CN1008040B - Improved amplitude-modulated stereo radio system - Google Patents

Abstract

The present invention relates to a system which can transmit and receive a compatible amplitude modulation stereophonic signal. The system provides an improved output quality by a controlling the electric level of a pilot signal on a transmitter, matching with a filter and controlling the phase of an audio output signal.

Description

Improved amplitude-modulated stereo radio system

What the present invention relates to is that amplitude-modulated stereo radio transmits and receives, and specially refers to the improvement of the existing system that generally uses.

Am stereo system is the system that comprises transmitter and receiver, and when sound is divided into two sound sources in the space, for example when the left side of music hall or theater stage and the right, this system uses two information signals of expression sound.These two information signals are combined into and signal and difference signal, and are used for the modulated RF carrier signal.CF signal reception and demodulation with emission reaches the purpose that offers a pair of audio converter as output signal then, and two signals are as broad as long with original information signal basically.Must be in possible limit any obstruction final purpose is partly in the improvement system.Existing system has provided all accurate recovering signal than system in the past, but still leaves room for improvement.

(I+L+R) cos(Wct+ φ of native system use) signal of form.Wherein L and R are a left side and right audio information signal, and Wct represents radio-frequency carrier, and φ is arctan(L-R+P)/(I+L+R), and P is a low-frequency pilot signal.Signal L-R is called difference signal, and L+R is called with signal or claims envelope signal.Signal and the monophony receiver compatibility of emission just, are used for receiving the receiver of amplitude modulation monophonic signal, and audio frequency input will be a common monophony programme signal and less than because of having added the caused distortion of stereo information.

One of place that needs the improvement system is the control of the pilot signal in the system transmitter part.Pilot signal is normally used for indicating stereophonic signal to exist in receiver, and makes and stereoly work in receiver.Pilot signal (p) level with respect to the emission carrier wave is very crucial, this be because signal too the young pathbreaker can not drive receiver circuit rightly, and signal too senior general disturb the demodulator circuit of difference signal passage, in other words, pilot signal just can not drive stereo work a little less than too, and when too strong, pilot signal will make last output signal distortion, and suitable scope is very narrow.

What problem may take place in the system is the intermediate-frequency strip of receiver on the other hand.Because stereophonic signal is in having intermediate frequency (IF) grade process of passing through that limits bandwidth, restituted signal comprises the appreciable additional high-frequency audio signal of part, and ideal situation is that this high-frequency audio component should be eliminated before information signal is coupled to as audio converters such as loud speakers fully.If can not eliminate this class distortion fully, must reduce the also acoustic mass of sound signal.

What influence system output signal in addition is that the input that is difficult in loud speaker obtains correct signal polarity.Because the audio output signal of demodulation is the carrier wave in the receiver and the function of local oscillation relative frequency, this just need control for the phase place of exporting in the receiver.

Therefore, an object of the present invention is to transmit by improvement in the control of pilot signal level and suitable filtering and improve above-mentioned am stereo system in the phase control of receiver sound intermediate frequency output signal.

The improvement of native system comes from the improvement to circuit, promptly offer reference signal of not demodulation carrier level, and finish this improvement by pilot signal level being maintained with respect to the device on that reference signal predetermined level by device in the transmitter.Remaining improvement is the device in the receiver, one of these devices are to be used for eliminating because intermediater-frequency stage finite bandwidth and at the matched filter of the high-frequency signal that audio output signal added, also comprise the device of control audio output signal with respect to the phase place of the relative frequency of local oscillator and reception carrier.

Fig. 1 is the transmitter-side block diagram according to system of the present invention.

Fig. 2 is a receiver block diagram that is used for system of the present invention.

System of the present invention is to initial No.4 at U. S. application, the improvement of the am stereo system of 218,586 patents, and former patent is for the invention provides the reference background data.

The assembly (left side and the right side are called L and R) that the transmitter of Fig. 1 and the representative of the receiver of Fig. 2 are broadcasted the system of two audio signals by standard amplitude modulation broadcasting passage, its channel range is from 500KHz to 1500KHz usually.Two signal L and R can be any two information signals independently, and passage can be of any suitable.

