V.H.S. and Betamax versus 8 m.m.

[ARC Tech-109]

Not a very subjective test, a test DVD would have a good NTSC resolution chart along with many others that will run the tapes thru their paces.

[DVtyro]

Another dumb question about color-under formats, I think I haven't asked it before:

• why raising "sync tip" of luma frequency without increasing the luma frequency bandwidth? Like, Umatic low band has it at 5.4 MHz, high band at 6.4 MHz, and SP at 6.6 MHz, but the deviation is 1.6 MHz in all three cases. What is the benefit?
• What exactly is improved and why by increasing chroma carrier frequency? I suppose the deviation remains the same? Hi-8 has the highest frequency of all the color-under formats, in fact all numbers are better than Umatic SP.

[ARC Tech-109]

Do keep in mind it was more about the prevailing technology of the day along with other factors like tip writing speed (drum diameter) and tape characteristics. The color under (downshift carrier) isn't as important as the actual bandwidth of said carrier and its transient time window to shift the phase, this window is roughly 10% of the actual color subcarrier before down conversion which would be about 100KHz wide giving a working differential of 50KHz or 30 horizontal lines of chroma resolution in the real world.

[DVtyro]

Um, thanks, but I think I am still in the dark. Once again, the question is why moving the whole luma band to the right, if its width is not changed? What does it improve? Similarly, does moving chroma carrier to the right improve color resolution if the width is the same? Unless the luma was moved to the right to free more bandwidth for chroma and to reduce the interference with hi-fi audio? Could you explain to someone who understand algebra and calculus, but is not a radio engineer?

[Electronic M]

There's probably noise and headspeed factoring into those choices. All tape has noise characteristics that vary with head speed and head speed varies with drum size and tape speed. There's probably noise advantages in placing the color and video at certain places with certain drum sizes/head speeds.

[DVtyro]

Ok, so simply moving the bandwidth up does not improve the picture (aside of maybe tape noise)?

[ARC Tech-109]

Has to do with the Nyquist frequency rate at the high end of the carrier frequency, moving the white clip (high end) will give an increase in the resolution (slight) and improve the white dynamic range without clipping or compromising the signal/noise ratio. Take a look at the attached pdf as this may give you some insight.

[DVtyro]

Originally Posted by ARC Tech-109

Has to do with the Nyquist frequency rate at the high end of the carrier frequency, moving the white clip (high end) will give an increase in the resolution (slight) and improve the white dynamic range without clipping or compromising the signal/noise ratio. Take a look at the attached pdf as this may give you some insight.

I have this document. It was one of the reasons I am asking the question

Regarding VHS, it says: "Two limitations occur due to the frequency and deviation limits selected. The lower frequencies of the luminance signal mix slightly with the down-converted chroma signal."

Ok, I can see that chroma mixes with luma.

Then they say "In addition, the frequency range limits the resolution to 240 lines", is it the same as deviation? Because if deviation defines white to black ratio (S/N, right?), then how both S/N and resolution can be defined by the same parameter?

They say, "Super VHS and Super VHS-C VCRs have higher resolution capabilities by raising the overall frequency of the luminance signal. Thus, the FMed signal is placed farther away from the chroma to reduce luminance and chrominance mixing." So, by moving luma to the right, they have increased the resolution of luma, because it is not smeared with chroma? Chroma resolution remained the same because the chroma bandwidth is not changed? This I can understand.

On the first picture with letters they say A is sync tip. A is at the start of the band. So, is the left side of the band always the sync tip or not?

Next, they say "In the case of Super VHS and Super VHS-C, the sync tips produce a modulator frequency of 5.4 MHz with 100% white level producing 7.0 MHz. Maximum deviation of the modulated signal equals 1.6 MHz." So, deviation is S/N? Absolute frequency is luma resolution? How exactly absolute frequency and resolution are related? I wish I saw this graph over time to understand how it looks for each line, for each sample.

If the deviation is 1.6 MHz, and sync tip is 5.4 MHz, what is happening between 1-something... let's say 2 MHz and 5.4 MHz?

[old_tv_nut]

FM modulation is described by Bessel functions - college level theory for sure.

What this means is that video frequencies produce a spectrum covering the sync tip to white frequency range, PLUS some sidebands beyond that range.

Low video frequencies produce only a little extra spread beyond the deviation, while higher video frequencies produce additional sidebands that are at least as far away from the background gray frequency as the higher video frequency, above and below the low-frequency deviation they are riding on.

So, for example: say sync tips are 5.4 MHz and white is at 7 MHz. If there is a high frequency pattern of 2.5 MHz video riding on a gray background that produces an FM frequency of 6.2 MHz, sidebands will be generated that extend at least from 3.7 MHz (6.2-2.5) to 8.7 MHz (6.2+2.5) (well beyond the deviation of 5.4 to 7).

Because of the limited upper frequency of the heads and tape, the high-side 8.7 MHz sideband will be attenuated, and the signal becomes a sort of single sideband (or vestigial sideband) narrow-deviation FM when read from the tape.

The lower sideband of 3.7 MHz MUST be present in order to demodulate the entire video including the gray background video component at that point AND the 2.5 MHz video component. If the system bandwidth was strictly limited to the deviation, no high frequency video patterns could be recorded.

No matter what deviation you choose, you must have these sidebands to reproduce higher video frequencies.