W
William Sommerwerck
"Die, die, my darling!"
As both PAL and NTSC are basically dead systems (NTSC in the US, at least),
there is little point in discussing their differences. But as Mr. Alison
insists on displaying his ignorance in public, I'm going to, anyhow.
The first color TV system approved by the FCC was a field-sequential (or
frame-sequential -- I forget which) system proposed by CBS. It was developed
by Peter Goldmark, the same man given credit for the modern LP phonograph
record. (I say "given credit for", because there have been questions as to
whether he was the principal designer.)
The CBS system is a classic example of a design botched from the get-go. At
that time (not long after WWII), there was no practical way to display three
color images simultaneously with a single CRT. So Goldmark went with a
spinning color wheel, a system that had been tried 25 years earlier for
color motion pictures, and found wanting.
The problems with such a system are obvious, but I'll describe them. One
problem is that it requires three times as much film (or in the case of TV,
three times the bandwidth). Another is that moving objects show color
fringing.
Then there was the problem of the spinning color-filter disk. A 10" TV would
require one at least 2' in diameter. Imagine the disk needed for a 21" set!
(Not to mention the noise, and the possibility, however remote, of
decapitating the cat.)
These obvious (and lethal) deficiencies didn't deter Goldmark or CBS,
because they were in competition with RCA/NBC. The CBS argument was... Why
limit TV to B&W? Why not /start/ with a color system, and be done with it?
CBS pressed the FCC (as one writer pointed out, every sale of an RCA B&W TV
would be another nail in the coffin of the CBS color system), and in 1950
the CBS system was approved, despite the fact it was wholly incompatible
with the 480i system already in use. *
David Sarnoff ("the most-nasty name in electronics") was naturally upset.
RCA had to make CBS look bad, while completing development of their own
color system. Sarnoff gleefully pointed out that the CBS system was
"mechanical", and subject to all the limitations accruing thereto. Though
this was literally true, it overlooked the fact that one can have
all-electronic field-sequential color. But -- on the other hand -- CBS had
nothing other than a mechanical system to offer.
RCA was working on a "dot-sequential" system. Each line of the image was
divided into 300 (or so) pixels **, with red, green, and blue samples
alternating. This system worked fairly well -- it produced an acceptable
picture on B&W sets. But (for reasons I don't remember) color receivers had
problems displaying B&W images. As color receivers would (initially) be used
mostly for B&W viewing, this was not acceptable
The breakthrough came when engineers at Hazeltine and GE remembered Monseuir
Fourier, and recognized that sampling the colors was equivalent to a
"continuous" signal at the sampling frequency. They "slipped a note under
RCA's door" (so to speak), and NTSC/PAL came into existance. The color
information was transmitted on a subcarrier whose sidebands were interleaved
with the luminance sidebands, to minimize interaction. ***
"...complete with bad commercials that repeat all night, both in compatible
color and black and white." -- Stan Freberg
The brilliance of NTSC/PAL is that their signals produce as good (or better)
an image on B&W sets, and display excellent color on a color set -- without
making any existing equipment obsolete, and without requiring additional
bandwidth.
So... why is NTSC "better" than PAL? For one thing, it has "better" and
"more" color. Although the original NTSC proposal used red and blue color
signals of equal bandwidth, it was recognized that this didn't fit with the
way the eye actually sees color.
It turns out that for a 480-line system displayed on a 21" tube, the eye
sees full color (red/green/blue) only to about 0.5MHz. From 0.5MHz to
1.5MHz, the eye sees only those colors that can be matched with red-orange
and blue-green primaries. **** The system was therefore changed to the
red-orange/blue-green and yellow-purple primaries, the former of 1.5MHz
bandwidth, the latter of 0.5MHz bandwidth.
PAL uses equal-bandwidth (1.0 MHz) red and blue primaries. If an NTSC set
fully demodulates the 1.5MHz color signal (most limit it to 0.5MHz to make
the set cheaper), more of the original image's color detail will be
displayed (though this will be visible mostly in graphics).
