarks in the programming contributed to the constant burst practice, since if the CEO tells you the watermark has to be in color and not change no matter what the program is then the burst has to stay on.
IIRC in the 70s didn't PBS do a thing where they made their bursts ultra precise so NIST could use them as a lab calibration reference?..If that was the case they may have had to keep burst on during monochrome shows to make that work, and may have been doing constant burst before cable.

You've got a close idea, but missing some points.

The monochrome standard had wide tolerance for scan frequencies; the FCC broadcast rules required the vertical and horizontal to be locked to each other for a precise interlaced line count, but the master rate could vary; in fact, the rate could be tied to the station's 60 cycle power and still be within tolerance, as the power companies maintained average correct frequency so synchronous motor electric clocks would tell the right time. In practice, stations adopted crystal references for the scanning frequencies, which were 60.00 Hz and 15750 Hz. An additional requirement was placed on the carrier frequencies such that the undeviated (silence) frequency of the audio carrier was 4.5 MHz above the video carrier with a rather tight tolerance. This was to allow TVs with separate audio IFs to fine tune the audio and video correctly simultaneously.
When color came along, there was concern with ~ 920 kHz video beat beween the color subcarrier and the undeviated audio being visible in monochrome sets. Because of the precise scan and audio frequencies used in monochrome, you could choose the color subcarrier to interleave (be less visible) with the video, but then it would be worst case for the 920 kHz; or vice versa. Mathematically you could not optimize both. The decision was made to change the scan rates and keep the 4.5 MHz audio spacing exactly as before, to prevent sound problems in legacy monochrome sets. So, the scan rates were changed by the ratio 1000/1001. Now the color frame rate ran slow compared to a 60 Hz wall clock, necessitating the invention of time code to make the conversion from frame count to seconds, minutes, and hours for program duration.

[old_tv_nut]

Details continued:

The color burst had to be present for color programs, of course, but there's a gray area for monochrome. If the monochome broadcast was at the old monochrome scan rates (as often happened in the early days) then the color burst had to be omitted because it would become a color broadcast at illegal scan rates for color. Early on, there could be a lot of hot switching between different sources.

If the local station was equipped for color and always operated at color scan rates, then I suppose they could leave the burst on and argue that they were sending a color broadcast of a monochrome image, but I believe they always followed the practice of turning off the burst for monochrome programs even if they used the color scan rate throughout their plant. This allowed color sets to turn off the color circuits on monochrome programs and eliminate any color noise as well as cross-color (high frequency monochrome info being interpreted as color "twinkles").

Things really got messed up in analog cable TV systems. Regulations required that a clean color burst and sync be inserted into all incoming signals before they went out on the cable. Headend equipment, as far as I know, typically did not include the capability to turn the outgoing burst on and off, so that monochrome programs were sent to the home with the burst on.

Regarding the use of burst for a high precision frequency reference, this started prior to the use of frame synchronizers at local stations. The three major networks each installed a master atomic clock, and the local stations would lock to the network, so that the scan and burst frequencies had accuracy approaching that of the National Bureau of standards. You could actually sync the video from one network with the video of another except for a constant offset in time delay. NBS then published a paper on using the color burst as a tight-tolerance frequency reference. The networks' purpose had been to prevent vertical flipping due to hot-switching sources, but that also created the precise reference frequency.

After the development and installation of frame synchronizers in local studios, the locals went back to their own crystal references, so the use of the burst for precise frequency reference was lost.

Today, digital broadcasts are all locked to GPS time, even more precise than the original network atomic clocks. This allows precision offset of stations' carrier frequencies and phase for minimum co-channel interference, and even the implemention of single-frequency networks where multiple transmitters cooperate to cover a given service area.