VHS: Speed of the tape NTSC vs PAL

A

aths

Newcomer
For PAL, the VHS linear tape speed is 2.339 cm/s, for NTSC-VHS it is 3.335 cm/s. Can someone explain that to me?

To get the 50 half-frames per second for PAL, the video heads spin with 50 rps. For NTSC, the head rotates with 60 rps. This is 20% more. However, the NTSC tape speed is 42% over the PAL-VHS tape speed.

Can someone explain me why?
 
There's also matters of resolution. While there are distinct vertical scanlines, analog storage is essentially continuous resolution on the horizontal, so it's limited really by analog bandwidth. For NTSC VHS, the bandwidth, IIRC, is about 3.2 MHz. Divide this by fps (29.97), and by the number of lines per frame (525), and you get about 200-ish cycles per line of video. Assume that about 20% are used for sync info, and only the remaining 80% are used for actual video content, and you get 160 cycles per line.

I don't know offhand how it works out for PAL, but I imagine that a fair bit of difference is showing up here and the fact that you've got 625 lines per frame and 25 fps.
 
I am quite sure that VHS stores only the rows used for actual image content, hence 576 lines for PAL and 480 for NTSC. In both cases we have 14400 rows per second as image content. I also never heard of different resultion per line. As far as I know, both NTCS and PAL versions of VHS provide the same detail per row.
 
It's actually quite simple... One rotation of the video head drum writes a frame (each head (2) writes a field (with some overlap)). Since PAL runs at 25fps (50Hz) the head needs to spin at 1500rpm. NTSC requires the head at 1800rpm (actually I think it's 1798.2rpm or something like that because NTSC is 29.97fps (59.94 fields/sec)). That causes the necessary linear tape speed for PAL to be 83.4% that of NTSC. However, NTSC tracks are also wider (58microns) vs. PAL tracks (49microns), yielding a 16.5 percent less space required for PAL tracks meaning less space between tracks required and thus a lower linear tape speed. As to why the PAL tracks are narrorwer? I can only presume that PAL tracks require less guard band to prevent chroma cross talk because of PAL's alternating phases. Other theories are that head design improved more by the time PAL decks become more common (not sure how much water that holds though).
 
aths said:
I am quite sure that VHS stores only the rows used for actual image content, hence 576 lines for PAL and 480 for NTSC. In both cases we have 14400 rows per second as image content. I also never heard of different resultion per line. As far as I know, both NTCS and PAL versions of VHS provide the same detail per row.
Click to expand...

Nope. The whole tv signal is encoded, syncs and all.

CC
 
Captain Chickenpants said:
Nope. The whole tv signal is encoded, syncs and all.

CC
Click to expand...
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.
 
archie4oz said:
It's actually quite simple... One rotation of the video head drum writes a frame (each head (2) writes a field (with some overlap)). Since PAL runs at 25fps (50Hz) the head needs to spin at 1500rpm. NTSC requires the head at 1800rpm (actually I think it's 1798.2rpm or something like that because NTSC is 29.97fps (59.94 fields/sec)). That causes the necessary linear tape speed for PAL to be 83.4% that of NTSC. However, NTSC tracks are also wider (58microns) vs. PAL tracks (49microns), yielding a 16.5 percent less space required for PAL tracks meaning less space between tracks required and thus a lower linear tape speed. As to why the PAL tracks are narrorwer? I can only presume that PAL tracks require less guard band to prevent chroma cross talk because of PAL's alternating phases. Other theories are that head design improved more by the time PAL decks become more common (not sure how much water that holds though).
Click to expand...
The noise-to-signal ratio is my main concern about the tape speed. The noise in the color is terrible on my PAL-VHS-VCR. With faster tape speed the signal should be recorded in better quality.

I also don't know how the color is stored in VHS tape. I only know that it is not FM like them monochrome brighness signal. So it es encoded with QAM or what?

While PAL uses phase alternating for the color's V signal, as far as I know it still uses a complete color line per brightness line. The "downmix" is done in the TV.
 
Captain Chickenpants said:
Nope. The whole tv signal is encoded, syncs and all.

CC
Click to expand...
To get sync signal on the tape, one have not to store all the lines. Wikipedia also states that only the image-holding content is stored.

Fruitfrenzy said:
Which is how they manage to get macrovision copy protection to work. If the syncs weren't stored then you wouldn't have anything to mess with when trying to prevent copying.
Click to expand...
Macrovision works with VHS' Auto Gain Control. That AGC improves the recorded quality from non-protected sources.
 
aths said:
To get sync signal on the tape, one have not to store all the lines. Wikipedia also states that only the image-holding content is stored.

Macrovision works with VHS' Auto Gain Control. That AGC improves the recorded quality from non-protected sources.
Click to expand...

I am fairly sure that wikipedia is wrong.

Macrovision does not just use the AGC
It adds dummy syncs and messes with the color burst. The amount that it messes with these depends on the level of macrovision.

CC
 
Captain Chickenpants said:
I am fairly sure that wikipedia is wrong.

Macrovision does not just use the AGC
It adds dummy syncs and messes with the color burst. The amount that it messes with these depends on the level of macrovision.
Click to expand...
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.
 
Last edited by a moderator:
The Wikipedia information is adapted and (too) simplified from this document, which I remember reading ages ago on USENET.
 
Last edited by a moderator:
Zaphod said:
Any guide explaining composite video will do: http://www.doom9.org/index.html?/capture/analogue_video.html.
Click to expand...
I still dont got it. Which modulation type is used for the color in VHS recording? Since the color signal is stored with a carrier, it has to be a modulated. With QAM one can store two signal on a single carrier, which makes me believe that color is may be modulated that way (to have U and V.) Your linked article talks only about "C" for colour.
 
aths said:
Is that also true for storage on VHS?
Click to expand...
Unless my memory is betraying me, my point is that (apart from filtering to reduce the required bandwidth - effectively reducing the number of lines of colour information) the storage format on VHS (composite analog video) is essentially the same as the transmition format. Thus, the question is more how is colour stored in PAL, NTSC, or SECAM, than it is how it is stored on VHS.
 
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