Analog video recording principles

[DVtyro]

First off, thanks to everyone who have been answering my questions about analog video recording formats. I re-read the answers and digged into a couple of books, but I still have questions!

I would appreciate if someone fills the blanks in my understanding of video signal modulation, storage, bandwidth, resolution, etc taking into account that I am not an electronics engineer and don't plan to become one, just want to have a little clearer understanding of how things work and why one format is better than another.

I've been reading some introductory texts on the topics of analog radio, television and color-under recording systems. I'll go with what I've learned (or I think I've learned), and please correct me where I am wrong.

First, how many samples per second we need to send, this is a rather simple idea: take frame size times frames per second, say 525 lines total, 4/3 aspect ratio with square pixels will give 525 * 4/3 = 700 samples per line, time 30 fps: 525*700*30 = 11 million samples per second.

Each wave period has two half periods, and I thought that each of them could be used to describe brightness of a pixel, so 11 million / 2 = 5.5 MHz. At this point I had a naive idea of how things work, I thought that this wave can be manipulated (a.k.a. modulated) so that amplitude of the each of its half-periods could be increased or decreased, this would give us 5.5 million unique samples. Based on this, I could not understand why do we need a RANGE of frequencies, why we cannot use ONE frequency modulated this way.



Apparently, this is not how things work in real life Instead, I need 5.5 million of DIFFERENT FREQUENCIES, each frequency representing a pixel on the screen. To help me understand this, I was reading a book on analog radio, so the example was, like, real music has range of frequencies, say from 100 to 10 KHz, and want to send all of them. So, if I modulate the carrier with this signal, I will get a BAND with two sidebands, corresponding to the carrier minus signal and carrier plus signal. Ok, I get this.

OTOH, I am not sure that sending audio over the air is exactly the same as sending samples on a screen… But if this analogy is correct, then each of the samples on screen is represented by its own frequency, just like each tone in music is represented with its own frequency. Is this correct so far?

So, the position of a sample is represented with frequency, the intensity can be represented with amplitude. This is how AM radio works, and this is how OTA TV works - luminance use amplitude modulation (NTSC/PAL color uses AM, SECAM color uses FM, audio uses FM).

Amplitude modulation is easy to represent with a picture and to understand. I've read arguments for FM modulation, so it seems that all signals on tape are FM modulated: luminance, chrominance and audio, correct?

Next, broadcast video is converted for recording on tape. Because of various technical difficulties, AM signal cannot be used for quality video recording, so first the luminance is separated from the chroma, then the luminance bandwidth is reduced, then the luminance signal is then used to FM modulate a carrier before it is recorded on the video tape.

It is harder for me to make a mental picture how FM modulation works. The first part of the modulation is the same with AM, two frequencies are added together, producing two sidebands. When recording on tape, carrier is chosen high enough for the upper sideband to basically be removed automatically just because such high frequencies cannot be recorded on tape. So this issue solves itself. Lower sideband is used for luminance signal, I get it so far.

I suppose everything else is FM-modulated as well including chroma? Including NTSC and PAL chroma?



But the sideband represents the POSITION of sample onscreen, right? To represent the brightness, the signal modulates the carrier frequency, not the carrier amplitude. This is what DEVIATION is about, it is represents brightness. The higher the deviation, the larger the difference between black and white, or in other words, better contrast and lower S/N.

What I don't get is how brightness corresponds to the sample position (or in case of audio, how loudness corresponds to audio frequency)??? If the carrier is modulated by frequency, it is moved up and down, but the sideband that describes the position of a sample, is relative to the carrier, how does it work???

The book on radio that I am reading, re-iterates: the "rhythm" (well, frequency) of the carrier deviation corresponds to the signal frequency, while the amount of the deviation corresponds to the brightness (loudness). But I still don't get how the two parts: the sideband, that describes the POSITION, and the deviation that describes BRIGHTNESS are reconciled together. I can that with AM the carrier does not change, so whatever the distance from the carrier is my frequency (audio tone or sample position). But in case of the FM the carrier constantly changes, how do I know what actual audio tone or sample position it corresponds to??? Please, explain!

Now looking at this picture, which is one of the series of graphs about VHS/SVHS/Hi-8, I have several questions.



First, what is "220% peak white limit", orange question mark. It is my translation from non-English text, but it corresponds to Panasonic info, which says about SVHS: "A 210% increase in the peak white level enhances the picture quality even more, with excellent delineation of image borders and highly faithful reproduction of detail." - does this mean that SVHS has 210% increase in the peak white level compared to VHS, and the above picture that describes VHS is incorrect? Also, "white level" is brightness, and brightness is defined by deviation of carrier frequency, not by amplitude, but the picture shows the amplitude. Is it a mistake, or what does it mean?

Second, red question mark: what is this frequency, from which the sideband is measured? It is not black level, not white level, not the middle. Is it 70% level of the deviation band, because the average value of a pure sine wave is 0.7 of max value?

