CT-100 Color Setup

1953 NTSC CTC-2 Color Setup
rev2

By Pete Deksnis

Here's something started in 2009 that I never published on my CT-100 website, which, as you may know, is available now on the ETF site.


If you are going to set up a vintage color set for accurate 1953 NTSC color, you must be able to recognize or measure Illuminant C.

Probably the toughest challenge you will face is to know when Illuminant C is beaming from your 15GP22 (or perhaps even the early 21AXP22; vintage documentation describes an identical spectral response for both tubes). The test for illuminant C can be a measurement by a colorimeter or a visual comparison.


Measurement
It is time-consuming, but accurate and rewarding, to use a colorimeter (such as the ColorVision Spyder2PRO used in the 2007 ETF demo) to set the white point of a Vintage NTSC color television set to Illuminant C, which is considered to be Average Sunlight: 6774°K (x=0.310, y=0.316).

By comparison: Illuminant D6500 is considered to be Daylight: 6504°K (x=0.313, y=0.329).


Visual Comparison

I now use visual comparison to set illuminant C: I recall vividly the Illuminant C hue I witnessed in 2007 because it was so much more red than the bluish P4 white I had been accustomed to. The contrast between P4 and illuminant C is so striking that the difference makes a lasting impression.


Adjustments

Here are the step-by-step procedures I use to set up a CT-100 or a 21-CT-55 (CTC-2 or CTC-2B chassis).

The controls involved are:
Color
Brightness
Contrast
R Screen
B Screen
G Screen
B Gain
G Gain
B Background
G Background


Presets:

Color down (CCW)
Contrast down (CCW)
Brightness near maximum (CW)
R Screen down (CCW)
B Screen down (CCW)
G Screen down (CCW)
B Gain (n/a)
G Gain (n/a)
B Background down (CCW)
G Background down (CCW)


Procedure:

The procedure is performed with a luminance-only image and assumes the purity has been set up.


1, Advance R Screen control fully clockwise, then back off slightly. The red screen is typically dim, but do not retard the red screen control to ‘save’ the red gun. Engineering information suggests the screen controls were designed to be set to the high side of the visible range of the control, not the low side.


2. Use the G Screen and B Screen controls to achieve Illuminant C without adjusting the R Screen control.


3. Set Brightness and Contrast for a normal picture.


4. Use the B Gain and G Gain controls to adjust the luminance-only image for Illuminant C. NOTE: When a properly set up vintage color set is displaying black and white programming, the picture will have a noticeably warm (reddish) appearance.


5. Use the Brightness control to adjust the luminance-only image for a dimmer-than-normal picture.


6. Use the B Background and G Background controls to adjust the dimmer-than-normal luminance-only image for Illuminant C. Check your setup by rotating the Brightness control through its useful range for the distinctive Illuminant C hue (not necessarily extreme brightness positions, however, as the focus may lose regulation).


7. Add some color and enjoy.


Further Thoughts:

1. 1953NTSC green phosphor is described as P1. [Perhaps modern CRT's still use the same spectrum phosphor?]

2. 1953NTSC blue phosphor is toward cyan, but was changed relatively soon to the more violet blue. [i.e., there is a little bit of green in 1953NTSC blue, and it is easily demonstrated by viewing color bars through a green filter: the blue and magenta bars are visible on a 15GP22 but barely so on a modern CRT.]

3. 1953NTSC red phosphor is, in my perception, far a more vivid red than the orange-tinted red of modern CRT television . [This is evident in side-by-side demonstrations of a 15GP22 and a modern CRT: facial tones are clearly more 'rugged' and orange-ish on contemporary CRT's. The 15GP22 skin-tone reproduction is more accurate to my eye. This can also be a result of matrix changes in later years, I hear. Comments welcome...]


And Final Thoughts:

A wide-spectrum nicely saturated color source (e.g., 30's and 40's 3-strip Technicolor films) reproduced on a 15GP22 driven by a calibrated CTC-2 chassis delivers astonishing color images.

For best images, the 15G must be viewed in a dark room -- not in subdued or dim light -- or that light-grey 15G phosphor screen will bounce light back into your eyes and desaturate (add white) to those amazing colors. I do use a 9 watt D6500 fluorescent lamp behind the CT-100. Emphasis on behind.

The 15G in my set has very little time on it (other than what I have added) and has been environmentally fortunate all its life. I feel very lucky!

Pete Deksnis
11-March-2009
08-August-2013
25-January-2014