#1
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ABC Network Color - not from some affiliates
In the early days of Color Broadcasting in the USA (62-65),
some ABC affiliates were not able to pass through the ABC Network Color Broadcasts. I don't understand why. According to RCA, realignment of the equipment at the local station cost about $20,000 (probably verifying the phase accuracy of the gear). Anyone have more info about why some ABC affiliates couldn't transmit ABC Color? Kirk Bayne |
#2
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Existing monochrome transmitters had much looser tolerances for frequency response.
FCC monochorme response limits allowed a response of +/-2 dB at 1.25 MHz, +2/-3 dB at 2 MHz, +2/-6 at 3 MHz, +2 / -12 at 3.5 MHz, and +2 / - infinity above 3.5 MHz. The RETMA monochrome lower limit was -3.6 dB at 3.6 MHz. The FCC limit for color transmitters was +/- 2 dB throughout. In addition, color tranmsission required attention to differential phase and differential gain (the variations of subcarrier phase and amplitude depending on the luminance level). Getting these within the barely tolerable +/-2 dB and +/-10 degrees required a transmitter with distortion of a few percent or less. Many monochrome transmitters were terrible in this regard, as the effect would be unnoticeable in black and white. |
#3
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That's true. I worked at an ABC affiliate in the 70's and they were using a converted B/W transmitter (TT25BL). I saw the manual for the retrofit and basically it replaced the ENTIRE modulator panel. It was about 20 inches high and 16 inches wide and had more tubes. Transmitters were high level modulated back then and it took about 200 Watts to modulate to 100% (grid modulation).
The TT25EL was built for color and the station across town had one of those. But it was high level modulation too. In '77 we installed a Harris BT25L2 which, as I recall, was the first low level modulated transmitter. It used a transversal sideband filter, at an IF frequency, which was a sort of SAW filter.
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#4
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Just FYI, when digital came in, the existing color transmitters could be used, but the exciters had to compensate for the remaining non-linearities. The goal was for the non-linearities to produce an error rate equivalent to a noise floor of -27 dB. This amount would not cut into coverage area with receivers having the usual 15 dB threshold. (Distortion equivalent to -15 dB would put all receivers at threshold, no matter how close to the transmitter. Distortion between -15 and -27 would add significantly to thermal noise and bring the threshold contour closer to the transmitter.)
The digital exciters use feedback of an output signal sample to automatically linearize everything. This could also have some effect on out-of-band splatter, but typical setups use a multi-pole sideband filter to really quash the lower sideband and then depend on the exciter non-linear and linear distortion corrections to make the inband error floor below -27 dB. Typical gear gets well into the minus 30s last time I paid attention. This means that the distortion margin is entirely available to the receiver to accomodate ghost conditions, antenna response, receiver noise figure and receiver IF frequency response. Last edited by old_tv_nut; 03-05-2019 at 04:05 PM. |
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