FM Multiplex Broadcasting (DARC) Related Patent
Patent No.2085423 (Japanese Patent Publication No. Hei 8-2044)

FM Multiplex Broadcasting Transmitting Device and Receiving Device

This invention relates to an FM multiplex broadcasting system suitable for mobile reception, including in-vehicle reception, of the FM multiplex broadcasting of text/diagram/sound services by means of multiplexing digital signals on an upper adjacent channel of the present FM stereo sound broadcasting signals.
In FM multiplex broadcasting mobile reception, there is a problem with the higher FM multiplex signal bit error rate. This occurs as mobile reception multipath distortion becomes severe, especially when the L-R signal level increases, resulting in harmonic distortion elements coupling into the FM multiplex signal frequency to twice the amount of stereo sub-channel signals (L-R balanced modulation wave signals on 38kHz subcarrier). On the other hand, lowering the L-R level induces the problem of crosstalk interference on the FM multiplex signal side.
This invention is designed to overcome these problems by controlling the FM multiplex signal levels in correspondence to the L-R signal level. Figure 1 illustrates the structure of the FM multiplex broadcasting transmitting device, and Figure 2 the level control relationships between the L-R signals and the FM multiplex signals.
In Figure 1, dn is digital input data for FM multiplex broadcasting, and Cn an FM multiplex signal, generated by an MSK digital modulation scheme FM multiplex signal generating circuit (consisting of serial-parallel converter section 2 through binder section 16 in the figure). As seen in Figure 2, the FM multiplex signal Cn is level controlled at level controller section 18 by L-R signals supplied by stereo signal generator 22 and smoothed at LPF20. It is then multiplexed with FM stereo broadcasting signals at stereo signal generator 22. As Figure 2 shows, the lower multiplexing limit is determined as the point where the FM multiplex signal does not interfere with the primary FM sound signals, even when no sound is being broadcast. The upper limit is determined by the designated maximum frequency deviation of the primary FM carrier.

Figure 1 Multiplexing level control
On the receiving device side, MSK detection can be conducted after adequately amplifying sampled FM multiplex signals and processing them through an amplitude limiter.
This invention improves the compatibility between FM stereo broadcasting and FM multiplex broadcasting by preventing FM multiplex signal bit error rate deterioration due to multipath interference during mobile reception, and also suppressing FM multiplex signal interference, even in the case of low level FM primary sound modulation.

Figure 2 Example of an FM multiplex broadcasting transmitting device block

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