Much has been made of PAL's phase alternation, especially its supposed
ability to eliminate the need for a tint [sic] control. (It should be hue
control.) When was the last time you adjusted the hue control on an NTSC
receiver? 30 years ago?
This issue is confused by two factors -- the differences between European
and American distribution systems, and their studio standards.
If the transmission network has constant group delay, the hue setting should
be set 'n forget, and never need to be changed. The American system had good
group-delay characteristics -- the European did not. So switching channels
could require twisting the hue knob. But that's not all there is to it.
Non-linear group delay changes the colors in a way that cannot be corrected
simply by adjusting the hue control. All the colors cannot be "correct" at
the same time. The advantage of PAL is that these color errors "flip" with
the phase, and are complementary -- the eye "averages" them to the correct
color.
So what's wrong with that? Well, the averaging also reduces saturation.
(Mixing an additive primary with its complement pushes it toward white.)
With severe group-phase error, the image shows bands of varying saturation.
(In NTSC, there are bands of varying hue.)
The other point of confusion is that, for many years, US broadcasters didn't
pay much attention to signal quality. Cameras weren't set up properly, and
burst phase wasn't properly monitored. So when you changed channels, you
sometimes had to change the hue setting. Broadcasters finally got their acts
together, and color quality has, for some time, been pretty consistent from
channel to channel.
In short, PAL's phase alternation is an advantage with transmission systems
having poor group-delay characteristics -- a problem that did not exist in
the US. In every other respect, it is inferior to NTSC.
All of this is true, to the best of my knowledge. Corrections and additions
are welcome.
* Some dishonest manufacturers sold B&W TVs with a "color converter" jack on
the back. It wouldn't have worked, because these sets didn't have the
required IF bandwidth (AFAIK).
** No, the term didn't exist at the time.
*** Some interaction is visible with objects having fine B&W detail. The set
"misinterprets" this detail as color information.
**** This is why two-primary color-movie systems (such as the original
Technicolor) could give acceptable -- though hardly great -- results.
As both PAL and NTSC are basically dead systems (NTSC in the US, at least),
there is little point in discussing their differences. But as Mr. Alison
insists on displaying his ignorance in public, I'm going to, anyhow.
The first color TV system approved by the FCC was a field-sequential (or
frame-sequential -- I forget which) system proposed by CBS. It was developed
by Peter Goldmark, the same man given credit for the modern LP phonograph
record. (I say "given credit for", because there have been questions as to
whether he was the principal designer.)
The CBS system is a classic example of a design botched from the get-go. At
that time (not long after WWII), there was no practical way to display three
color images simultaneously with a single CRT. So Goldmark went with a
spinning color wheel, a system that had been tried 25 years earlier for
color motion pictures, and found wanting.
The problems with such a system are obvious, but I'll describe them. One
problem is that it requires three times as much film (or in the case of TV,
three times the bandwidth). Another is that moving objects show color
fringing.
Then there was the problem of the spinning color-filter disk. A 10" TV would
require one at least 2' in diameter. Imagine the disk needed for a 21" set!
(Not to mention the noise, and the possibility, however remote, of
decapitating the cat.)
These obvious (and lethal) deficiencies didn't deter Goldmark or CBS,
because they were in competition with RCA/NBC. The CBS argument was... Why
limit TV to B&W? Why not /start/ with a color system, and be done with it?
CBS pressed the FCC (as one writer pointed out, every sale of an RCA B&W TV
would be another nail in the coffin of the CBS color system), and in 1950
the CBS system was approved, despite the fact it was wholly incompatible
with the 480i system already in use. *
David Sarnoff ("the most-nasty name in electronics") was naturally upset.
RCA had to make CBS look bad, while completing development of their own
color system. Sarnoff gleefully pointed out that the CBS system was
"mechanical", and subject to all the limitations accruing thereto. Though
this was literally true, it overlooked the fact that one can have
all-electronic field-sequential color. But -- on the other hand -- CBS had
nothing other than a mechanical system to offer.
