GB1183338A - Improvements in or relating to Information Transmission Systems - Google Patents

Abstract

1,183,338. Television. MULLARD Ltd. 7 April, 1967 [16 June, 1966], No. 26870/66. Heading H4F. Television transmission systems, such as those described in Specifications 1,147,603, 1,147,604 and 1,149,051, in which a plurality of frames of information are transmitted in a cyclically repeated succession are modified in that the identification signals, which are provided to permit a desired frame to be selected from the succession, are formed of a plurality of components which differ from each other in frequency. The components may be transmitted simultaneously and may thus last throughout a substantial part of a line in the frame flyback which is allocated to the identification signals. In one arrangement, Fig. 2 (not shown), fifteen frequencies are available and the identification signals are code combinations formed of different selections of frequencies. If desired an identification signal may be repeated in two or more lines; however, if only a single line is used for a code combination, subsequent lines may convey further combinations of frequencies so permitting an increase in the number of possible identification signals. For this purpose a number of the frequencies serve as "routing" frequencies and the remainder provide the code combination. Thus, in a simple case. Figs. 3a and 3b (not shown), an identification signal may occupy two lines. Two frequencies are selected as "routing" frequencies, one for first line and the other for second line, whilst the remaining frequencies are selected to form code combinations. In a more complex arrangement, Fig. 4 (not shown), three frequencies are selected as "routing" frequencies so permitting eight lines to be distinguished during which the remaining frequencies (twelve where a total of fifteen is available) are selected to form code combinations. Further extension of this technique is limited since the number of blank lines available during the frame flyback interval is twelve and some of these may be required for other purposes. However, it is possible to use the flyback intervals of both the odd and even fields of a television picture and the specification refers to an arrangement based on four "routing" frequencies together with eleven code frequencies and occupying sixteen line intervals to form one identification signal. The specification also discusses more generally the optimum selection of the numbers of "routing" and code frequencies. Receivers for use in the systems, Fig. 5 (not shown), are identical to those described in Specification 1,147,603, except for the replacement of the sequence switch and associated pulse generator by a:rays of parallel connected filters and detectors to determine the presence of frequency components in the identification signals. The detector outputs prime the input register. Where "routing" frequencies are used, separate identical groups of filters and detectors corresponding to the code frequencies are provided for each line used for the identification signals and are brought into use in turn during the corresponding lines by gate stages operated by the "routing" frequencies.