Among Fig. 1, shown in transmitting oscillator 10 provide a carrier frequency signaling to 90 ° of phase shifters 12, original signal and phase shift signal are coupled to respectively on a pair of frequency raiser or the modulator 14,16.Second input 18 that is added to modulator 14 is expressed as I+L+R, i.e. monophonic signal.This monophone or envelope signal are the synthetic of L and R signal and direct current (DC) signal of representing the unmodulated carrier amplitude.The second stereo input signal L-R also obtains from L and R signal, the latter probably from for example two place suitable microphone (not shown)s.This difference signal is coupled on the modulator 16 via adder circuit 22 from an input 20.

Pilot signal is for example 25Hz of a typical low frequency signal.When difference signal, or when launched in the stereo equipment radio station, it just was added on the poor passage, and receiver detects this pilot signal, and utilizes it in several respects, and relevant situation map 2 will be described.Pilot signal is produced by audio frequency oscillator 24, and is coupled on a control circuit or the voltage-controlled amplifier 26.Because pilot signal level with respect to unmodulated carrier level very crucial (important), just must be set up reference level in order to control effectively.The control of this pilot signal can be a quite simple hand gear, or complicated automatic circuit.Represented a possible embodiments among the figure, wherein, a signal takes out and compound frequency raiser 27 with a signal that takes out from 90 ° of phase shifters 12 from the output of modulator 14, then in a very filtering in the low pass filter 28.The cut-off frequency of filter is very low, for example, and the part of 1Hz.Because frequency raiser 27 is output as 1+L+R, thereby filter produces a direct current signal of representing the unmodulated carrier level.This direct current signal is used for controlling control circuit 26, and it can be a voltage-controlled amplifier, thereby has adjusted and be added in the 25Hz semaphore on the difference signal in the adder 22.In this preferred embodiment, this level has been defined as the 5%+0.1% of unmodulated carrier.Second input of modulator 16 is coupled in the output of adder 22.The output of modulator 16 be one with L-R+p modulated carrier signal (difference signal adds controlled pilot signal).

When the output of modulator 14,16 is synthetic in adder circuit 30, the result be one by the pure friendship signal of modulating respectively with signal and difference signal.If do not require and monophony receiver compatibility that this signal in fact can be as broadcast singal.Because signal must be compatible, it is to be coupled to amplitude limiter 32, and removes the modulation of look network within it, will only be coupled on the radio-frequency modulator 34 at carrier wave modulated on the phase place then.Modulator 34 comprises the common high-power amplifying stage of transmitter.Second input that is transported to modulator 34 is a 1+L+R envelope signal from end points 36, so that be with (1+L+R) cos(Wct+ φ at the broadcast singal of transmitting antenna 38) compatible signal of form.

Among Fig. 2, receiving broadcast signal selectively on reception antenna 40 and tuned radio frequency amplifying stage 42.With the same from the CF signal of local oscillator 46, the mixer stage 44 of selected signal coupling.Below also in detail the mixer stage function will be described in detail.The intermediate-freuqncy signal (IF) that produces is coupled on demodulating equipment that a first information signal L+R envelope detector 50 and one second information signal L-R synchronous detector 52 constitute and the amplitude limiter 54 by intermediate-frequency strip 48.The L+R output signal of envelope detector 50 is coupled on the matrix circuit 56.Synchronous detector 52 provides ((L-R+p) cosine φ) output signal, and this output signal is coupled on the frequency divider 58, and is corrected within it, and this problem also will be in following explanation.The L-R signal just is coupled on the matrix 56 by monophony/stereo switch 60 as a result.

The phase modulation output of amplitude limiter 54 is coupled on a cosine phase discriminator 64 and phase-locked loop (pLL) 66.The phase-locked loop comprises a phase discriminator 68, one low pass filters 70 and a voltage controlled oscillator 72.The phase change of amplitude limiter 54 output signals is represented in the output of phase discriminator 68.Low pass filter 70 has been eliminated the fast phase variation, by removing control generator 72 mutually.68 feedback networks that turn back to oscillator again force the frequency stabilization of oscillator on the intermediate frequency carrier frequency from oscillator 72 to phase discriminator.SIN function signal (WCt) from oscillator 72 is coupled on the synchronous detector 52, and is compound by the output of intermediater-frequency stage 48 on wave detector.Cosine signal from oscillator 72 is coupled on the cosine phase discriminator 64, compares in the output signal of this and amplitude limiter 54.The calibrating signal of Huo Deing (cosine φ) is divided into synchronous detector 52 output like this, produces accurate L-R+p signal at the output of frequency divider 58.

