| 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.
On the receiving device side, MSK detection can be conducted after adequately
amplifying sampled FM multiplex signals and processing them through an
Multiplexing level control
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