A method is described of adding a real-time TV signal to an existing cable multiplex from a towed vehicle where the existing multiplex is such that (a) the multiplex extends down to low frequency and therefore the TV cannot be sent at baseband, (b) the cable attenuation at the upper end of the required TV signal transmission band is in excess of that normally considered a candidate for amplitude equalization. The method provides from 70 – 80 dB of amplitude equalization, while maintaining an acceptable group delay, and modulates the TV video on to a remotely variable frequency carrier. The lower sideband of the modulated carrier is used for the cable transmission so that the high frequency components of the video are transmitted at the lower end of the transmission band. Because a large amount of amplitude equalization is employed, when the vehicle is being checked out prior to launch, the unavoidable small signal leak through that bypasses the cable becomes proportionally large. By remotely varying the carder frequency in small increments of frequency, the effect of the signal leak through can be minimized. Using the technique, a standard 30 frames per second picture with a horizontal resolution of 100 - 130 lines has been obtained over a 28,000 ft tow cable using a 1.85 MHz carrier. At this frequency the cable attenuation was in excess of 77 dB.
The practicality of transmitting a standard TV signal over a long coaxial tow cable depends principally on whether an adequate RF signal bandwidth can be obtained. If this is not the case, then there is the classic frame rate vs signal bandwidth vs picture resolution tradeoff. Although frame rates of 15 per sec have been used, they are annoying because of the flicker generated. To avoid the flicker, either a lower resolution picture has to be accepted or a flickerless picture can be restored using a scan converter. If the frame data rate is too low, manual control of the vehicle using the TV display may be diffi9ult if not impossible. If the signal originates in a towed vehicle there are probably other telemetery channels and/or power which preclude the TV being transmitted at baseband, i.e., the TV video must be modulated onto a carrier. The carrier must be as far away in frequency from the maximum video frequency to maximize the sampling rate and to simplify the filter requirements after detection. In any multiplex system filters handling a TV signal the filters must be designed such that their group delay (rate of change of phase with frequency) does not cause ringing in the display.
A standard 30 frames per second 525 line TV picture requires approximately 1.0 MHz for every 100 lines of horizontal resolution or about 4.0 MHz for a picture of good commercial broadcast quality. A low resolution picture of about 80 lines in acceptable and a moving picture viewed at the optimum viewing distance under good conditions is limited to about 600 lines of resolution by the human eye.
