Hi, everyone.
>It's true. PAL has 625 lines of resolution, compared to 525 for NTSC,
>so the resolution is better. Not that either ever really achieved their
>full theoretical resolution. Especially on 1/2-inch consumer-level VHS
>videotape.
>Many US TV sets had more like 250 lines (pixels).
Don't forget the 525-625 lines are _vertical_ resolution (horizontal
lines), whereas VHS' 250 lines represent the _horizontal_ resolution (pairs
of points/pixels on a line). From the 625 lines of PAL, only 575 are
visible because the rest is needed for sync pulses -- most TV sets show
even less. I'm not sure about NTSC values, but it should have around 480
useful lines.
Same for horizontal resolution: the screen only shows about 800f each
line. So, 250 "lines" = 500 pixels would show about 400 _visible_ pixels.
Another issue is colour resolution. The chroma (colour) signal has _much
less_ horizontal resolution than the luminance (B&W) signal. I've read
somewhere that for the PAL broadcasting (4.43 MHz colour subcarrier) the
figures are 4.5 MHz for luminance and only 1.5 MHz for chroma -- that's 288
and 96 "lines", respectively. I'm afraid that some digital cameras do
something similar.
When recording to tape, the thing gets much worse... VHS downsamples colour
subcarrier to just 627 kHz, so it's no surprise that colours are rendered
as fuzzy blobs ;-). IIRC, Beta and Video-8 are somewhat better with 689 and
732 kHz, resp. SuperVHS keeps the 627 kHz, though. Playback circuitry also
does "nasty" things with chroma signal, too.
>The big difference is that NTSC codes color as the absolute phase of the
>color subcarrier, while PAL uses the frame-to-frame difference, so PAL color
>is far more robust.
AFAIK, the reason is a bit different... PAL uses a, say, "negative" coding
for even lines, instead of the "positive" coding of the odd lines of each
field. Phase errors will make a colour shift, like NTSC, but with opposite
directions between even/odd lines, so the global effect would be negligible.
On the other hand, SECAM colour system has a completely different approach:
the colour subcarrier is frecuency-modulated. But it halves colour
_vertical_ resolution and compatibility with B&W sets is not as good as in
NTSC/PAL.
>I don't know the image size of Super8, but if it scales from 35mm movie film,
>at 24x18, then the image size will be something like 1/4-size, or 6mm by
>4.5mm,
Here's a list of movie film formats:
8mm: 4.9 x 3.55 mm
Super-8: 5.36 x 4 mm
16mm: 10.05 x 7.42 mm
35mm: 16 x 22 mm
70mm: 52.2 x 23 mm
>which is the rough equivalent of (6*100)(4.5*100)= 270,000 tricolor pixels
>per frame,
>or 0.54 Mpix, as digital cameras are rated. In terms of lines, this is
>equivalent
>to 520 by 520, far more than VHS ever could do.
Based on the above table, and assuming a conservative 50 lp/mm (the limit
may be higher with the short focal lenghts of Super-8), my calculations
give 214400 tri-color pixels, or about 230 lines -- roughly the same of
VHS. However, colour reproduction will be better on film.
Happy holidays to all,
...
Carlos J. Santisteban
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