JPH0740642B2 - SSB demodulation circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention is directed to removing amplitude distortion received in a propagation path.
The present invention relates to a circuit system for removing distortion included in a demodulated signal in an SSB modulation / demodulation system in which an amplitude limiter is used in a demodulation unit.

[Conventional technology and problems]

After SSB modulation with a voice signal on the transmission side, a carrier wave is added to this to form a transmission signal, and after the signal is received on the reception side, the amplitude is fixed using an amplitude limiter and then frequency detection is performed. In the communication system, in order to reduce the distortion of the demodulated signal, it is necessary to increase the carrier wave level compared to the SSB signal level.

However, since the carrier wave does not directly transmit information, such a system has a drawback that the information transmission efficiency in the modulated signal is low.

[Object of the Invention]

In order to eliminate these drawbacks, the present invention aims to remove distortion in a demodulation section, and to provide a demodulation method capable of obtaining a high-quality demodulated signal without increasing the carrier level. I am trying.

Hereinafter, the configuration and the like of the present invention will be described in detail with reference to the drawings of the embodiments.

〔Example〕

FIG. 1 is a block diagram of an embodiment of the present invention, in which 1 is a signal input terminal, 2 is a Hilbert transformer, 3 is a subtraction circuit,
Reference numeral 4 is an adder circuit, 5 is a multiplication circuit, 6 is a delay circuit, 7 is an adder circuit, and 8 is an output signal terminal.

On the receiving side, the modulated wave in which the carrier wave is added to the SSB (lower sideband) signal is received, and the frequency is detected after passing through the amplitude limiter to remove the amplitude distortion received in the propagation path. The original signal is reproduced by equalizing / oct, and this demodulated signal is guided to the signal input terminal 1 of FIG.
Then, the signal is divided into four, and one of them is introduced into the Hilbert transformer 2.

The output of the Hilbert transformer 2 is further divided into two, and a subtraction circuit 3 and an addition circuit 4 are provided between the Hilbert converter 2 and the signal from the input signal terminal 1.
Then, the difference signal and the sum signal are made respectively. The multiplication circuit 5 multiplies the outputs of the subtraction circuit 3 and the addition circuit 4 as inputs.

On the other hand, one of the four divided input signals passes through the delay circuit 6, and the adder circuit 7 calculates the sum with the output of the multiplication circuit 5,
This is the output.

The delay time of the delay circuit 6 is determined so that the distortion component contained in the signal at the output signal terminal 8 is minimized.

Next, the operation principle of the present invention will be described in detail using mathematical expressions.

First, the voice signal m (t) is represented by an analytic signal as m (t) = f (t) + j (t).

(T) is a Hilbert transform pair of f (t).

When the carrier frequency is ω ₀ , the SSB signal plus the carrier output S (t) is S (t) = Ccosω ₀ t + f (t) cosω ₀ t + (t) sinω ₀ t = Acos (ω ₀ t−ω (t )) Can be written. here ω (t) = arctan (t) / (C + f (t)). For the amplitude limiter to be used, it is necessary to satisfy C> | f (t) |, | (t) |. The carrier output S (t) = A cos (ω ₀ t−ω (t)) is input to the amplitude limiter to remove the amplitude modulation component. Then, the signal having a constant amplitude, which is the output of the amplitude limiter, is demodulated by the frequency detector. The function of the frequency detector is mathematically differentiating, and it suffices to extract the low frequency component of its output e (t), which can be expressed as the following equation.

Where ω (t) is as described above Given in. For ω (t), c> | f |, c> ｜|
And Can be deployed. Therefore, Becomes Here, 0 (c ^-3 ) means a minute amount having the order of c ^-3 .

The signal excluding the DC component (ω ₀ ) of this output is equalized at −6 dB / oct, or its meaning is as follows. That is, If we simply use the integral (= ∫ dt) to equalize, it will be accompanied by phase rotation. For example, if cost is integrated, it will rotate by 90 ° with ∫ cos dt = sin t. Therefore, −6 dB / oct equalization is used for equalization without phase rotation. Using (integration) + (Hilbert transform) as an example of this equalization, Becomes

Therefore, the signal component at terminal 1 is Is.

Since the first term −f (t) / C is the main component of the signal, the operation of the embodiment shown in FIG. 1 will be described based on this, and each signal is −f ( The output of the Hilbert transformer of t) / C becomes − / C, the output of the subtraction circuit 3 is (− (t) / C) + (f (t) / C), and the output of the addition circuit 4 is − (((t ) / C) + (f (t) / C)).

When these two outputs are calculated by the multiplication circuit 5, the output is If this output is added to equation (1), Therefore, the secondary distortion can be removed.

The delay circuit 6 delays the main signal so as to match the delay time generated by the signal passing through various circuits before the output of the multiplication circuit 5, and the delay time is set so that the secondary distortion is minimized.

As is clear from the mathematical formula used in the above description,
As a means for realizing a function equivalent to that of the present invention, a signal of a difference between a signal obtained by squaring a demodulated signal and a signal obtained by squaring a Hilbert-transformed demodulated signal and a signal obtained by delaying the demodulated signal for a certain period of time are added. It is also possible to adopt a configuration in which an output is obtained by doing so.

However, in general, multiplying circuits (squares are performed by multiplying circuits)
Has a complicated configuration, and particularly when a plurality of sets are provided as in the above configuration, matching for compensating for the difference in the characteristics of each multiplication circuit is required and the handling becomes extremely complicated. , Good results can not be expected.

As can be seen in the embodiments, the present invention requires only one set of multiplying circuits, and therefore can realize a circuit which is easy to handle and has good characteristics.

〔The invention's effect〕

As described above, according to the method of the present invention, it is possible to remove the distortion included in the reproduced signal, and in particular, it is possible to remove twice the frequency component of the original sound that is audibly audible. The demodulated voice quality is improved. Further, when there is no distortion removing circuit, it is necessary to lower the distortion rate of the demodulated voice by raising the carrier level, but according to the method of the present invention, it is necessary to raise the carrier level as in the conventional case. Since it does not exist, there is an advantage that the information transmission efficiency can be improved.

[Brief description of drawings]

FIG. 1 is a block diagram of an embodiment of the present invention. 1 …… Input signal terminal, 2 …… Hilbert converter, 3 ……
Subtraction circuit, 4 ... Addition circuit, 5 ... Multiplication circuit, 6 ... Delay circuit, 7 ... Addition circuit, 8 ... Output signal terminal

Claims (1)

[Claims] 1. A SSB-modulated audio signal on the transmitting side, a carrier wave is added to this to form a transmission signal, and after the signal is received on the receiving side, its amplitude is made constant using an amplitude limiter. A circuit added to the receiving side of the communication system configured to obtain a demodulated signal by frequency detection from the A subtraction circuit (3) for obtaining the difference from the demodulated signal; an addition circuit (4) for obtaining the sum of the output of the Hilbert transform circuit (2) and the demodulation circuit; and an output of the subtraction circuit (3) and an addition circuit ( A multiplication circuit (5) for multiplying the output of 4), a delay circuit (6) for delaying the demodulated signal, an output of the delay circuit (6), and an output of the multiplication circuit (5) SSB characterized by comprising an adding circuit (7) Tone circuit.