JPH0153946B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for measuring a transmission (carrier) frequency on a receiving side in voice communication using a carrier suppressed single sideband communication method (hereinafter referred to as SSB).

In wireless communications, radio stations must emit radio waves at a predetermined frequency in order to conduct orderly communications. Generally, most radio stations operate as required by law, but there are a large number of radio stations, and not all of them necessarily comply with this. Regarding frequencies, it may be necessary to receive emitted radio waves, measure the transmission frequency, and apply some kind of regulation.

By the way, in recent years, in shortwave communication and very short wave communication,
SSB is widely used to effectively utilize transmission power and frequency bandwidth. However, when SSB is used, unlike AM or FM, a carrier wave is not sent, so it is difficult to measure the transmission frequency on the receiving side. By the way, normal
In SSB communication, the receiver is carefully tuned to ensure the best quality of received audio. The accuracy of the reception frequency at this time is about 30Hz, even for very experienced users, and this is also the accuracy of the transmission frequency measurement. In addition, there are individual differences in this measurement, and the reliability of the measurement results is inevitably problematic.

A more accurate method is to pass the received audio signal through two narrowband filters in an octave relationship, and then apply the respective output waveforms to the X and Y axes of an oscilloscope, while observing the resurge waveform. , a method of tuning the receiver has been proposed (Radio Control Bureau, Radio Technology Research Report No. 284, April 1978). In this case, if the tuning is done so that Lissages can be observed during stable vowel pronunciation, the frequency measurement accuracy will be about 3 to 4 Hz. However, even this method cannot be said to have sufficient accuracy, and its major drawback is that it must rely on the experience and intuition of the measurer. Furthermore, with this method, it is almost impossible to measure when the signal-to-noise ratio of the signal is less than 10 dB.

On the other hand, if AFC can be applied to SSB,
The transmission frequency can be automatically determined from the reception frequency. For example, a method in which the waveform obtained by linear envelope detection of the SSB waveform is compared with the waveform obtained by normal detection, and the reception frequency is controlled by a type of phase synchronization (US Patent No. 2938114, May 24, 1960)
There are proposals for similar methods, including . However, these have problems both in principle and technically, and there is currently no practical SSB AFC.

The present invention focuses on the fact that voiced sound has a harmonic structure with respect to its fundamental frequency, detects the difference between the transmitting frequency and the receiving frequency from the cepstrum of the received voice, and converts this frequency from the receiving frequency without human observation. The purpose is to measure the SSB transmission frequency more accurately and automatically.

First, the short-time frequency spectrum when audio is transmitted using SSB will be explained with reference to FIG. In Figure 1,
1 is the short-time frequency spectrum of the original voice, 2 and 3 are the short-time frequency spectra when 1 is received after being detuned by F, and the amplitude is shown as an amplitude spectrum expressed logarithmically. Here, 2 is F<0, and 3 is F>0, and when completely tuned, a signal with the same amplitude spectrum as 1 is obtained. Note that the positive/negative of F and the vertical relationship between the transmitting and receiving frequencies differ depending on the rise and fall of the SSB sideband waves, but here, the positive/negative of F will be considered based on the relative relationship between the spectrum of the original voice and the received spectrum. Speech can be divided into voiced sounds and unvoiced sounds depending on how it is generated. FIG. 1 shows an example of voiced sounds, and voiced sounds will be explained below.

By the way, a voiced sound has a waveform caused by the vibration of the vocal cords as its sound source, and is a sound wave that resonates in the vocal tract and is radiated into the air. Since the vibration of the vocal cords is quasi-periodic, speech has a quasi-periodic waveform with the vibration frequency of the vocal cords as the fundamental frequency f ₀ , and its short-time frequency spectrum has a harmonic structure consisting of the fundamental frequency and its harmonics. are doing. 1, the local peaks on the amplitude spectrum are harmonically related. On the other hand, in the case of 2 and 3, there is no harmonic relationship even if a peak similar to that of 1 is observed.

The embodiment of the first invention is shown in the block diagram of FIG.
This will be explained with reference to the flowchart in FIG. 3, the amplitude spectra in FIGS. 4 and 6, the cepstrum in FIG. 5, and the cross-correlation function in FIG. Here, FIG. 2 is a block diagram showing the configuration of a measurement system that implements the method of the present invention, where 7 is an SSB receiver, 8 is a frequency meter, and 9 is a
is the detuned frequency measuring section. FIG. 3 shows the flow of processing performed in step 9. 10 is an audio input terminal as well as an output terminal of the SSB receiver, and 20 is a detuned frequency signal output terminal. Also, 4 and 5 in Figure 4 are 2 and 3
This is a rewrite of .

In Figure 2, we receive SSB radio waves with a transmission frequency of ft, and tune as accurately as possible at 7. Measure the receiving frequency (local oscillation frequency) at that time with 8,
Let this frequency be fr. On the other hand, SSB received at 7
The audio waveform signal 10 is added to 10 in FIG.
This waveform is cut out with a section length (window length) T and subjected to Fourier transformation to be converted into a short-time frequency spectrum G(f). When this is converted into a logarithm, the amplitude spectrum log|G(f)| becomes 4 when the detuning frequency F is positive and 5 when it is negative. When this amplitude spectrum is Fourier-transformed again, the cepstrum shown in FIG. 5 is obtained.

By the way, in the cepstrum of a voiced sound, a remarkable maximum value is observed in the quefrency corresponding to the fundamental period (1/f ₀ ) of the voice. By the way, even in the case where there is no harmonic structure as in 4 and 5, if the fluctuation of the amplitude spectrum is periodic, the cepstrum that is the result of frequency analysis will have a maximum value corresponding to the fundamental period. Therefore, f ₀ can be easily determined from the que frency of this maximum value.

