JPS61292411A - Ssb automatic tuning system - Google Patents

Abstract

PURPOSE:To attain automatic tuning by detecting a difference between a transmission frequency and a reception frequency from a spectrum of a received sound so as to control the reception frequency of a receiver. CONSTITUTION:A waveform signal of a prescribed period length of a received sound signal 10 is extracted by a receiver 8 of the carrier suppression single side band communication system (SSB) and transformed by the Fourier transformed into a short time frequency spectrum G(f), the spectrum is calculated, a speak value M corresponding to the basic period of the spectrum is obtained, then (f) is shifted by (h) to form (f-h), the spectrum is calculated to obtain the valve M, and the process calculating the spectrum while varying the (h) is repeated to obtain the (h), (referred to as hp) corresponding to the maximum value of the value M, the reception frequency of the receiver is controlled by the (hp) to tune automatically the SSB receiver 8. Further, in order to apply the AFC without caring for the level of the side band, a prescribed factor (k) (0<k<1) is multiplied with the (hp) and the reception frequency of the receiver 8 is controlled.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention utilizes a carrier suppressed single sideband communication system (hereinafter referred to as SSB).
) Automatic tuning force type (AFC') that automatically matches the transmitting (carrier) frequency and receiving frequency in voice communication
It is related to.

In recent years, SSB has been widely used in shortwave communications and very high frequency communications in order to effectively utilize transmission power and frequency bandwidth. However, when SSB is used, unlike FM, a carrier wave is not sent, so AFC cannot be applied to achieve stable reception. Also, unlike AM and FM, the difference between the transmitting frequency and the receiving frequency directly causes distortion of the received audio, so quality deterioration due to detuning is a problem.
Being in sync is extremely important. Therefore, normally the receiver is carefully tuned to ensure the best quality of received audio during communication. In this case, even if the user is quite skilled, the accuracy of tuning is said to be about 3QHz. Further, even once the device is tuned, it is necessary to re-tune it over time because the transmitting frequency and the receiving frequency gradually change.

By the way, in cases where manual reception is not always required, or when automatically received audio is recorded and used later, it is necessary to listen to it as it is even if the detuning is large and the quality deteriorates. No. SS received out of tune
There is no other way than to perform SSB modulation and demodulation again on the B audio and modify it during that time. However, this is not an easy task and requires skilled manpower.

Therefore, AF'C is also proposed for 88B. For example, a method has been considered in which a carrier wave is left in transmission, this is detected by a narrow band filter on the receiving side, and the receiving frequency is controlled using the output signal as a guide to apply AFC. However, this method cannot be applied if the carrier wave is sufficiently suppressed, and there are various problems in stably detecting the carrier wave using a narrowband filter.

In addition, a method (U.S. Pat. No. 2,938,114,
(May 24, 1960) and other similar methods have been proposed. 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 automatically controls the receiving frequency of the receiver. It is characterized by synchronization,
The purpose is to introduce AFC into the 88B receiver and perform stable SSB communication.

First, the short-time frequency spectrum when audio is transmitted using SSB will be explained with reference to FIG. In FIG. 1, 1 is the short-time frequency spectrum when 2, 3, and 1 of the original voice are received with a detuning of F, and the amplitude is shown as an amplitude spectrum whose amplitude is expressed logarithmically.

Here, 2 is F<, O13 is F'>0, and when the synchronization is complete, 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 upper and lower sides 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, voiced sounds have a waveform caused by the vibration of the vocal cords as the sound source.
Sound waves that resonate in the vocal tract are 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 fO, 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, even if a peak similar to that of 1 is observed,
There is no harmonic relationship. Speech with a distorted harmonic structure not only gives a sense of discomfort, but also degrades intelligibility when F is large.

The embodiment of the first invention will be explained with reference to the block diagram in FIG. 2, the processing flowchart in FIG. 3, the amplitude spectrum diagram in FIG. 4, and the cepstrum diagram in FIG. 5. Here, FIG. 2 is a block diagram showing the configuration of a receiver that implements the method of the present invention, where 8 is an SSB receiver and 9 is a detuned frequency measuring section.

