NTSC versus PAL
in [Repair]
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From: Geoffrey S. Mendelson on 5 Apr 2010 11:09 Sylvia Else wrote: > If you then negate the resulting red difference component of the second > line, and average with the red difference component of the first line, > the parts received from the blue difference component cancel out, > leaving a red different component that equals the original, multiplied > by the cosine of the phase error. The same applies to the blue > component. The result is that the hues are correct, but not as saturated > as they shoud have been. Since PAL TV sets have a saturation (color level) control, isn't that a "non-problem". If it matters, you just adjust it to compensate. My experience is that people set the color saturation too high, if I hold my hand up to the screen my skin looks pale in comparison to everyone on it. Geoff. -- Geoffrey S. Mendelson, Jerusalem, Israel gsm(a)mendelson.com N3OWJ/4X1GM New word I coined 12/13/09, "Sub-Wikipedia" adj, describing knowledge or understanding, as in he has a sub-wikipedia understanding of the situation. i.e possessing less facts or information than can be found in the Wikipedia.
From: Dave Plowman (News) on 5 Apr 2010 14:07 In article <slrnhrjv4t.l1t.gsm(a)cable.mendelson.com>, Geoffrey S. Mendelson <gsm(a)cable.mendelson.com> wrote: > My experience is that people set the color saturation too high, if I hold > my hand up to the screen my skin looks pale in comparison to everyone > on it. Especially CSI. ;-) -- *If I throw a stick, will you leave? Dave Plowman dave(a)davenoise.co.uk London SW To e-mail, change noise into sound.
From: Geoffrey S. Mendelson on 5 Apr 2010 14:59 Dave Plowman (News) wrote: > In article <slrnhrjv4t.l1t.gsm(a)cable.mendelson.com>, > Geoffrey S. Mendelson <gsm(a)cable.mendelson.com> wrote: >> My experience is that people set the color saturation too high, if I hold >> my hand up to the screen my skin looks pale in comparison to everyone >> on it. > > Especially CSI. ;-) That's funny, I was thinking of last Thursday night's episode of CSI when I wrote that. Geoff. -- Geoffrey S. Mendelson, Jerusalem, Israel gsm(a)mendelson.com N3OWJ/4X1GM New word I coined 12/13/09, "Sub-Wikipedia" adj, describing knowledge or understanding, as in he has a sub-wikipedia understanding of the situation. i.e possessing less facts or information than can be found in the Wikipedia.
From: Sylvia Else on 5 Apr 2010 22:15 On 6/04/2010 12:53 AM, William Sommerwerck wrote: >>> However... If the burst phase is wrong, then there is no cancellation of >>> errors, because there are no "errors" /in the signal itself/. (Right? > (???)) >>> Therefore, I don't see how line averaging can be used to eliminate the > need >>> for a manual hue control. > >> Think of the chroma signal as a vector with its y coordinate equal the >> red difference component, and the x coordinate equal to the blue >> difference component. A phase error rotates that vector about the z >> axis. Effectively, the blue difference component receives a bit of the >> red difference component, and vice versa. > >> On alternate lines the phase of the red difference component *only* is >> inverted. In our view, this has the effect of reflecting the vector in >> the x axis - what was a positive y value becomes negative. > >> The same phase error causes this vector to rotate in the same direction >> about the z axis, but because of the reflection, the mixing of the >> components has the opposite sign. > >> If you then negate the resulting red difference component of the second >> line, and average with the red difference component of the first line, >> the parts received from the blue difference component cancel out, >> leaving a red different component that equals the original, multiplied >> by the cosine of the phase error. The same applies to the blue >> component. The result is that the hues are correct, but not as saturated >> as they shoud have been. > > No argument. That's always been my understanding. But... > > If the burst phase gets screwed up somewhere along the line, no amount of > line averaging will fix the problem, because there's nothing "wrong" with > the subcarrier to fix. If the burst has a random phase relationship to the colour subcarrier on each line, then my analysis falls apart because the vectors would have random orientations. In such a situation a PAL receiver would do no better than NTSC, and they'd both perform awfully. If the burst just has a fixed phase offset from the true colour subcarrier, then the averaging will work. Indeed it will work if the colour subcarrier drifts in a consistent way relative to the burst - or if the receiver's oscillator similarly drifts. The effect of such a drift on an NSTC picture would be a variation of tint from left to right. However, a tint control wouldn't be able to address that problem - it would simply move the horizontal position on the screen where the colours are accurate - suggesting that it doesn't occur in practice except in equipment that is recognisably broken. > > Granted, this problem hardly ever happens. But the argument that a fully > implemented PAL set is inherently immune to color errors is hard for me to > swallow. > > I don't think there's a claim that it is inherently immune to all colour errors, only those caused by consistent differences between the phase of the subcarrier and the burst. Sylvia.
From: Sylvia Else on 5 Apr 2010 22:23
On 6/04/2010 1:09 AM, Geoffrey S. Mendelson wrote: > Sylvia Else wrote: >> If you then negate the resulting red difference component of the second >> line, and average with the red difference component of the first line, >> the parts received from the blue difference component cancel out, >> leaving a red different component that equals the original, multiplied >> by the cosine of the phase error. The same applies to the blue >> component. The result is that the hues are correct, but not as saturated >> as they shoud have been. > > Since PAL TV sets have a saturation (color level) control, isn't that > a "non-problem". If it matters, you just adjust it to compensate. If it's a fixed phase error, yes. If the phase error is changing slowly over time the the picture will have a saturation that varies over time which would be annoying if the effect were high enough. However, I've never noticed such an effect. Sylvia. |