Electrical audio signal processing systems and devices – Binaural and stereophonic – Broadcast or multiplex stereo
Patent
1997-05-13
1999-06-22
Loomis, Paul
Electrical audio signal processing systems and devices
Binaural and stereophonic
Broadcast or multiplex stereo
381 16, 455337, 329347, 329348, 375261, H04H 500
Patent
active
059150289
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention concerns a method of demodulating a quadrature-amplitude modulated stereo signal for radio receivers.
BACKGROUND INFORMATION
The object of the present invention is a method of amplitude-demodulation for radio receivers. Since such receivers are usually executed in digital technology, a digitally realized amplitude demodulator can make use of the timing signals present in the receiver.
SUMMARY OF THE INVENTION
A digital radio receiver for quadrature-amplitude demodulated signals is known from European Patent Application Number A 0 540 195. The received signal is digitalized and, in an initial stage of the circuity, blended and filtered and its sampling rate decreased, whereupon in-phase and quadrature components of the signal leave through an output terminal. Envelope detection and elimination of the direct component leaves the stereo sum signal (L+R), and the stereo difference signal (L-R) is generated by renewed blending and quadrature-amplitude modulation and conversion.
One advantage of the amplitude demodulator in accordance with the present invention is that the reference frequency does not need to be coupled to the frequency of the unmodulated carrier when matched if amplitude-modulation stereo broadcasts that employ a quadrature-amplitude modulated carrier are received. Such coupling is known for example from IEEE Transactions on Consumer Electronics, Vol. 40, No. Feb. 1, 1994, pages 64 to 74.
A particular feature of one embodiment of the present invention is that the phase signal can be corrected prior to construction of the tangent when there are deviations in frequency and phase in the intermediate frequency that has been converted into the baseband. Frequency is accordingly corrected by open loop control instead of closed loop control (PLL).
Frequency deviations can be advantageously corrected by subtracting from the phase signal a correction signal derived by measuring variations over time in the phase signal, subsequent low-pass filtering, and integration of the low-pass filtered signal.
In another embodiment of the present invention a correction signal can be derived by frequency-correcting and low-pass filtering a phase signal and subtracting the result from a phase signal that has already been corrected for frequency deviations.
Still another correction signal can in one embodiment of the present invention be constructed by obtaining the magnitude of and low-pass filtering a phase signal that has already been corrected for frequency deviations. A downstream threshold circuit supplies hysteresis. This third correction signal can then be added to a phase signal that has already been corrected for frequency deviations. Since phase angle is limited to between -.pi. and +.pi. in the arithmetic stage that computes the magnitude signal and the phase signal, the third correction signal can assume two different levels representing phase deviations of .pi..
Still another advantageous way of deriving the stereo difference signal from the phase signal according to the present invention is by low pass filtering the stereo difference signal to obtain a feedback signal, low pass filtering the signal that characterizes the variations in the phase signal over time, adding these two low pass filtered signals, adding the resulting signal to a phase signal that has been corrected for frequency deviations, and routing the resulting signal through a limiter in order.
Since the direct component depends in this event only on phase error, the nonlinear-distortion factor is particularly low.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates an embodiment of the present invention intended for monophonic reception.
FIG. 2 illustrates an embodiment of the present invention intended for stereophonic reception.
FIG. 3 shows a detailed illustration of the stereo-demodulation part of the circuitry of FIG. 2.
FIG. 4 illustrates the regulating circuit according to the present invention illustrated in FIG. 3.
FIG. 5 illustrates a component for decoding the phase compon
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Loomis Paul
Mei Xu
Robert & Bosch GmbH
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