FIG. 3 shows the flow of processing performed in step 9. 10 is an audio input terminal and an output terminal of the 88B receiver, and 20 is a reception frequency control signal output terminal. 4 and 5 in FIG. 4 are rewritten versions of 2.3.

The SSB audio waveform signal received at 8 is added to 10 in FIG. Cut out this waveform by section length (window length) T,
7-lier transform to convert into a short-time frequency spectrum G(f). Its amplitude spectrum logarithmically converted log l
G (f) I is 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/fo) of the voice. The method to find the fundamental period from this maximum value is
This is widely used as a so-called 1 cepstrum pitch extraction method. 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. However, the maximum value (denoted as M) is large when F is 0 or when F is equal to the harmonic of half the fundamental frequency (fo/2), and small in other cases. . This means that in cases like 4- and 5, if the amplitude spectrum is viewed as a periodic wave,
This is due to the fact that the waveform phases differ on the positive and negative sides of the frequency axis. On the other hand, when F is 0, the amplitude spectrum is periodic across the positive and negative sides of the frequency axis, and 9M becomes large. Utilizing these properties, the unknown F is approximately estimated.

Shift f of 1og|G(f)| by h and calculate the cepstrum as f−h. Change h one after another and find the relationship between h and M. Among them, h when M is maximum is hp
This is the frequency corresponding to F. In addition, h
Considering that the initial tuning is done manually during reception, the range of movement is normally ±(about fO/2);
There is no problem even if it is fixed at about 50 to ±100 Hz.

The reception frequency of 8 is controlled by the HP obtained in 20.

In this case, 8 must be equipped with a circuit that changes the reception frequency using voltage or a synthesizer that can be controlled externally. It is sufficient to generate a required voltage, signal, or digital signal in the control signal generating section, and there are no restrictions on the method.

By the way, the direction of control of the reception frequency differs depending on the upper and lower sideband waves of 88B. For example, the receiving frequency must be fr-)-hp for the upper sideband and fr-hp for the lower sideband. When tuning manually or when the communication partner is known, this control is easy because the upper and lower sideband waves are also known. In order to apply AFC without worrying about the rise and fall of sideband waves, the following method can be considered. The reception frequency of 8 is controlled by multiplying hp by a constant coefficient k (0<k<1'). If the next HP obtained is smaller than the previous HP,
Continue similar control. When it becomes large, control is performed by reversing the sign of hp. If you repeat this process several times, f
r converges to ft.

Although the principle of the present invention has been explained above in an analog manner, a supplementary explanation will be given of an embodiment using digital processing. 10 is sampled at l Q kHz. Take out the signal of T = 51.2 m5 (sample number 512 points), convert it to a short-time frequency spectrum by fast Fourier transform, and repeat the conversion to cepstrum to obtain h
Find p. In this case, since 512-point fast Fourier transform is used, the unit of the frequency step of the amplitude spectrum is approximately 20 Hz, and the unit of the movement of h is also equal to this. Therefore, the error between F and hp will be at most half this unit, about 10 Hz. SSB manual tuning accuracy is 30
Considering that it is about Hz, this is sufficient accuracy. In addition, even with 256-point fast Fourier transform, 20Hz
There is no problem in practical use since the accuracy within the range can be obtained. More accurate tuning is possible if processing is performed with a longer window length, but since audio is a quasi-stationary signal, processing with a window length of 20 to 50 m5, which is used in normal audio information processing, is appropriate. be.

In the first invention, since the accuracy of AFC is within 10 to 20 Hz, it is not satisfactory when more accurate tuning is required. Therefore, the embodiment of the second invention with higher accuracy is based on the processing flowchart shown in FIG. 6, the amplitude spectrum shown in FIG.
This will be explained using the diagram of the cross-correlation function in FIG. Note that in the system of the second invention, the configuration of the receiver is the same as that in FIG. 2.

In Fig. 6, the SSH audio waveform signal added to 10 is cut out with section length T and subjected to Fourier transformation! Convert to 1lG(f). This amplitude is converted into a logarithm to calculate the cepstrum, and the process up to this point is the same as the process shown in FIG.

