[opendtv] Re: PAL; 405 line pictures

  • From: "Dale Kelly" <dalekelly@xxxxxxxxxxx>
  • To: <opendtv@xxxxxxxxxxxxx>
  • Date: Fri, 15 Dec 2006 09:18:54 -0800

Thanks, that is generally how I recall it and much more....

> -----Original Message-----
> From: opendtv-bounce@xxxxxxxxxxxxx
> [mailto:opendtv-bounce@xxxxxxxxxxxxx]On Behalf Of S J Birkill
> Sent: Friday, December 15, 2006 2:33 AM
> To: opendtv@xxxxxxxxxxxxx
> Subject: [opendtv] Re: PAL; 405 line pictures
> Walter Bruch and his PAL team at Telefunken tackled the major 
> shortcoming of NTSC: hue errors due to distortion in video amplifiers 
> and modulators. It was the cumulative phase errors through dozens of 
> amplifiers in a TV studio complex, hundreds in a terrestrial 
> distribution chain, plus transmitter modulators and receiver demods, 
> essentially non-linear circuits, that spoiled NTSC. It's easy to 
> forget that before satellite distribution in the 1970s, Americans 
> seldom saw accurate color on their screens.
> Differential phase was the killer. Those non-linear stages 
> systematically shifted chroma phase according to the instantaneous 
> luminance level (above black, burst reference) the chroma was riding 
> on: flesh tones took the brunt, yielding NTSC's traditional green, 
> orange or purple faces; hence the system's need for a 'hue' control. 
> PAL eliminated this, converting phase error to a small amplitude 
> error -- diff phase distortion looked just like diff gain, 
> introducing only a slight desaturation at the higher luminance 
> levels. One less knob on the set.
> PAL's line-wise alternation of the V (R-Y, give or take small 
> differences in scaling, matrixing and gamma-correction) chroma 
> component's modulation phase, serves to correct chroma phase errors 
> by optical (simple PAL, long obsolete) or electrical (delay-line PAL) 
> averaging. The delay-line method halves vertical color resolution, 
> which then matches (again approximately, so don't take me to task on 
> the numbers -- this is from memory) its band-limited horizontal 
> resolution. NTSC retains the higher vertical color resolution -- 
> that's the *only* loss from PAL coding. In fact there's a slight 
> collateral gain: the phase switching dithers the CSC pattern, making 
> for a less visible diagonal 'dot crawl' in saturated picture areas; 
> CSC notch filtering in the receiver could be relaxed a little.
> ['Composite video over telephone lines' is misleading: 625-line video 
> distribution in the UK (for instance) was from its very beginning 
> (1964) via 5.8-MHz equalized co-axial cable within studio centers and 
> across cities, and via 2GHz or 7GHz FM microwave links between 
> studios and transmitters. Even in the 405-line days when video 
> bandwidth was 3MHz, STLs on the main distribution backbone were in 
> Post Office co-axes using an HF carrier system, with a combination of 
> rebroadcast and microwave radio links for the more remote stations.]
> Hue error correction is just one component of the reason the PAL 
> pictures looked so much better than NTSC in the early days. There 
> *were* different production values: European lighting and set 
> designers avoided the gaudy sets used in US studio productions to 
> 'show off' color TV, instead letting natural color prevail (alas 
> those values have largely been lost!) The 576 active lines (versus 
> 480) plus the increased video bandwidth (by about the same ratio) 
> helped a lot. In PAL-I countries at least, the bandwidth could be 
> better exploited, as there was space for HF luminance components 
> above the 4.43MHz CSC notch, before cut-off at about 5.5MHz and the 
> receiver's 6MHz sound (aural) notch; things were much tighter between 
> 3.58 and 4.2MHz in NTSC land. And of course for PAL the distribution 
> was shorter: rather than syndicates 'out west', BBC TV's furthest 
> outpost was Shetland, a mere 600 miles from London.
> I don't feel the black level/blanking level difference was 
> noticeable. IIRC it was designed to minimise the visibility of sync 
> pulse overshoots during the line flyback (horizontal retrace) period, 
> but with improved blanking (suppression) in color TVs it became 
> unnecessary. In any case it was accepted practice even in PAL studios 
> to apply a set-up ('lift') of between 2 and 5% to the picture, which 
> amounted to much the same thing. The reduction of video dynamic range 
> (an SNR delta of <1dB) due to the IRE blanking level was surely 
> insignificant in analog days.
> 405-line pictures of course looked sharp because there *was* no CSC. 
> But still it needed good STLs, careful transmitter alignment and a 
> good TV to realise the system's potential 3MHz video bandwidth -- 
> most of the time you'd be lucky to see 2.5MHz in the home. 377 active 
> lines was only three-quarters of what US viewers were accustomed to, 
> and it showed. Don't forget also that sync separators were more art 
> than science in the pre-color days: TV sets frequently lost interlace 
> totally, giving the effect of seeing the world through a stationary 
> 188-line raster! And let's not forget the 10.125kHz line whistle and 
> the 50Hz field flicker on those fast-phosphor B&W tubes. That 
> delegation must have been used to some pretty rough 525-line stuff!
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