RDS data demodulator capable of precisely attenuating ARI...

Details RDS data demodulator capable of precisely attenuating ARI... RDS data demodulator capable of precisely attenuating ARI...

1999-03-01

2003-04-29

Pham, Chi (Department: 2631)

Pulse or digital communications

Receivers

Particular pulse demodulator or detector

C375S316000

Reexamination Certificate

active

06556631

ABSTRACT:

BACKGROUND OF TIE INVENTION
1. Field of the Invention
The present invention relates to a data demodulator, and more particularly, to a data demodulator for demodulating the radio data system (RDS) broadcast operable in Europe.
2. Description of the Related Art
The Auto-fahrer Rundfunk Informations (ARI) broadcast system is popularized as one of the information providing services capable of mitigating traffic jam problems in Europe. In the ARI broadcast system, a broadcast station for broadcasting the road traffic information multiplexes a subcarrier having a frequency of 57 kHz, i.e., an “SK signal,” onto a speech signal. A receiver including a detection unit can recognizing this SK signal. This detection unit can detect as to whether or not a traffic information broadcasting program can be received from the presently tuned broadcast station based upon this SK signal detection result.
Furthermore, the amplitude of this subcarrier is modulated by using a specific frequency. The receiver can recognize that broadcasting of the regional information and the traffic information is commenced or finished by detecting this specific frequency. The signal regarding the regional information is referred to as the “BK signal,” and the signal regarding the start/end of the traffic information is referred to as the “DK signal.” The combination of the SK signal, BK signal, and DK signal is called the ARI modulation signal.
The RDS broadcast system is also known in this field. The RDS broadcast system is further developed from the above-explained ARI broadcast system, and is capable of providing various information services in the format of digital data. The technical specification of the RDS broadcast system is standardized by European Broadcasting Union (E.B.U.). On the transmission side, the transmission data is differentially encoded, and then a clock signal having the frequency of 1.1875 kHz is modulated in a 2-phase PSK modulation manner by using the differentially-encoded signal. Furthermore, the amplitude of the 57 kHz signal corresponding to the subcarrier is modulated in a subcarrier suppression type amplitude modulation manner by using this 2-phase PSK modulation signal. Then, a double-side-band (DSB) signal is multiplexed onto a speech signal. This double-side-band signal is referred to as an “RDS modulation signal.”
A receiver demodulates the DSB signal transmitted in accordance with the above-described technical specification, and is synchronized with the data in accordance with rules of E.B.U., so that the receiver can decode the message. It should be noted that the subcarrier of the RDS modulation signal has an in-phase relationship, or a quadrature-phase relationship with the third higher harmonic wave of the pilot signal (19 kHz) indicative of the stereophonic broadcasting program.
Both the RDS signal and the ARI signal can be simultaneously transmitted. For such a simultaneous transmission, the respective subcarriers are set to the same frequency of 57 kHz, and the quadrature-phase relationship can be continuously established between the phases of these carriers. The frequency shift of the RDS modulation signal with respect to the main carrier is usually +2 kHz to −2 kHz However, in the case that both the RDS modulation signal and the ARI modulation signal are transmitted at the same time, the frequency shift of the RDS modulation signal with respect to the main carrier is set to +1.2 kHz to −1.2 kHz, whereas the frequency shift of the ARI signal with respect to the main carriers set to +3.5 kHz to −3.5 kHz.
In
FIG. 3
, there is shown a spectrum of an RDS modulation signal
2
and a spectrum of an ARI modulation signal
3
, which are multiplexed on a speech signal
1
. To recognize such an RDS modulation signal on a.receiver side, a demodulator designed for this specific purpose is required. This demodulator will now be explained with reference to
FIG. 4
which shows a schematic block diagram of a conventional RDS data demodulator. This conventional RDS data demodulator includes a filter means
4
and an RDS demodulating means
5
. The filter means
4
extracts an RDS modulation signal
7
from an analog FM demodulation signal
6
which is demodulated by using the analog signal processing technique. The RDS modulation signal
7
is outputted from the filter means
4
. The RDS demodulating means
5
demodulates this output signal from the filter means
4
to derive an RDS data signal and a reproduction clock signal used to demodulate the RDS data.
In general, the filter means
4
employs an analog filter such as a switched capacitor circuit. At the output terminal of this filter means
4
, the RDS modulation signal
7
which has been separated from the speech (audio) signal is outputted. It should also be understood that when the RDS modulation signal and the ARI modulation signal are simultaneously broadcasted from the broadcast station, both the RDS modulation signal and the ARI modulation signal are outputted at the same time.
Both the extracted RDS modulation signal and the extracted ARI modulation signal are supplied to the RDS demodulating means
5
. The RDS demodulating means
5
contains a costas loop type PLL for demodulating the DSB signal. As shown in
FIG. 5
, the costas loop type PLL includes multipliers
8
and
9
, a phase comparator
10
, a loop filter
11
, and a VCO
12
. This type of PLL circuit carries out synchronization even when there is no subcarrier. That is, a synchronization can be established when the subcarrier becomes 0 degree, or 90 degrees with respect to the VCO. Consequently, such a PLL circuit is suitable for demodulating an RDS modulation signal having no subcarrier.
In the above-explained conventional RDS data demodulator, if only a RDS modulation signal is transmitted, the RDS modulation signal, which has been DSB-demodulated, is outputted as the synchronization-detection output
13
. If both the RDS modulation signal and the ARI modulation signal are transmitted at the same time, such an RDS modulation signal, which has been DSB-demodulated, is outputted as the quadrature detection output
14
. This is because when both the ARI modulation signal and the RDS modulation signal are transmitted at the same time, only the ARI modulation signal is synchronized since the modulation factor of the ARI modulation signal is higher than that of the RDS modulation signal. As a result, the RDS modulation signal having the quadrature-relationship with the ARI modulation signal is outputted as the quadrature modulation output
14
.
Accordingly, when the costas loop type PLL circuit is used, one has to switch the ARI modulation signal and the RDS modulation signal in order to deal with simultaneous transmission of the ARI modulation signal and the RDS modulation signal. Japanese Unexamined Patent Publication No. 62-206929 discloses an improved method capable of switching the ARI modulation signal and the RDS modulation signal. As shown in
FIG. 5
, in a method disclosed in the above-referenced Patent Publication, an ARI signal detecting circuit
15
is provided to receive the synchronization-detection output
13
of the costas loop type PLL circuit for judging whether or not the ARI signal is present. Furthermore, a signal switching circuit
16
is employed to select between the synchronization-detection output
13
and the quadrature-detection output
14
. In response to a judgment result made by the ARI signal detecting circuit
15
, either the synchronization-detection output
13
or the quadrature-detection output
14
is outputted from the signal switching circuit
16
to a post-stage circuit (not shown), so that the RDS signal which has been DSB demodulated is derived.
However, this conventional circuit arrangement has the following problems. When both the RDS modulation signal and the ARI modulation signal are transmitted at the same time, the RDS signal cannot be derived until the ARI signal is detected by the ARI signal detecting circuit
15
. Therefore, a lengthy time period is required to obtain the RDS data.

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