EP1090469A1 - Simplified am stereo detector - Google Patents

Abstract

Left and right stereo audio information is reproduced from a compatible quadrature amplitude modulation (C-QUAM) broadcast using a simplified approximation to true C-QUAM decoding. A synchronous detection approximation is used which avoids generating an envelope signal or calculating a cosine correction factor as in true C-QUAM decoding. The method requires less intensive computation power thereby allowing a digital signal processor to perform other functions at the same time. The method produces no perceivable distortion or artifacts when reproducing normal broadcast material.

Description

SIMPLIFIED AM STEREO DETECTOR

The present invention relates in general to a radio receiver for receiving compatible quadrature amplitude modulation (C-QUAM) stereo radio signals, and more specifically, to a simplified technique for detecting AM stereo signals which requires less processing capacity while simultaneously improving signal quality for most typical broadcast signals and reception conditions. In commercial AM or medium-wave broadcasting, stereo stations broadcast using compatible quadrature amplitude modulation (C-QUAM) signals so that non-stereo capable receivers can still receive a compatible monophonic signal. As is known in the art, C-QUAM modulation involves phase modulating the stereo sum (L+R) and stereo difference (L-R) channels in quadrature followed by multiplying the phase components by a cosine correction factor. The signal is then limited to remove any amplitude variations and is finally amplitude modulated by the monophonic (L+R) signal. At the receiver end, a non-stereo capable receiver receives a compatible signal by recovering just the final amplitude modulation. In a stereo receiver, phase information is recovered in order to detect the stereo channels. In a typical receiver, the in-phase (I) signal component and the quadrature-phase (Q) signal component are synchronously detected. An envelope detector detects the envelope of the received AM signal. The I signal and the envelope signal are compared in order to recreate the cosine correction factor. The I and Q signals are multiplied by the correction factor to reverse the modulation process previously performed at the transmitter end. The cosine- corrected I and Q signals (or the envelope signal and the Q signal) are input to a stereo decoder for decoding left and right stereo channels. Many of the steps required in a typical C-QUAM receiver are computationally intensive which raises the cost of a receiver. For example, the envelope construction requires the calculation of a square root. In calculating the cosine correction factor, a divide calculation is required. In digital signal processing (DSP) hardware, these types of calculations require a relatively greater amount of processing resources than other calculations such as multiplication and addition.

An audio output of a typical C-QUAM receiver can be extremely distorted during adverse signal reception conditions such as when over-modulation or co-channel interference exists. When these errors are introduced into the received signal, the ideal C-QUAM calculations suffer from exacerbated distortion due to phase errors.

In one aspect, the present invention provides a method for reproducing left and right stereo audio signals in response to an AM stereo broadcast signal wherein a stereo sum signal and a stereo difference signal are modulated using compatible quadrature amplitude modulation including a correction factor. The broadcast signal is converted to an intermediate frequency (IF) signal. Coherent sine and cosine injection signals are generated in response to the IF signal. The stereo sum signal is approximated by mixing the IF signal with one of the injection signals. The stereo difference signal is approximated by mixing the IF signal with the other one of the injection signals. Left and right stereo audio signals are decoded by forming a sum and a difference of the approximated stereo sum and stereo difference signals without modifying the approximated stereo signals in response to the correction factor.

In another aspect, the invention provides a radio receiver comprising a digital signal processor including said coherent signal generator, said first and second mixers, and said decoder.

The present invention has the advantages of reducing processing resource requirements while introducing negligible, unnoticeable error during good reception conditions and providing improved performance during adverse reception conditions. The invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a block diagram showing a prior art C-QUAM receiver; Figure 2 is a block diagram showing a simplified C-QUAM stereo detector of the present invention; and

Figure 3 is a flowchart showing a method embodied in the apparatus of Figure 2.

