JPS6315778B2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention provides AM broadcasting that is compatible with existing AM broadcasting.
This invention relates to a stereo broadcast system and its receiver.

In order to realize such AM stereo broadcasting, the main signal (L+R) and sub signal (L-R)
as two types of modulation signals: AM-PM system, AM
- Modulation methods such as FM method and orthogonal carrier method have been proposed in the United States. Especially AM-PM
The AM-FM system is highly compatible with monaural broadcasting, and the receiver is relatively simple to configure, so it is highly likely to be implemented.

On the other hand, in Japan, AM is being used in exactly the same way.
Transmitting and receiving stereo broadcasts is desirable for both receiver users and manufacturers, but there are problems in terms of domestic radio regulations. That is,
By applying phase modulation or frequency modulation in addition to conventional amplitude modulation, the number of sidebands increases and the occupied bandwidth is expanded. In the United States, the permissible bandwidth is relatively wide, so even if the maximum phase shift due to sub-signals is set to 1 radian in the AM-PM system (Magnabox system), for example, transmission can be done within the permissible value. . However, in domestic AM broadcasting, the carrier wave frequency is ±7.5KHz.
Since 99.5% of all energy must be contained within, the above Magnabox method cannot be applied as is.

There are two methods to solve this problem:

(1) Attenuate the high frequency of the sub signal.

(2) Make the modulation by the sub signal shallower.

Method (1) attempts to suppress the spread of the spectrum by limiting the modulation frequency in phase modulation (or frequency modulation), but as a result, the degree of separation during stereo playback can only be obtained at low frequencies. This cannot be called true stereo broadcasting.

Method (2) attempts to suppress spectrum broadening by lowering the modulation index and lowering the level of secondary and higher order sidebands, which eliminates the problem in (1), but Sub signal S/
It has the disadvantage that N decreases. A more specific explanation will be given using the AM-PM method as an example. The carrier wave s(t) to be modulated by the Magnabox method is assumed to have a carrier angular frequency as ω _O , a left signal as l, and a right signal as r (the pilot signal is omitted).

s(t)=(1+l+r) cos(ω ₀ t+(l-r))
It can be expressed as Here, in order to limit the amount of phase modulation by the sub-signal (l-r), let a be a constant of 0a1, and s(t)=(1+l+r)cos(ω ₀ t+a・
(l-r)). If you decrease the value of a,
Sideband waves of secondary order and higher order drop to a level where they can be ignored. For example, when a=0.3, from the Betzel function,
The level of the secondary sideband is approximately -20dB compared to the carrier wave.
Therefore, even if it is removed by a filter at the time of transmission,
No large distortion occurs during demodulation. This allows both the main signal and sub signal to be modulated up to 7.5KHz, making perfect stereo playback possible. However, in terms of S/N of the sub signal, when a = 0.3, it is 10 dB
This results in a decrease in Especially when modulating a small signal, the influence of S/N reduction is easily detected.

Therefore, in order to solve the drawback of method (2), the present invention performs companding processing on the amplitude of the sub-signal during modulation and demodulation of the sub-signal to ensure a narrow occupied bandwidth and high S/N. AM in Japan
This makes stereo broadcasting possible.

FIG. 1 is a block diagram of a transmitter according to the invention. The sub-signal is multiplied by a by an attenuator 3 and then compressed by a compressor 4. Compression processing refers to non-linear processing that lowers the amplification degree of the large amplitude portion of the original signal and increases the amplification degree of the small amplitude portion. FIG. 4 shows the characteristics of the compressor 4 and the characteristics of the decompressor 11, which will be described later. It is well known that by lifting and transmitting a small amplitude region on the transmitting side as shown in the figure, the S/N ratio of this small amplitude signal is improved during reproduction. The carrier wave which is the output of the oscillator 1 is modulated by the output signal of the compressor 4 in the phase modulator 2, and then modulated by the main signal in the amplitude modulator 5.

