NTSC versus PAL
in [Repair]
|
From: William Sommerwerck on 6 Apr 2010 07:41 > If the burst just has a fixed phase offset from the true colour > subcarrier, then the averaging will work. Right. I missed that. > I don't think there's a claim that [PAL] is inherently immune to > all colour errors, only those caused by consistent differences > between the phase of the subcarrier and the burst.
From: Arfa Daily on 6 Apr 2010 20:08 "Sylvia Else" <sylvia(a)not.at.this.address> wrote in message news:4bba994c$0$15459$c3e8da3(a)news.astraweb.com... > 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. > Many years back, Bush in the UK produced a colour decoder which was 'revolutionary' compared to other manufacturers' efforts, in that the subcarrier was regenerated in the decoder directly from the burst, rather than being a free-running oscillator just locked to the burst with a PLL. They did this by deriving a phase-adjustable pulse from the H-flyback, and using this to 'notch out' the burst from the back porch period. The 10 cycles of burst thus recovered, were then applied directly to the 4.43MHz crystal, which caused it to ring at exactly the same frequency and in exactly the same phase as the original subcarrier. Always seemed to work pretty well, and they continued to use this system over a period of probably 10 years or more, covering three chassis designs / revisions. Arfa >> >> 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: Arfa Daily on 6 Apr 2010 20:10 "Sylvia Else" <sylvia(a)not.at.this.address> wrote in message news:4bba9b0d$0$9296$c3e8da3(a)news.astraweb.com... > 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. I would guess that you never would see such an effect, as all of the decoders that I can remember working on, had ACC circuits which worked very well ... Arfa
From: William Sommerwerck on 6 Apr 2010 20:55 > Many years back, Bush in the UK produced a colour decoder which was > 'revolutionary' compared to other manufacturers' efforts, in that the > subcarrier was regenerated in the decoder directly from the burst, rather > than being a free-running oscillator just locked to the burst with a PLL. > They did this by deriving a phase-adjustable pulse from the H-flyback, and > using this to 'notch out' the burst from the back porch period. The 10 > cycles of burst thus recovered, were then applied directly to the 4.43MHz > crystal, which caused it to ring at exactly the same frequency and in > exactly the same phase as the original subcarrier. Always seemed to work > pretty well, and they continued to use this system over a period of > probably 10 years or more, covering three chassis designs / revisions. This was first done by GE, circa 1966, in the Portacolor set, mostly because it was cheaper. Another way of looking at this system is that the crystal was an extremely narrow-band filter that removed the "Fourier sidebands" around the subcarrier frequency created by transmitting the 10-cycle burst only once on each scanning line.
From: Sylvia Else on 6 Apr 2010 22:29
On 7/04/2010 10:08 AM, Arfa Daily wrote: > "Sylvia Else"<sylvia(a)not.at.this.address> wrote in message > news:4bba994c$0$15459$c3e8da3(a)news.astraweb.com... >> 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. >> > > Many years back, Bush in the UK produced a colour decoder which was > 'revolutionary' compared to other manufacturers' efforts, in that the > subcarrier was regenerated in the decoder directly from the burst, rather > than being a free-running oscillator just locked to the burst with a PLL. > They did this by deriving a phase-adjustable pulse from the H-flyback, and > using this to 'notch out' the burst from the back porch period. The 10 > cycles of burst thus recovered, were then applied directly to the 4.43MHz > crystal, which caused it to ring at exactly the same frequency and in > exactly the same phase as the original subcarrier. Always seemed to work > pretty well, and they continued to use this system over a period of probably > 10 years or more, covering three chassis designs / revisions. > > Arfa I'm left wondering what exactly was the *real* problem that PAL was intended to fix. It appears that the NTSC tint control could only address a fixed phase offset between the colour burst and the subcarrier, with both transmitters and TV sets able to maintain that offset sufficiently closely that the hue wouldn't vary from left to right of the picture. Other issues, such as non-linear phase shift would have been a problem for NTSC viewers, regardless of the tint control. So were NTSC viewers tolerating colour pictures that couldn't be set right even with the tint control? Or is there something else that I've missed? Sylvia. |