JPH0671218B2 - Automatic tuning suppressed carrier SSB system - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (1) Description of technical field to which the invention pertains The present invention relates to a suppressed carrier SSB system capable of automatic tuning (AFC).

(2) Description of prior art SSB is widely used because it can communicate with half the bandwidth of AM. In SSB, if the reception frequency detunes from the transmission frequency due to the influence of the propagation path, etc., the quality of the reproduced sound may deteriorate significantly, so as a countermeasure, add a carrier wave or add a special pilot signal. Often added. However, in this case, the transmission power increases,
Disadvantages such as a narrow bandwidth allocated to the information signal. On the other hand, there is a suppressed carrier SSB method that does not add a carrier to save transmission power, but in this case, if detuning occurs, the reception frequency cannot be automatically matched with the transmission frequency. Therefore, under the present circumstances, the receiver normally tunes by sequentially changing the reception frequency while listening to the reproduced sound.

Up to now, several methods have been reported to automatically tune by the suppressed carrier SSB method, but few have been put to practical use.

(3) Object of the Invention The present invention automatically detects a detuning frequency by adding a weak standard signal (described later) to an information signal in advance on the transmitting side and transmitting the information signal, and performing predetermined signal processing on the receiving side. However, the purpose is to automatically tune the reception frequency to the transmission frequency based on the information.

(4) Description of Operation of the Invention In the present invention, the weak reference signal defined by the equation (1) is always added to the information signal on the transmission side and transmitted (first
See figure). At this time, since the energy of the additional signal is weak, it is masked by the information signal and does not disturb the hearing.

However, each variable in the equation (1) is defined as follows.

C = A value relatively smaller than the energy of the information signal.

(P ₁ , p ₂ , ..., PM ₎ = Random pattern consisting of 0 and 1 having sharp autocorrelation. For example, a maximum period sequence (M sequence) often used in the field of error correction code.

f ₀ = fundamental frequency.

Δf = frequency interval.

Random number uniformly distributed with θk = (-π / 2, π / 2) C = 1, fφ = 300Hz, Δf = 100Hz, period in equation (1)
Second example in which the short-time power spectrum of a (t) when 31 M-sequences are used is obtained by fast Fourier transform (FFT)
Shown in the figure. However, sampling frequency is 8 kHz, FFT analysis length is 512
The FFT coefficient interval is about 15.6 Hz (= 8000/512).

Looking at FIG. 2, a sharp peak is seen at the frequency corresponding to p _k = 1 (in this case, the number of k at which p _k = 1 is 16). The power spectrum pattern of FIG. The abbreviation "pattern" can be regarded as a kind of pseudo noise waveform. Therefore, if this pattern is reproduced by shifting on the frequency axis at the receiving side due to detuning, the detuning frequency can be determined by correlating the reference pattern and the playback pattern that are predetermined at the transmitting and receiving side. It becomes possible to detect.

Next, a specific procedure of the automatic detuning frequency detection method will be described (see FIG. 3).

First, the SSB receiver obtains the information signal r (t). FFT analysis is performed in the stop section of this information signal to obtain a frequency spectrum pattern (B ₁ , B ₂ , ..., _BN ). For convenience, this is called a reproduction pattern. Either the power spectrum or the amplitude spectrum may be used as the frequency spectrum. Note that N is 1/2 of the FFT analysis length.

Next, the correlation function between the reference pattern (A ₁ , A ₂ , ..., A _N ) and the reproduction pattern is calculated by the equation (2).

However, the change range (-h, h) of n corresponds to the maximum value (about ± 200 Hz) of the detuning frequency.

By the way, (p ₁ , ...
When a sharp autocorrelation such as an M sequence is used as p _M ), ρn in the equation (2) takes a large value only when n is sequentially changed and the reference pattern matches the reproduction pattern. , Otherwise it takes a small value. Therefore, if the maximum value ρm is searched from (ρ ₋ h, ..., ρh) obtained from the equation (2), the detuning frequency can be estimated by the following equation.

fd = m × (sampling frequency / FFT analysis length) (3) By feeding back the value of fm to the SSB receiver, the receiving frequency can be automatically tuned to the transmitting frequency.

(5) Effect of the Invention Sampling frequency is set to 8 kHz, coefficient C of the equation (1) is set to 1, basic frequency f _{0 is set} to 300 Hz, and frequency interval Δf is set to 100 Hz, and (p ₁ , ..., P _M ), A computer simulation experiment was performed using an M series with a period of 31. When the FET analysis length on the receiving side is set to 512 points, the frequency valve resolution is about 15.6 Hz, but it was confirmed by experiments that the present invention can properly estimate detuning frequencies that are integer multiples of this value. Although voice was used as the information signal in this experiment, the influence of the additional signal of the formula (1) on it could hardly be detected. Therefore, the present invention is sufficiently practical as an automatic tuning method for the suppressed carrier SSB, and automatically recovers the quality of the reproduced sound significantly deteriorated by the detuning of the reception frequency, without the need for human intervention as in the past. can do.

[Brief description of drawings]

FIG. 1 is a transmission block diagram, FIG. 2 is a power spectrum, and FIG. 3 is a reception block diagram.

Claims (1)

[Claims] 1. A transmission side always adds a reference signal composed of a large number of sine waves randomly selected from a large number of sine waves of the same amplitude arranged at the same frequency interval, and at the reception side. The frequency spectrum in the stop section of the information signal is obtained, the correlation function between the frequency spectrum of the information signal and the frequency spectrum of the reference signal is calculated, and the detuning frequency f _d is calculated from the maximum value of the function.
And control the reception frequency of the receiver by f _d
Automatic tuning suppressed carrier SSB system characterized by automatically tuning the B receiver.