Patent GB-894713-A (GB)
Title: Improvements in or relating to signal transmission systems
IPC: H04N5/919
Filed: 1958-06-23
Abstract: 894,713. Magnetic split-spectrum recordings. PHILIPS ELECTRICAL INDUSTRIES Ltd. June 23, 1958 [June 21, 1957], No. 20031/58. Class 40 (2). [Also in Group XXXIX] In a system for recording and playing back a signal of large band-width from a source 7 over a plurality of channels on a magnetic tape 14 having a smaller band-width the signals are fed to modulators 3 1  . . . 3 m  fed with cyclically staggered pulses derived from a pulse generator 1 and a tapped delay line 2, the outputs being fed to recording-heads 15 1  . . . 15 mÀ  The outputs from the playback heads 16 1 . . . 16 m  are gated in modulators 6 1  . . . 6 m  to a low-pass filter 10 to reconstitute the signal, the modulators being fed by cyclically staggered pulses received from a tapped delay line 9 fed by a pulse generator 8. A frequency divider 4 is fed by the pulse source 1 to produce a pilot frequency which is recorded on a separate track by head 15 and played back by head 16, the received frequency being multiplied at 6 to produce a synchronizing signal for the source 8. In the case of a T.V. signal the audio and the synchronizing signal may be transmitted on the same track and the video signal over the remainder of the tracks. If the frequency range to be transmitted is O-F c/s. then the bandwidth of each of the channels may F be approximately -. Owing to the fact that m the response of the channels to an impulse is of the nature shown in Fig. 7 so that undesirable overlap between the channel outputs occurs, the pulse repetition period of the sources 1 and 8 is made equal to the period # 2 -# 1 , where # 1  and # 2  represent the times of the first two crossings of the zero line by the response curve. The instant of sampling of the curve P m-2  due to one of the channel outputs thus occurs at time # 1  - -(# 2 -# 1 ),the value of the sample being aP m-2,  where a is constant for all channels if these are identical. It is now seen that since the samplings of the subsequent channel outputs P m-1,  P m  occurs at instans # 1  and # 2  these are unaffected by the presence of an overlap by P m-2. Since the response curve does not cut off equal portions of the time axis some effect #P m-2  on the pulse P m+1  may be expected. This may be eliminated by introducing a correction by means of the circuit of Fig. 3 in which the pulse output from each channel such as 5 1  (which represents generically the heads 15 1 , 16 1  and the associated track) is passed through a phase inverter 18 1  to gate 6 1  to be fed as the main signal to the common output. In addition the signal is fed to a device 17 1  which introduces a delay of 3(# 2  - # 1 ) and an attenuation of #/a. This emerges as a correction for next-but-two channel pulse. Alternatively, instead of attenuation being introduced at 17 1 , amplification could be introduced at 18 1 . Switching means may be provided to enable the same heads and modulators to be used for both recording and playback. A similar system for signal transmission over a plurality of communication channels is also described (see Group XXXIX).