Also be coupled to from the output signal of frequency divider 58 and detect on the pilot signal wave detector 76 whether the 25Hz pilot signal exist.The rectification and be used to drive an indicating device 80 and monophony/stereo switch 60 in rectifier 78 of the output signal of wave detector.Normally in order to the existence of indicating stereophonic signal to the user whether the miniature bulb on the broadcast receiver front panel or a light-emitting diode (LED) for indicating device 80.Monophony/stereo switch 60 is by the output of rectifier 78 control, and its effect is to indicate when not having stereophonic signal or stereophonic signal strong inadequately when pilot signal, stops on the L-R signal arrival decoder 56.

Should be realized that to have only and adopt the intermediate-frequency strip of infinite bandwidth just might not cause distortion in this one-level of receiver.This is because the selectivity of intermediate-frequency strip can partial loss be fallen the high order sideband that produces at modulated process.But, have now found that audio output signal is carried out suitable filtering can reduce this distortion.And before being coupled to as audio converters such as loud speakers, filter audio signal by phase-control circuit 86 and output 90,92 at signal, then can eliminate this distortion substantially by matched filter 82,84.Owing to compare with the needs signal, unwanted signal is positioned on the high audio, thereby filter 82,84 must have low-pass characteristic.Term " coupling " is in order to the special relationship between the characteristic of expression output response characteristic of intermediater-frequency stage and filter 82,84.Thereby filter 82,84 must be well-designed so that distortion is reduced to below the level that can perceive.In order to realize this design, the cut-off point of first and second matched filters 82,84 must be positioned on the slope that intermediater-frequency stage 48(sees Fig. 2 output response characteristic a).Certainly, if necessary, also might insert input to filter 82,84 at stereo matrix 56.

If filter 82,84 is placed on after the stereo matrix 56, filter output will be coupled on the phase-control circuit 86.Because it is height or low that the L on audio output 90,92 and the polarity of R output signal are got faster than local oscillator 46 carrier frequency that ratio receives on frequency, so phase control 86 is necessary.The foundation of this conversion can be at triangular equation cosAcosB=1/2 (cos(A+B)+cos(A-B)) in see that wherein A and B represent reception carrier frequency and local oscillation frequency.The compound tense in mixer stage 44 as A and B, its result produces the item that comprises A+B and A-B.The A+B item can not lose by remaining circuit owing to frequency is too high.Certainly the A-B item will be determined by a higher frequency, and because the final output of matrix 56 is determined in frequency mixer output, thereby require phase place or Polarity Control in matrix output place.Control circuit can be to include to determine the circuit of a source with respect to the frequency in other source and the complexity that automatic open close is exported on end points 90,92, perhaps simple manual operation circuit as double-point double-throw switch.

So far, showed and the improvement system that is used to broadcast and receive the amplitude-modulated stereo signal has been described.The present invention also has many other conversion and changes shape, all will be protected as long as they fall in the spirit and scope of the claim of enclosing.

Claims (5)

1, a kind of improved am stereo receiver, comprise a local oscillator (46), be used for radio-frequency stage and the intermediater-frequency stage (42 of reception form for (I+L+R) COS (Wct+ Φ) compatible signal, 48), L and R are audio signals in the formula, Wct represents CF signal, Φ is arctan[(L-R+p)/(I+L+R)], P is a low-frequency pilot signal, and be used for demodulation and transmit with the device (50,52,54 that obtains two information signals, 58,64,65) it is characterized by: the low-pass filter device of coupling (82,84) is coupled to eliminate the demodulating equipment because of the high audio distortion component that finite bandwidth caused of intermediater-frequency stage from the output of first and second information signals.

2, according to an improved am stereo receiver of claim 1, it is characterized by: matched filter device comprises first and second low pass filters (82 that receive L+R and L-R signal respectively, 84), and filter mates with the characteristic of the intermediater-frequency stage of only removing unwanted high frequency distortion component.

3, according to an improved am stereo receiver of claim 1, it is characterized by: the filter apparatus of coupling comprises and is coupled to first and second low pass filters (82 that receive L and R information signal respectively, 84), and the output characteristic of filter and intermediater-frequency stage coupling.

4, according to an am stereo receiver of claim 2 or 3, it is characterized by: first and second matched filters (82,84), cut-off point and the slope of intermediater-frequency stage characteristic on a bit match.

5, according to an improved am stereo receiver of claim 1, it is characterized by: receiver comprises the device (86) that is coupled to reception L and R information signal, and this device is to change the relative phase of said signal by the relative frequency requirement of emission carrier wave and oscillation signals according.

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