The Cine Technician (1953-1956)

February, 1954 THE CINE-TECHNICIAN 19 TV PICTURES on MAGNETIC TAPE Cine's report last month on RCA's tape recording of pictures aroused much interest among film and TV technicians in Britain. Technical problems posed and solved in the process are described in detail in the article below, for which ive are indebted to our friends of the International Photographers' Local of the I.A.T.S.E. of U.S.A. and Canada. O/^1 A 's method of TV picture recording is AX v^-fi. similar, in basic respects, to the technique used to record speech and music with present-day magnetic tape sound equipment. Electrical signals are impressed through a recording head — a small horseshoe electro-magnet — on to the magnetically treated surface of a plastic tape. As the tape is drawn across the recording head, the head continuously changes the magnetic polarity of the magnetic oxide particles on the tape so that they become a compact code of the original signal. For playback, the tape is drawn across the same, or a similar head. The magnetic " shorthand " on the tape causes an alternating current to flow in the windings around the reproducing head. The reproducing current closely duplicates the original signal. Although the principles are similar, the engineering problems are not; audio recording is today an easy task compared with TV recording. The reason is that audio signals are in the range of 20 to 20,000 cycles per second; while TV signals range up to 4,000,000 cycles per second. And colour television signals, as now fomulated, must carry at least twice as much pictorial information as black-and-white. Besides, TV tape must carry the associated sound signals. Among technical TV tape problems already solved by RCA or approaching solution, are: 1. High-Frequency Recording Heads. RCA research has resulted in specially developed recording and reproducing heads which respond to frequencies many times above the cutoff point for the recording heads used in sound recording on magnetic tape. This means that the speed of the tape across the head has been brought within manageable limits. The equipment demonstrated in the early part of December had a tape speed of 30 feet per second. Advanced equipment now under construction will move the tape at a lower speed, and with time, further reductions of tape speed appear likely. 2. Size of Magnetic Tape Reels The magnetic tape reels of the present laboratory equipment as demonstrated are 17 inches in diameter and will record 4 minutes of a television programme. RCA is working now for a reel 19 inches in diameter which will carry a 15-minute programme. 3. Electronic Amplifying and Equalizing Circuits Recording and reproducing amplifiers have been designed to handle the signal inputs and the signal outputs. These take into account and compensate for the characteristics of the heads and the magnetic tape materials when recording the very wide bands of frequencies used in television. Further development is in progress to obtain even better response characteristics. 4. Constant Speed Tape Transport Mechanism Since even small variations in the speed of the tape and in the pressure at which it bears on the head can create noticeable effects in the picture, it has been necessary to devise precision apparatus to control accurately the speed of tape at the recording and reproducing points. The laboratory TV tape equipment controls these many times more accurately than is necessary in magnetic tape recorders for sound. Even greater precision in regulating speed and pressure