Utilising unused TV picture frame lines - using supplementary audio transmission and code-responsive interrupt switch

摘要

Unused picture frame lines are exploited for the compressed digital transmission of supplementary audio information in teletext services. The system comprises a frame data store (UT) and a store (bt) for the associated audio information. The former supplies a re-coding circuit and buffer (VTCO) to be able to transmit a picture frame in a single data burst responsive to a t.v. line counter (ZZ) causing the change-over switch (TtU) to connect a sender (S) with that buffer store (VTCO). Groups of teletext picture frames are thus sent out at intervals, and there are often also empty teletext picture lines. This unused capacity may be utilised for transmitting digitised audio signals at the compressed bit rate of 36 Kbit per second. At a teletext subscriber station (T1n) the receiver (E) normally feeds the digitised and compressed audio signals into a buffer (P) which gives out the data at a slower rate into a d-a converter to a loudspeaker. The input to the buffer is interrupted when a new teletext frame is selected via a code responsive switch (UTt) to receive the frame video data into a recirculating store (VT Dec).