Patent GB-1229884-A (GB)
IPC: H04J1/00
Filed: 1968-05-22
Abstract: 1,229,884. Multiplex radio signalling; radio receiving apparatus. PHILIPS' GLOEILAMPENTABRIEKEN N.V. 22 May, 1968 [25 May, 1967], No. 24422/68. Headings H3Q and H4L. In a system for transmitting additional information on the sound channel of a conventional television broadcasting system, the sound channel having an available bandwidth of approximately the line recurrence frequency, the additional information is modulated on the lower sideband of a single sideband sub-carrier having a frequency on the verge of the normal available audio range of said sound channel. The system may be used for the transmission of special pulse coded messages, for the measurement of the relative size of the audience for a television programme, or for the transmission of the speech accompanying the television programme in a foreign language in addition to the language of the country of origin. Stereo transmission also is referred to. The line frequency or a signal derived therefrom by frequency changing is used for the sub-carrier, thus facilitating the regeneration of the sub-carrier at the television receiver for synchronous detection of the additional signal. As shown in Fig. 1, the sound channel has a normal bandwith Bg shown by the broken line 1 which is divided into a part having a bandwidth B N  with a cut-off frequency # a  for the normally transmitted sound signal 2 and a part having a bandwidth B for the additional signal 3 consisting of the lower sideband of sub-carrier # r . The composite signal frequency modulates the main sound signal carrier or alternatively amplitude modulation may be used. If the additional signal is used for the measurement of viewer response a much smaller bandwidth B v  is required, Fig. 3. In addition to the normal sound channel 1 ranging from 0 to # g  a second audio channel 6 is shown which may be transmitted as a single sideband or double sideband signal on a sub-carrier 2# g . The latter arrangement may be used for the difference signal in a conventional stereophonic transmission system. A second additional signal 7 may be transmitted as a sideband of a subcarrier #r 2 . Apparatus is described, Fig. 4 (not shown), in which the sub-carrier # g  of the additional signal is suppressed. Switches are provided so that the sub-carrier and the normal audio signal may be supplied from a television receiver or a studio and the resulting composite signal may be recorded for retransmission or broadcast directly. A receiver is described, Fig. 6, in which the output of the frequency demodulator 38 is supplied via a switch 39 to the normal sound reproducing apparatus 40, 41 and is also supplied to a demodulator 42 for the aditional signal. The demodulator 42 includes a synchronous detector 47 receiving the regenerated sub-carrier derived from the line deflection stage 33 via a filter 45 and the output of the detector is supplied via a filter 48 and switch S 3  to a recorder 49 for measuring the television audience size. A predetermined signal starts the recorder after which data associated with the programme in spectrum 2 is recorded. At predetermined intervals the tape is sent to a central office for assessment. Reversal of the switches S 3 , S 4  enables information such as a special commentary on the vision signal to be received via the normal audio equipment 40, 41.