Next, the frequency of f ₀ is taken as the fundamental wave, and the constant amplitude amplitude spectrum log |S(f)| is composed of its harmonics.
generate. This is shown in FIG. Next, a cross-correlation function R(h) between this amplitude spectrum and log|G(f)| is calculated. h is the frequency difference between both spectra. FIG. 7 shows an example of R(h). In R(h), h when the maximum amplitude is shown (this is referred to as hp) is the measured value of F. The range of movement of h at this time is
Considering that you have to tune manually when receiving,
Normally, ±(about f ₀ /2) is sufficient, and it may be fixed at about ±50 to ±100Hz.

The transmission frequency is estimated from the hp and fr obtained in step 20. In this case, the calculation formula differs depending on whether the transmitted signal is an upper side band wave or a lower side band wave. For upper sideband waves,
Use fr+hp, and fr−hp for lower sideband waves. Note that the upper and lower sides of the sideband waves are often known, but can be easily known when tuning.

Although the principle of the present invention has been explained in an analog manner, a supplementary explanation will be given of an embodiment using digital processing. Assume that the received signal applied to 10 is sampled at 10kHz. T=51.2ms (number of samples 512
The signal at point ) is extracted, converted to a short-time frequency spectrum using fast Fourier transform, and then converted to a cepstrum. Next, the fundamental period (1/f ₀ ) is determined from the quefrency indicating the maximum value on the cepstrum. This process is the same as the cepstrum-based "pituchi extraction method." Once the fundamental frequency f ₀ is determined,
Let f ₀ be the frequency of the fundamental wave, and generate a spectrum consisting of its harmonics with constant amplitude. Next, with the frequency difference h between both spectra as a variable, log|G
Calculating the cross-correlation function of (f)| and log|S(f)|, we get
From the maximum value, the frequency hp can be easily determined from the difference between the two spectra. When f ₀ is accurately determined, the accuracy of this method is dominated by the order of harmonics included in G(f) and the frequency resolution of the amplitude spectrum. If G(f) is calculated in 20Hz increments and up to the Nth harmonic clearly exists, hp can be calculated with an accuracy of approximately (10/N) Hz. If T=10, the accuracy is about 1 Hz. Note that the cross-correlation function can be calculated by setting h in small increments (for example, 1 Hz). By the way, when finding the fundamental period (1/f ₀ ) from the cepstrum, the increments of the quefrency are 0.1 ms, which does not have sufficient accuracy as the value of f ₀ . For more accurate determination of f ₀ , use the “cepstrum pitch extraction method”.
Various methods have been proposed, such as the in-line method and the method based on moment calculation, and these can be used.

The process of determining hp is possible when the speech is voiced and quasi-stationary. This process cannot proceed in the case of silence, unvoiced sound, or transient sound, but this can be easily determined from the value of M.

However, when the signal-to-noise ratio of the received signal is low, errors may occur in determining the fundamental period or hp, or the peak of hp may not be noticeable. If F is constant and f ₀ is calculated correctly, R(h) is calculated one after another using the fact that each has a maximum value at a constant h.
(h) is accumulated n times. The accumulated R(h) is easier to determine by emphasizing the peak of hp, and
The HP measurements determined from this will be more accurate.
If n is set large, R indicates incorrect hp individually.
Even if (h) is included, the effect will be almost eliminated.
The method of determining hp after accumulating R(h) in this way is the method of the second invention.

As described above, according to the method of the present invention, when receiving SSB radio waves in which no carrier wave is transmitted, after initial manual tuning, the detuning frequency can be automatically measured. It is possible to measure transmission frequencies with high accuracy, and is widely useful for radio wave supervision and monitoring.
Can be used for AFC.

[Brief explanation of drawings]

Figures 1, 4, and 6 are amplitude spectra, Figure 2 is a block diagram, Figure 3 is a process flow diagram, and Figure 5 is a block diagram.
The figure is a cepstrum, and FIG. 7 is a diagram of a cross-correlation function. 1...Original audio (correctly received audio),
2 and 4...Sound received with detuning (detuned frequency F<0), 3 and 5...Same as above (F>0).

Claims (1)

[Claims] 1 Carrier suppression single sideband communication system (hereinafter referred to as SSB)
In the receiver, a waveform signal of a fixed section length of the received audio signal is extracted and converted into a short-time frequency spectrum G(f) by Fourier transformation, its cepstrum is calculated, and the fundamental period (1/f ₀ ) is calculated from the cepstrum. Next, for the amplitude spectrum consisting of harmonics of the fundamental frequency f ₀ and log|G(f)|,
Calculate the cross-correlation function R(h) regarding the frequency difference h,
An SSB transmission frequency measurement method characterized in that the frequency hp of the difference between both spectra is obtained from h indicating the maximum value of R(h), and the transmission frequency is measured from the reception frequency of the receiver and hp. 2 In the SSB receiver, a waveform signal of a certain section length of the received audio signal is extracted and converted into a short-time frequency spectrum G(f) by Fourier transformation, its cepstrum is calculated, and the fundamental period (1/f) is calculated from the cepstrum. ₀ ), then calculate the cross-correlation function R(h) regarding the frequency difference h for the amplitude spectrum consisting of harmonics of the fundamental frequency f ₀ and log|G(f)|, and calculate the cross-correlation function R(h) regarding the frequency difference h, Similarly for R
(h), repeat this operation to accumulate n R(h), obtain the frequency hp of the difference between both spectra from h that indicates the maximum value of the accumulated R(h), and calculate the frequency hp of the difference between both spectra. An SSB transmission frequency measurement method that measures the transmission frequency from the reception frequency and hp.