FIG. 2 is a block diagram of a receiver compatible with the broadcasting system to which the present invention is applied. The received signal is RF
The main signal is amplified by the IF section 6 and detected by the amplitude detector 7. After passing through the limiter 8, the phase is detected by the phase detector 9, and the amplifier 10
After being multiplied by 1/a, the signal is input to an expander 11 whose characteristics are opposite to those of the compressor 4. The output signal of the expander 11 becomes a demodulated sub-signal, which is processed by the matrix circuit 12 together with the main signal from the amplitude detector 7.
Separated into left and right signals.

Here, 13 is a changeover switch, which allows 1/
The a-fold and extension processes can be bypassed.
In this way, the same receiver can support AM stereo broadcasts in both the United States (without decompression processing) and Japan (with decompression processing).

FIG. 3 is a diagram showing the output level of the sub-signal when compressor 4 is used. Line A is the sub-signal level in the normal Magnabox system, line C is the characteristic when a=0.3 and no compressor, and line C is the characteristic when a compressor is used. From a comparison of straight lines A and C, it can be seen that the S/N ratio can be improved more than the normal Magnabox method when the signal is small.

As compressors and expanders, instantaneous companding of waveforms may be used, for example, (1) using diode characteristics, (2) using logarithmic functions, and (3) using polygonal characteristics. death,
Performs companding in response to changes in the average level of the signal.
Syllable companding may also be used. The compressor 4 in this case includes a voltage controlled amplifier inputting the sub-signal,
It is also configured with a level detector which inputs a sub-signal and controls the amplification factor of this voltage-controlled amplifier. Furthermore, a system that takes frequency characteristics into consideration in the companding effect is also possible; for example, it is also possible to use a noise reduction system that is already in use on the market.

As described above, according to the present invention, by applying companding to the sub-signals in a stereo signal, it does not affect compatibility with monaural signals, and also provides maximum support for AM stereo receivers in the United States and Japan. It is possible to have commonality.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a transmitter according to an embodiment of the present invention, FIG. 2 is a block diagram of a receiver according to an embodiment of the present invention, and FIG. 3 is a sub-signal output characteristic when a compressor is used. 4 are input/output characteristic diagrams of the compressor and decompressor. 1... Oscillator, 2... Phase modulator, 3... Attenuator, 4... Compressor, 5... Amplitude modulator, 6...
RF and IF section, 7...amplitude detector, 8...limiter, 9...phase detector, 10...amplifier, 11
...Extender, 12...Matrix circuit, 13...
Changeover switch.

Claims (1)

[Claims] 1 For the same carrier wave, one of two modulated signals is amplitude-modulated, the other is subjected to amplitude compression processing according to a predetermined rule, phase-modulated or frequency-modulated, and transmitted, and upon reception. is an AM stereo broadcasting system characterized in that a phase detection signal or a frequency detection signal is expanded with characteristics opposite to the compression processing described above. 2. The AM stereo broadcast system according to claim 1, wherein the compression and expansion are instantaneous compression and expansion of a waveform by a non-linear circuit. 3. The AM stereo broadcast system according to claim 1, wherein the compression and expansion are syllable compression and expansion using a level detector and a variable gain amplifier. 4 A receiver that receives an AM stereo signal that is phase-modulated or frequency-modulated after amplitude-modulating one of two modulated signals and compressing the other according to a predetermined rule for the same carrier wave, the receiver receiving the phase-modulated or frequency-modulated AM stereo signal. an amplitude detector that detects the modulated signal; a phase or frequency detector that detects the phase modulated or frequency modulated signal; and an expander that expands the output of the phase or frequency detector with characteristics inverse to the compression process. and a matrix circuit that matrixes the output of the amplitude detector and the output of the expander to extract left and right signals. 5. The AM stereo receiver according to claim 4, further comprising a changeover switch for switching between the output of the phase or frequency detector and the output of the expander and inputting the same to the matrix circuit.