Third, green question mark: chroma. It has both sidebands. Is it FM or AM modulated? If it is FM modulated, it should have its own deviation? Is chroma bandwidth of all color-under systems the same? If not, why it is not specified? The increase in chroma bandwidth means the increase in chroma resolution? But apparently this is not what they did, they only ever increased luma resolution. Why increasing chroma carrier is touted as a big deal, improving picture? Isn't the only thing that matter is that chrominance and luminance do not collide?

P.S. Please excuse me for using chroma and chrominance interchangeably.

[old_tv_nut]

"Instead, I need 5.5 million of DIFFERENT FREQUENCIES, each frequency representing a pixel on the screen."

Stop right there! Each spot on the screen does not have its own frequency.
You need to understand Fourier transforms, which calculate the frequencies produced by a sequence of dark and light spots.

So, a particular frequency (5.5 MHz in your example) produces a continuous sequence of alternating dark and light spots. A frequency of 2.75 MHz would produce a continuous sequence of two light spots followed by two dark spots, and so on. (In analog, since there are not discrete pixels, this would just be a coarser pattern of wider light and dark areas.)

So, each frequency in the signal affects ALL the spots on the screen, but in patterns of different coarseness and strength (contrast). The overall spectrum of a signal represents how much of each frequency is present such that when all these corresponding time patterns are combined, they form the sequence of light, dark and gray spots to make a scanned picture.

If you can first understand this, then we can discuss your further questions, and maybe see why some of them make sense and some don't.

[DVtyro]

Ah! Ok, I get this now. This is the relationship between frequency and resolution. Higher frequency creates a finer pattern of bright and dark samples, which means more detail. And a complex image is made of a combination of different patterns. I think I get it now without learning too deep about Fourier transforms. BTW, this video is awesome! I think I understand the concept now.

So, there is a "cloud" of frequencies around the carrier representing a complex pattern. This cloud changes every 1/60 of a second for interlaced video. Well, actually it changes constantly, as analog video is just one big string of samples. I guess my attempts to imagine how it would look like in static are pointless, because frequency by definition presumes something happening over time.

I am not sure this helps me understand the relationship between the pattern onscreen and some of its parts being brighter or darker.

[old_tv_nut]

Ok, so you now have the idea that you need to keep in mind whether you are discussing time domain or frequency domain.

So:
"What I don't get is how brightness corresponds to the sample position (or in case of audio, how loudness corresponds to audio frequency)??? If the carrier is modulated by frequency, it is moved up and down, but the sideband that describes the position of a sample, is relative to the carrier, how does it work???"

You should now understand that loudness does not correspond to audio frequency, but to audio amplitude. And that a particular video sideband does not describe the position of one sample, but a repeating pattern of light and dark.

[old_tv_nut]

Regarding the 220% peak white limit:

Before the Freqency Modulation, the luma signal is "pre-emphasized." Thiis means the high frequencies are amplified more than the lower frequencies. Since the high frequencies are involve in producing sharp edges, this means that (in the case of a black to white transition) instead of going from the normal black level to the normal white level, the pre-emphasized luma signal continues to rise above the normal white level for a while and then comes back down to normal white. During playback, the demodulated luma signal is subject to a corresponding de-emphasis that restores the transition to its normal black-to-white transition. Because de-emphasis is reducing the high frequency components of the signal, it also reduces any high frequency noise generated by the tape. A similar technique is used in FM radio broadcasting, and even in recording LP records. Dolby noise reduction and other noise reduction methods go further and use a variable pre-emphasis and de-emphasis to adjust the amount of it depending on whether the audio is soft or loud.

Here is a web site that shows a little-known (these days) video recording system that used variable pre-emphasis, and compares it to VHS's fixed pre-emphasis (Figure 1):
http://www.digiommel.fi/Improving%20...%20Quality.pdf

[DVtyro]

Well, ok, a sideband describes a pattern of light and dark.

Thank you for explaining the pre-emphasis thing. Yes, I know how Dolby works, this helps!

But what about the deviation It defines brightness. How does it correspond to the pattern? Does it define the brightness of the whole pattern (frame) or for each sample? I suppose it should somehow act on separate samples to make certain parts darker or brighter.

Regarding 220%, here is a Sony picture. Here the luminance bandwidth goes from the carrier to the left, not from like 0.7% of the deviation.

[old_tv_nut]

"Second, red question mark: what is this frequency, from which the sideband is measured?"

The video signal has 40 "IRE units" of sync and 100 units of black to white, so a mid gray sits at 90 units, which is about 68% of the total deviation from sync tips to white. So, that frequency corresponds to middle gray.

[old_tv_nut]

Look at the Sony diagram again - 7.0 MHz is the FM frequency for mid gray, and the bandwidth of the signal is shown measured down from that. If you think about this for a while, you will see that there is more bandwidth for high frequency detail when that detail is riding on a higher brightness background, and some what less when it's riding on a dark background.