RCA was working on a "dot-sequential" system. Each line of the image was
divided into 300 (or so) pixels **, with red, green, and blue samples
alternating. This system worked fairly well -- it produced an acceptable
picture on B&W sets. But (for reasons I don't remember) color receivers had
problems displaying B&W images. As color receivers would (initially) be used
mostly for B&W viewing, this was not acceptable
The breakthrough came when engineers at Hazeltine and GE remembered Monseuir
Fourier, and recognized that sampling the colors was equivalent to a
"continuous" signal at the sampling frequency. They "slipped a note under
RCA's door" (so to speak), and NTSC/PAL came into existance. The color
information was transmitted on a subcarrier whose sidebands were interleaved
with the luminance sidebands, to minimize interaction. ***
"...complete with bad commercials that repeat all night, both in compatible
color and black and white." -- Stan Freberg
The brilliance of NTSC/PAL is that their signals produce as good (or better)
an image on B&W sets, and display excellent color on a color set -- without
making any existing equipment obsolete, and without requiring additional
bandwidth.
So... why is NTSC "better" than PAL? For one thing, it has "better" and
"more" color. Although the original NTSC proposal used red and blue color
signals of equal bandwidth, it was recognized that this didn't fit with the
way the eye actually sees color.
It turns out that for a 480-line system displayed on a 21" tube, the eye
sees full color (red/green/blue) only to about 0.5MHz. From 0.5MHz to
1.5MHz, the eye sees only those colors that can be matched with red-orange
and blue-green primaries. **** The system was therefore changed to the
red-orange/blue-green and yellow-purple primaries, the former of 1.5MHz
bandwidth, the latter of 0.5MHz bandwidth.
PAL uses equal-bandwidth (1.0 MHz) red and blue primaries. If an NTSC set
fully demodulates the 1.5MHz color signal (most limit it to 0.5MHz to make
the set cheaper), more of the original image's color detail will be
displayed (though this will be visible mostly in graphics).
Much has been made of PAL's phase alternation, especially its supposed
ability to eliminate the need for a tint [sic] control. (It should be hue
control.) When was the last time you adjusted the hue control on an NTSC
receiver? 30 years ago?
This issue is confused by two factors -- the differences between European
and American distribution systems, and their studio standards.
If the transmission network has constant group delay, the hue setting should
be set 'n forget, and never need to be changed. The American system had good
group-delay characteristics -- the European did not. So switching channels
could require twisting the hue knob. But that's not all there is to it.
Non-linear group delay changes the colors in a way that cannot be corrected
simply by adjusting the hue control. All the colors cannot be "correct" at
the same time. The advantage of PAL is that these color errors "flip" with
the phase, and are complementary -- the eye "averages" them to the correct
color.
So what's wrong with that? Well, the averaging also reduces saturation.
(Mixing an additive primary with its complement pushes it toward white.)
With severe group-phase error, the image shows bands of varying saturation.
(In NTSC, there are bands of varying hue.)
The other point of confusion is that, for many years, US broadcasters didn't
pay much attention to signal quality. Cameras weren't set up properly, and
burst phase wasn't properly monitored. So when you changed channels, you
sometimes had to change the hue setting. Broadcasters finally got their acts
together, and color quality has, for some time, been pretty consistent from
channel to channel.
In short, PAL's phase alternation is an advantage with transmission systems
having poor group-delay characteristics -- a problem that did not exist in
the US. In every other respect, it is inferior to NTSC.
All of this is true, to the best of my knowledge. Corrections and additions
are welcome.
* Some dishonest manufacturers sold B&W TVs with a "color converter" jack on
the back. It wouldn't have worked, because these sets didn't have the
required IF bandwidth (AFAIK).
** No, the term didn't exist at the time.
*** Some interaction is visible with objects having fine B&W detail. The set
"misinterprets" this detail as color information.
**** This is why two-primary color-movie systems (such as the original
Technicolor) could give acceptable -- though hardly great -- results.