A typical implementation for a C-QUAM AM stereo receiver s shown in Figure 1. A broadcast signal is received via an antenna 10 and provided to a tuner 11 for producing an intermediate frequency (IF) signal at a standard intermediate frequency of 455 kHz, for example. Depending upon the broadcaster, the IF signal may be modulated in C-QUAM format to provide stereo listening for C-QUAM receivers. A receiver may be implemented either in analog or digital form. For a digital implementation, an analogue-to-digital converter 12 digitizes a C-QUAM IF signal which is provided to an envelope detector 13. An envelope signal equal to 1+L+R is provided from envelope detector 13 to a DC blocking filter 14. After the DC offset is removed by DC blocking filter 14, an L+R signal is provided to a stereo decoder matrix 15 which produces right and left channel audio outputs.

The remainder of Figure 1 converts the C-QUAM IF signal to a regular QUAM IF signal in order to generate a difference signal L-R for input to matrix 15. Thus, the C- QUAM IF signal is provided to a variable gain amplifier 16. The output of amplifier 16 is coupled to an in-phase synchronous detector 17 and a quadrature-phase synchronous detector 20. The output of detector 17 provides an output signal corresponding to 1+L+R+ERROR to one input of a difference amplifier 18, where ERROR represents the cosine correction factor. The other input of difference amplifier 18 receives the 1+L+R signal from envelope detector 13. Difference amplifier 18 is a high gain amplifier and provides an ERROR signal to the gain control input of variable gain amplifier 16. The output of amplifier 16 is corrected for the ERROR and thus corresponds to a QUAM signal . The output of quadrature-phase synchronous detector 20 is connected to matrix 15 and to a phase detector 21. The QUAM signal from amplifier 16 is connected to another input of phase detector 21. The output of phase detector 21 is connected to a voltage controlled oscillator (VCO) 22 which provides a cosine signal to in-phase synchronous detector 17 and a sine signal (phase shifted by 90% with respect to the cosine signal) to quadrature-phase synchronous detector 20. Thus, phase detector 21 and VCO 22 form a phase-locked loop. The cosine and sine injection signals may also be generated using an adaptive line enhancer as is taught in U.S. Patent No. 5,357,574, which is incorporated herein by reference.

In order to accurately reverse the modulation process performed at the transmitter, the receiver in Figure 1 performs envelope detection along with synchronous detection in order to determine the cosine correction factor. When digital signal processing is used in Figure 1, the envelope detection requires performing a square root calculation and the correction factor calculation requires a division, which adds complexity to the DSP implementation. When the transmitted signal consists of a pure sinewave, these operations are essential in order to accurately decode the C-QUAM modulation with low distortion (e.g., less than about 2% THD) . However, commercial broadcasts rarely include any such pure sinewave as part of either a normal band-limited music or voice transmission. For the actual types of audio signals broadcast over commercial transmitters, an accurate reproduction of the audio is achieved by the present invention with far less computation by eliminating the envelope detection and calculation of the cosine correction factor, and instead using synchronous detection alone as an approximation . When receiving under adverse reception conditions, such as during over-modulation or co-channel interference, phase information is corrupted. In normal C-QUAM decoding, the calculation of the cosine correction factor is greatly impacted by the phase errors leading to large signal distortion in the detection. Under these conditions, the approximation of the present invention is less affected by the phase errors and so produces an audio output of better perceived quality to the listener. Referring to Figure 2, a preferred embodiment of the present invention employs a coherent signal generator 25 receiving a C-QUAM IF signal from an A/D converter (not shown) . Generator 25 may be comprised of a phase-locked loop or an adaptive line enhancer as described above. A sine and cosine injection signals are provided to inputs of mixers 26 and 27, respectively. Mixers 26 and 27 also receive the C-QUAM IF signal. By mixing the sine and cosine injection signals with the IF signal, an in-phase demodulated (I) signal and a quadrature-phase demodulated (Q) signal are produced. In the present invention, the Q signal from mixer 26 approximates the stereo difference signal L-R. However, the Q-signal from mixer 26 includes a 25 Hz stereo pilot signal which is removed by a pilot rejection filter 28. The approximated stereo difference signal is supplied to one input of a stereo decoder matrix 29. The I signal from mixer 27 provides an approximation of the envelope signal 1+L+R. The DC component is removed in a DC blocking filter 30 and the approximated stereo sum signal L+R is input to a second input of stereo decoder matrix 29. The sum and difference of the I and Q signal approximations is formed in matrix 29 by adders 31 and 32, respectively, thus producing the left and right audio signals.

