Circuit for decoding traffic information message tone signals

• US 4,633,517 A
• Filed: 05/21/1985
• Issued: 12/30/1986
• Est. Priority Date: 06/01/1984
• Status: Expired due to Fees

• Current Assignee: Deutsche ITT Industries GmbH (TDK Corporation)
• Original Assignee: Deutsche ITT Industries GmbH (TDK Corporation)
• Inventors: Pfeifer, Heinrich
• Primary Examiner(s): NG, JIN FAY
• Application Number:
  US06/736,633
• Time in Patent Office: 588 Days
• Field of Search: 455/35, 455/36, 455/45, 455/226-228, 340/905, 340/825.44, 340/825.48
• US Class Current: 455/228
• CPC Class Codes: G08G 1/094 Hardware aspects; Signal processing or signal properties, e.g. frequency bands

Abstract

An essentially digital circuit is disclosed in which a demodulated broadcast signal is digitized by an analog-to-digital converter and processed in three signal paths each including a tuned filter. The tuned filters have closely adjacent resonance frequencies, the same resonance curves, and the same resonance rises. The signals at the outputs of these three signal paths are so evaluated by means of four comparators and an RS flip-flop that the message tone signal appears at the Q output of the flip-flop only in the presence of the message tone frequency.

Claims

1. A circuit for decoding traffic information message tone signals each having a message tone frequency, marking a traffic information message, said tone signals being contained, in the form of a carrier amplitude-modulated therewith, in a received broadcast signal demodulated with a conventional radio receiver, said circuit comprising:

means responsive to the demodulated broadcast signal for deriving, by further demodulation and analog-to-digital conversion, a digital signal in the baseband;

a first signal path to which said digital signal is applied for each message tone frequency;

second and third signal paths to which said digital signal is applied for frequencies differing from said message tone frequency by a maximum of +1% and -1%, respectively;

each of said first, second and third signal paths including a tandem arrangement of a digital filter tuned to the respective frequency, the digital filters for all of said first, second and third paths having the same bandwidth and the same resonant rise, a digital absolute-value stage, and a digital low-pass filter having an upper cutoff frequency lower than twice the message tone frequency;

a series combination of a first additional digital low-pass filter and a second additional low-pass filter receiving said digital signal, said first additional low-pass filter having an upper cutoff frequency equal to that of the digital low-pass filters, and said second additional digital low-pass filter, having an upper cutoff frequency equal to the frequency corresponding to the transient-time constant of the tuned filters;

first, second, third and fourth digital comparators, each having a minuend input and a subtrahend output, said first digital comparator having a minuend-greater-than-subtrahend output, said second, third and fourth digital comparators each having a minuend-smaller-than-subtrahend output, said first signal path being coupled to the minuend inputs of said first, second, third, and fourth digital comparators, the subtrahend inputs of said third and fourth comparators being connected to the outputs of said second signal path and said third signal path, respectively;

a first constant multiplier coupling the output of said second additional low-pass filter to said first comparator subtrahend input;

a second constant multiplier coupling the output of said second additional low-pass filter to said second comparator subtrahend input;

an RS flip-flop having its S input coupled to the minuend-greater-than-subtrahend output of said first comparator and providing the message tone signal at its Q output;

an OR gate coupling the minuend-smaller-than-subtrahend outputs of said second, third, and fourth comparators to the R input of said RS flip-flop; and

wherein

said first constant multiplier has a constant smaller than one, said constant being equal to the nominal modulation factor of the message tone signal;

said second constant multiplier having a constant smaller than one, said constant being equal to a presettable fraction of the nominal modulation factor.

2. A circuit in accordance with claim 1, wherein:

said means for deriving comprising a mixer for mixing

said demodulated broadcast signal with a local-oscillator frequency higher than the sum of the message tone frequency and the carrier frequency of the message tone signal,

an analog-to-digital converter having a sampling signal input, an analog low-pass filter having a maximum upper cutoff frequency equal to half the frequency of the sampling signal of said analog-to-digital converter for coupling the output of the mixer to said analog-to-digital converter and an additional digital absolute-value stage coupled to receive the output of said analog low-pass filter.