"Keyed Rainbow" bars: Standard across manufacturers?

[N2IXK]

Is there any formal/informal standard to the "keyed rainbow" color test pattern produced by the typical service shop pattern generators? The older RCA/Amphenol/Sencore units or the later Sencore VA62 (without the addon EIA pattern generator accessory).

Wondering if there is an "official" representation of how it should look on a modern vectorscope?

[old_tv_nut]

The keyed rainbow is very simply a keyed CW signal that has a frequency offset from nominal 3579545 Hz by one cycle per horizontal scan, that is, different by 15734 Hz.

Then, each keyed burst is the same amplitude as color burst, and the one that directly follows horizontal sync is used by the receiver as color burst.

The keying uses a square wave that is "on" 12 times per horizontal scan, giving a 30 degree phase shift between gated bursts. One burst occurs during horizontal blanking, the next is used as the color sync burst, and the remaining 10 are visible bars.

On a vector scope, this looks like 11 color bursts separated by 30 degrees, with the 12th burst missing. Since the phase is actually changing during the on-time of each burst, and the duty cycle is 50%, there is actually a 15 degree phase change across each gated burst, which ideally will show up as a smear of the phase, rather than a single point on the vector scope. Combined with the narrow bandwidth of the color demodulators, this results in a 12-lobed "flower petal" pattern on the vector scope, with one petal missing.

A generator conceivably could skip the nicety of blanking the burst that occurs during horzontal retrace; but I don't know if any particular designs skipped blanking or not.

[old_tv_nut]

I just went googling, and found a model that used an odd horizontal frequency (15816) , I guess either in order to get particular pulse widths or to accommodate a simple counter circuit to lock things. So I guess there were variations on the basic idea, but all had to work by slipping color by one cycle per horizontal, whatever the horizontal was that they used. 15750 (the monochrome horizontal rate), locked to a 189 kHz pulse rate, is another likely choice.

[Celt]

[lnx64]

Originally Posted by Celt

Not quite the generator pattern they are talking about.

I believe they are talking about this one.

[Findm-Keepm]

The July, 1972 edition of Electronic Servicing Magazine has a detailed article called "How Color Bars are Produced." I just read most of it, and they hit on the various methods of producing the color bars, including that used in the RCA WR-64. If you want the 25-cent version, read old_tv_nut's two earlier posts - he nails in several concise paragraphs what Electronic Servicing took 4 pages to detail.

I can scan the article and post a PDF, but old_tv_nut's explanation is the nutshell....


Cheers,