In this preferred embodiment, the C-QUAM IF signal is generated at an intermediate frequency of zero Hz. Filters 28 and 30 may preferably be comprised of second order high pass filters. The overall method of the present invention is shown in Figure 3. In step 35, a C-QUAM IF signal is generated. In step 36, coherent sine and cosine injection signals are generated. In step 37, the injection signals are mixed with the IF signals to get the I and Q signals. Stereo is decoded from the I and Q signals in step 38 without using the C-QUAM cosine correction factor or performing envelope detection. Therefore, less computation is required in the DSP receiver and reception performance is improved during adverse signal conditions.

When receiving a C-QUAM encoded stereo broadcast, the I signal of the present invention approximates the L=R information plus a small error while the Q signal approximates the L=-R information plus the same small error. The small error introduces little perceivable distortion. For a left only or right only stereo broadcast, the distortion introduced by this small error is equivalent to the distortion that would be present in a receiver using an ideal C-QUAM decoding. When a broadcast consists primarily of stereo difference information (i.e., L=-R modulation), the error introduced by the approximation can become quite large (approximately 20%

THD) especially at low frequencies (i.e., less than 300 Hz).

However, this type of modulation is rarely involved at low frequencies due to limitations in normal recording techniques as well as phase cancellation which results in a playback for such signals. Thus, the approximation of the present invention may provide overall reception performance better than true C-QUAM detection under many reception conditions.

Claims

1. A method for reproducing left and right stereo audio signals in response to an AM stereo broadcast signal wherein a stereo sum signal and a stereo difference signal are modulated using compatible quadrature amplitude modulation including a correction factor, said method comprising the steps of: converting said broadcast signal to an intermediate frequency (IF) signal; generating coherent sine and cosine injection signals in response to said IF signal; approximating said stereo sum signal by mixing said IF signal with one of said injection signals; approximating said stereo difference signal by mixing said IF signal with the other one of said injection signals; and decoding said left and right stereo audio signals by forming a sum and a difference of said approximated stereo sum and stereo difference signals without modifying said approximated stereo signals in response to said correction factor.

2. A method as claimed in claim 1, wherein said one injection signal is said cosine injection signal and said other one of said injection signals is said sine injection signal .

3. A method for reproducing left and right stereo audio signals in response to an AM stereo broadcast signal wherein a stereo sum signal and a stereo difference signal are modulated using compatible quadrature amplitude modulation including a correction factor, said method comprising the steps of: converting said broadcast signal to an intermediate frequency (IF) signal; generating coherent sine and cosine injection signals in response to said IF signal; mixing said sine and cosine injection signals with said IF signal to produce an in-phase demodulated signal and a quadrature-phase demodulated signal, respectively; and coupling said in-phase and quadrature-phase demodulated signals to a stereo decoder matrix for decoding said left and right stereo audio signals without performing an envelope detection on said in-phase and quadrature-phase demodulated signals and without regenerating said correction factor.

4. A radio receiver for reproducing left and right stereo audio signals in response to an AM stereo broadcast signal wherein a stereo sum signal and a stereo difference signal are modulated using compatible quadrature amplitude modulation including a correction factor, said receiver comprising: a tuner for converting said broadcast signal to an intermediate frequency (IF) signal; a coherent signal generator (25) for generating coherent sine and cosine injection signals in response to said IF signal; a first mixer (27) for mixing said IF signal with one of said injection signals to generate an in-phase (I) signal; a second mixer (26) for mixing said IF signal with the other one of said injection signals to generate a quadrature-phase (Q) signal; and a decoder (29) for forming a sum and a difference of said I and Q signals without modifying said I and Q signals in response to said correction factor.

5. A radio receiver as claimed in claim 4, comprising a digital signal processor including said coherent signal generator (25), said first and second mixers (27,26), and said decoder (29) .

6. A radio receiver as claimed in claim 4, wherein said coherent signal generator is comprised of an adaptive notch filter.

7. A radio receiver as claimed in claim 4, wherein said coherent signal generator is comprised of a phase- locked loop.