How do phase errors appear in the NTSC color subcarrier?

Question

So, NTSC uses a YIQ color space, with the I and Q channels encoded in a color subcarrier using QAM with the carrier suppressed.

NTSC had a reputation for poor color reproduction—“Never Twice the Same Color”—and when I look up articles that explain the reasoning for this, it says that the color problems are caused by phase errors in the color subcarrier. PAL, so I read, addresses the problem by alternating the phase on successive lines and averaging them out on the receiver, so phase errors result in reduced saturation rather than a hue shift.

My question is—how does the hue shift in NTSC appear in the first place? The color subcarrier is carrier suppressed, but synchronized to the color burst on the back porch. This means that there shouldn’t be any first-order phase errors.

So, what gives NTSC its bad reputation for color reproduction? Is it higher-order phase errors? What would cause a phase shift in the color subcarrier that wouldn’t cause an equal phase shift in the colorburst? Or does NTSC’s reputation have a different explanation?

Apparently the PAL tag is an alias for programmable-logic… — Dietrich Epp, Jun 20 '21 at 05:53
PAL doesn't address the problem, as in solve it, but it does make it less noticeable. It's one of those situations where two opposite Phase wrongs, as long as they are on Alternate Lines, do make a right, or at least a less wrong than either would look by itself. — Neil_UK, Jun 20 '21 at 09:00
PAL is ... "Phase Alternating Line" for EU television ... (It is also "Programmable Array Logic".) It use a burst ( 8 periods ?, on each line) of 4.43 MHz of the original clock for resynchronisizing local one. NTSC does not have this circuitry. So, clock shift slowly ... and colors change ... — , Jun 20 '21 at 12:21
I forget saying that there are two bursts with initial phases of +135° and -135° on two succeeding lines. — , Jun 20 '21 at 13:30
OK. But was it "alternate phase" on every line ? NTSC and PAL has been upgraded at a moment ... PAL with a 64us delay line giving PAL-DL ... for NTSC ... These TV are not used in EU. I don't know. — , Jun 20 '21 at 20:53
@Antonio51 NTSC burst is always into -U direction, or 180 degrees. As it happens there are 227.5 color carrier cycles per line, so even if the phase does not alternate, it creates a checkerboard pattern. PAL alternates +/- 45 degrees around the 180 degrees -U direction, and does not even have integer amount of color carrier cycles per line. — Justme, Jun 20 '21 at 21:15
Synchro is only made on the 2 bursts of consecutive lines wich are "simultaneously" present with the help of the 64 us delay line. Synchro of the clock need only be made at the moment of the bursts. Clock then is free ... but only for one line ... hoping that it will not shift (it does not, xtal used) during one 64us line. Sorry for my bad and poor english. — , Jun 20 '21 at 21:35
Is NTSC burst always in the -U or -I direction? The NTSC color space is YIQ, so I would expect it would be -I. — Dietrich Epp, Jun 21 '21 at 05:48
I also forget that "bandwith color signals" has ben reduced to max 1 MHz (?), because color information is "accessory" towards base video signal white/black which is 5 (?) Mhz large. — , Jun 21 '21 at 09:15

score 1 · Accepted Answer · answered Jun 20 '21 at 16:40

The NTSC system is sensitive to non-linear distortions, which cause dynamic phase changes, so that is why there can be differential phase errors.

Differential phase error simply means how much the brightness change affects the phase of the signal.

The burst is sent at blanking level, and thus color applied to bright levels can have these phase changes when compared to color applied to dim levels.

These nonlinearities may happen during amplification and RF modulation at the transmitter, during travel of the RF signal, or at reception and demodulation of the RF signal to baseband composite video.

The PAL system is just as sensitive too, but it simply works around the differential phase errors with the phase alternation to average out the phase difference.

@ Justme - Last phrase. Do not forget the 64 us delay line. Correction works with 2 succeeding lines at the same time. — , Jun 20 '21 at 21:01
@Antonio51 There are many ways of decoding PAL. Simple decoders do not even use a delay line, it just relies on human eyes to average out the difference. Standard decoders do use a delay line (1D), more sophisticated decoders use two delay lines (2D) or even more complex setups (3D, motion adaptive, etc). — Justme, Jun 20 '21 at 21:25

score 1 · Answer 2 · edited May 06 '23 at 10:05

I disagree. While phase distortions with amplitude do exist they are not why NTSC color images changed color when the source changed. The problem was the NTSC standard did not fully spec the requirements for the relationship of the color burst and horizontal sync. Much of the gear used in the earlier days of NTSC color stripped out the original burst and sometimes even the whole sync, backporch and color burst and replaced them with the local sync. That often resulted in a phase difference between the substituted color burst phase and the actual color phase in the active area of the scan line or picture area.

As switching took place to insert a commercial coming from another studio, city or other side of the country this substitution of new sync and color burst might happen multiple times. The result was constantly shifting color phase as you went from program to program or from program to commercial to another commercial and back to the program. If you were a manic type like me you were constantly tweaking the 'tint' control. BTW there were other problems too. The edge of the horizontal blanking would sometimes creep into the left edge of the picture if the substitution process took place too many times.

Decades later the RS-170 fixed much of the definition of the relationship between horizontal sync and burst issues, or at least the implementation did. Also, later a more complete color reference was added to the vertical blanking that equipment along the chain could 'read' and use to correct chroma phase and level issues automatically. In the last decade before the NTSC system was replaced by digital broadcasting NTSC/RS-170 color was surprisingly stable. Most people never had to get up off the couch to tweak the hue.

Both PAL and SECAM provided another form of correction. PAL's phase-alternate-line was considered as a part of the original NTSC standard but they saw some weird artifacts on screen and ran out of time to figure out how to fix it. By the time the BBC started broadcasting PAL in 1967 they found that adding 1 Hertz per frame to the subcarrier frequency would fix the problem so there is an extra 25 Hz added to the PAL subcarrier frequency beyond being a multiple of half the horizontal scan frequency.

How do phase errors appear in the NTSC color subcarrier?

2 Answers2