VHS: Speed of the tape NTSC vs PAL

[aths]

Because I am now two days off to visit the IFA, I have no access to the b3df the next days. When I am back I will post some additional questions about VHS =)

This thread so far already helped, but there are some more things about VHS I am eager to know. The most internet ressources I found so far are either erroneous or too sketchy.

 

[Colourless]

Yes, my parents VHS even saved the Teletext signal. There was lots of transmission errors, and you usually had to wait for several loops before the full page loaded (still with errors), but it was there. It was of course of little use, but could be nice if you wanted to see when you recorded an old tape (Teletext include date and clock).

Teletext is stored in the vertical retrace time, around where the VHS recorder switch head. So there's no wonder that there's a lot of transmission errors.

My parents VCR 'sometimes' records teletext too. It seems dependant on the Tape as much as anything too. They sometimes turn on Teletext subtitles when watching recorded programs. Most of the words are ok, but occasionally they get random character in them.

 

[Captain Chickenpants]

Can you give me a source for that? Since wikipedia is written and reviewed by many people, I consider wiki quite reliable.

Still, you dont have to store any line of the full TV signal to get sync signals on tape.

I have a rather large electronics book which is basically a collection of IEEE papers. It has a section on VHS tape.

With regards to Macrovision, I worked on the drivers for the various TV encoders that our graphics cards have supported. With a expensive reference TV you can actually offset the picture horizontally so that you can see into the blanking regions. Doing that you can actually see the AGC in action as well as the dummy syncs etc.

CC

 

[aths]

Some more questions

So far, this thread helped a lot to understand some issues about VHS. Now I have some more questions. (Since my english is not that good I am also thankful for corrections about my writing.)

•(1)• How to calculate the resolution per line depending on the video bandwidth?

Lets say we have 3.47 MHz pure video bandwidth and 576 lines for 25 full frames per seconds (or 288 lines for 50 half-fields per seconds.) Since we have 14400 lines per second, 240 Hz per lines are left (if we just forget about HSync signals.)

Does that mean we have 480 "pixels" per line (since we can display 240x white + black) or do I have to consider that the phase information is eating the half of the bandwidth, leaving only 120x white + black (per line) left?



•(2)• For a PAL-TV-signal, I read about the QA-modulated color: "1.3 MHz from the lower sideband, and 0.64 MHz of the upper sideband are transmitted. (Original: "Es werden etwa 1,3 MHz des unteren Seitenbandes und 0,65 MHz des oberen Seitenbandes davon übertragen." from http://de.wikipedia.org/wiki/Phase_Alternating_Line#Wahl_der_PAL-Farbtr.C3.A4gerfrequenz)

Does one sideband contains the amplitude spectrum while the other sideband contains the phase spectrum? As far as I know, both sidebands contain the same information, so I wonder why the upper sideband is neither entirely cut, nor transmitted with full bandwidth.



•(3)• Longplay (LP) for VHS – how does it work? Are the tracks even narrower so more tracks are on the tape (416 instead of 208 for SP on a PAL-VHS) or are the tracks just as half as long as in SP mode? Is the horizontal resulution lowered by a low-pass or does the magnetic particle densitiy limit the image resolution in VHS LP?



•(4)• What horizontal resolution VHS does really have? I often read "240 lines". That is more than confusing. Do they mean 240 "pixels" per line? (I write "pixels" instead of pixels because due to the analogue recording I think it is not ok to talk about real pixels. As far as I recognized, due to its analogue nature an VHS image can show a black-white-transistion at any position inside a line, so that is not bound to a pixelized pattern. Buf of course, because of bandwidth limitations, there is a smallest detail which can stored on VHS. Lets talk about the width of that smallest possible single detail as "pixel size".) If I look at my VHS recordings, I consider that number of 240 too low. While checking my recorded images also saw that the horizontal resultion seems to be quite low for irregular patterns, while regular patterns got a higher resolution. Is that observation correct?



•(5)• I also still got some problems understanding the bandwidth consumption of modulation. Lets say we have a signal with 2 MHz bandwidth and a carrier frequency of 8 MHz. To transmit the modulated signal, we need the band of 6-10 MHz. (from 8-2 to 8+2.) Is that true for AM as well as FM? If we have two signals with 2 MHz bandwidth each, and modulate them on an 8 MHz Carrier with QAM, how broad is the modulated signal?



•(6)• I experience intensive "dot crawling" while watching VHS recordings. It is the same image quality degradation as if I use the composite cable instead of an RGB cable with my gaming consoles, only that the crawl patterns of VHS recordings are bigger. Therefore I think it is because of my recorder (connected with a SCART cable) does not support RGB or S-Video output and uses composite instead. Am I right or is that dot crawling a problem of VHS recording in general? Or is it a problem regarding the quality of the Scart-cable? If I watch TV with the VCR-tuner, I also recognize a degradation in image quality compared to the picture with my TV-tuner, but I think the difference is much smaller. So I am not sure about the influence of using a composite signal only instead of S-Video or RGB.



•(7)• Different sources talk about a colour noise reduction for VHS playback, but I still don't know how it is done. Is it simple color-averaging of two lines like in PAL?

 

[aths]

... is nobody willing to answer at least some of my questions?

 

[EC8010]

You probably don't care any more, but... VHS didn't have the bandwidth to record a full PAL (or even NTSC) signal, so colour had to decoded and recorded differently. Luminance was recorded narrowband FM and had a recorded bandwidth of about 3MHz (slightly better for SVHS that used metal tape having finer particle size). Colour bandwidth was limited to 300kHz and recorded under the luminance. To get a feel for the bandwidths and why VHS was so soft and soggy, a broadcast PAL signal occupied 5.5MHz of video bandwidth. Colour was interleaved with the luminance signal by double sideband suppressed carrier modulation on a 4.43361875MHz subcarrier. Colour bandwidth was therefore 1MHz. U-Matic was a colour-under system like VHS used by the broadcasters for news gathering and allowed for 600kHz of colour bandwidth and about 4MHz of luminance bandwidth, so it wasn't too bad. Halving the colour bandwidth again really crippled colour definition for VHS. Most VHS machines did not have a time base corrector to clean up the video (making vertical lines jagged). The dot crawl you have seen is endemic to PAL and NTSC and exacerbated when you go through multiple generations of coding and decoding. Multiples generations (required by editing) always degraded analogue video and was always limited to two generations, or at worst, three. 240 lines of definition means that the vertical definition is half that of a broadcast signal (480 active lines). Horizontal definition in a well-engineered system will be similar (240 x 4/3) lines. (The 4/3 comes about because of the 4:3 aspect ratio.) in LP mode, linear tape speed is halved, so track width halves, so signal to noise of the RF signal off-tape is halved. Because the RF off tape is an FM signal, RF noise does not relate linearly to video noise, but video noise can be expected to degrade, sometimes catastrophically. As far as I remember (this was a long time ago), only NTSC had asymmetric colour bandwidth; PAL was symmetric. I suspect that's why poorly decoded NTSC looked so bad. As for your noise reduction question; probably. It was pretty amazing that a bits of pressed tin and moulded plastic could record video at all, so everything had to be done on the cheap. I forget what the price of the first broadcast videotape machines was (other than an awful lot), but the machine was the size of an upright piano and needed a separate full height rack of electronics. Tape was 2" wide and ran at 15.625 inches per second, necessitating 13" spools.