Now suppose the image has a full black-to-white fine repeating pattern in some area. In that case, the average background in that area is mid gray by definition, since it is the average of alternating black spots and white spots. This will produce a sideband frequency at the lower limit shown in the diagram. It would be impractical to draw a diagram showing every case of detail amplitude and background gray level, so systems are compared using the kind of diagram you have posted.

[old_tv_nut]

Chroma is quadrature AM modulated. In fact, it is the same as the chroma signal in the original NTSC signal, just shifted in frequency. It could not be FM modulated unless it was first split into two separate color components with each modulated on a separate FM carrier.

[DVtyro]

"there is more bandwidth for high frequency detail when that detail is riding on a higher brightness background" - indeed! I see it!

The VHS pre-emphasis picture shows 14 dB gain, which is about 500%, not sure where 220% came from.

"Now suppose the image has a full black-to-white fine repeating pattern in some area. In that case, the average background in that area is mid gray by definition, since it is the average of alternating black spots and white spots. This will produce a sideband frequency at the lower limit shown in the diagram. It would be impractical to draw a diagram showing every case of detail amplitude and background gray level, so systems are compared using the kind of diagram you have posted." - this I did not quite get. What is "a sideband frequency at the lower limit", does it have numerically low frequency closer to the left, or does it mean that the difference (I don't want to use deviation here) between this frequency and the carrier will be narrower, so the actual frequency will be closer to the carrier? I am looking at the part to the left of the carrier. And I still haven't reconciled in my mind how the same pattern can be reproduced with different brightness.

Say, I have a pattern, that is, a complex frequency. If I want to make it brighter, I increase the carrier frequency, but then my whole pattern moves to the right, so a frequency that was, say 2 MHz, becomes 2.5 MHz, which means it is a different pattern!

Looks like I am still missing something. I am going to shut up for a while and let you finish

[old_tv_nut]

"The VHS pre-emphasis picture shows 14 dB gain, which is about 500%, not sure where 220% came from."
1) The pre-emphasis is in the frequency domain. The picture sledom will include a full amplitude single frequency, so a 100% black to white excursion of one frequency is uncommon. More commonly, this is emphasizing the high frequencies that are part of making a step edge between black and white.
2) If the pre-emphasis does produce a waveform that goes over 220%, the luma signal is limited to 220% in the time domain, this preventing overload of the FM demodulator circuit. This means that if the original signal was a very high frequency stripe pattern of high amplitude, it would be clipped to a lower amplitude before FM modulation and therefore appear as a lower amplitude when demodulated. This is a good trade-off for better-defined, less noisy edges.

[old_tv_nut]

"Say, I have a pattern, that is, a complex frequency. If I want to make it brighter, I increase the carrier frequency, but then my whole pattern moves to the right, so a frequency that was, say 2 MHz, becomes 2.5 MHz, which means it is a different pattern!"

Exactly right. It is a different frequency spectrum.
This should not bother your understanding, as in the luma input time domain the high frequency ripple pattern is riding on an average background level; in the frequency domain, the luma baseband input consists of the high frequency plus some DC (zero frequency) energy representing the average background brightness.

After FM modulation, in the frequency domain, the sideband frequency, determined by the high frequency pattern frequency, is "riding on" an average carrier frequency determined by the average brightness of the background. In other words, if the average brightness is different, the average carrier frequency is different, and all the associated sidebands move to keep their same frequency distance from the average.

You should also understand that the diagrams show the frequency bands that are available to carry the FM signal, not the actual FM spectrum that is being sent in the available band in a particular case. The flat top of the FM range just indicates that the FM carrier and sidebands will be recorded if they fit in that range. If the sidenbands go too low in frequency, they will be cut off and their pattern will not appear in the output image. That's what happens in the extreme example where a very high frequency luma pattern is riding on a dark background. In this case, the average carrier frequency is lower, the sideband shifts lower by the same amount as the average, and it gets cutoff by the low end of the system response curve.

[DVtyro]

Originally Posted by old_tv_nut

if the average brightness is different, the average carrier frequency is different, and all the associated sidebands move to keep their same frequency distance from the average

This I do understand. What I do not understand how these sidebands will be represented onscreen. The distance from the average is the same, but the absolute frequency is different. Unless a TV knows how to represent the picture in relation to the changing average I just don't get how the picture will look the same, but brigher, because absolute frequencies are different. And as you said before, frequencies in the sideband represent the pattern, then the pattern will change with a change in brightness. I guess I am missing some important info about obtaining the average frequency for any moment in time.

[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.

[old_tv_nut]

Note: because upper sidebands are cut off by the recording/playback bandwidth, the recovered FM signal will have both frequency and amplitude variation; but the FM demodulator ignores the amplitude variations.