Next, the fundamental period (1/fo) is determined from the quefrency indicating the maximum value on the cepstrum. This process is the same as when using the cepstrum in the so-called Qi pitch extraction method. Once fo is determined, the frequency of fO is set as the frequency of the fundamental wave, and a spectrum of constant amplitude consisting of its harmonics is generated. This amplitude spectrum log I 8 is shown in Fig. 7.
(f) Show I. Next, using the frequency difference between both spectra as a variable, logl G (f) I and log
Calculate the cross-correlation function R(h) of I S (f) l. This is shown in FIG. The frequency hp of the difference between both spectra can be easily determined from the maximum value. The processing after HP determination is the same as the first invention.

As in the first invention, the accuracy of this method will be considered in the case of digital processing. When fO 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 20H2 increments and up to the Nth harmonic clearly exists, hp can be calculated with an accuracy of approximately (10/N) Hz.

If N=10, an accuracy of about I Hz can be expected. Note that, different from the first invention, the cross-correlation function can be calculated by setting h to a small increment (for example, IH2). Regarding the accurate determination of fO, various methods have been proposed such as the "pitch extraction method using N-cepstrum", the inside-out method and the method using 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 when there is silence, unvoiced sound, or transient sound, but the judgment is as follows.
This can be easily done from the value of M. Furthermore, if the HP is not determined, the known HP may be used as is. Also, SSB
Since the changes in the transmitting frequency and receiving frequency are usually not so rapid, it is possible to significantly skip the process of calculating HP to reduce the load on the computer, use a slow calculation speed computer, or use a single 9 It is also possible to control the reception frequencies of multiple receivers.

In principle, the method of the present invention may make an error when F and fo or harmonics of fO match. However, this probability is small, and over a long period of time, a constant f
It's never spoken in O, so it's not a problem.

As described above, according to the method of the present invention, it is possible to automatically synchronize with SSB, so it can be widely used not only for communication between fixed stations but also for mobile radios, fishing radios, etc. to improve call quality. In addition, the labor of the receiver operator can be reduced. Furthermore, by measuring the receiving frequency, it is possible to accurately know the SSB transmitting frequency, which is normally difficult to measure, so it can be useful for establishing orderly communication and monitoring radio waves.

[Brief explanation of drawings]

Figures 1, 4, and 8 are amplitude spectra, Figure 2 is a block diagram, Figures 3 and 6 are processing flow diagrams, Figure 5 is a cepstrum, and Figure 8 is a diagram of a cross-correlation function. be. 1...Original audio (or correctly received audio),
2 and 4...Sound received with detuning (detuned frequency F<0), 3 and 5...Same as above (F>0). Patent applicant Maki Hiba, Director of the Radio Research Institute, Ministry of Posts and Telecommunications No. 25117 2
Title of invention SSB automatic tuning method 3 Person making the amendment 5 Number of inventions increased by amendment None 1
, the brief description of the drawings section should be changed as follows: (1) From the 2nd line to the 5th line on page 14 of the specification, “1st
Figures 4 and 8 are amplitude spectra, -1111--
It is a diagram of -------. ” to “Figure 1, 41!I
, Figure 7 is the amplitude spectrum, Figure 2 is the block diagram, Figure 3 is the amplitude spectrum.
6 and 6 are flowcharts of processing, FIG. 6 is a cepstrum, and FIG. 8 is a diagram of cross-correlation numbers. ”.

Claims (3)

[Claims] (1) In a carrier-suppressed single-sideband communication system (hereinafter referred to as SSB) receiver, a waveform signal of a certain interval length of the received audio signal is extracted and converted into a short-time frequency spectrum G(f) by Fourier transformation, Calculate its cepstrum,
Find the peak value M corresponding to the fundamental period on the cepstrum, then move f by h and calculate the cepstrum as f-h to find M. Repeat the process of changing h and calculating the cepstrum to find the maximum of M. h corresponding to the value (supposed to be hp)
The receiving frequency of the receiver is controlled by HP, and the SS
SSB characterized by automatically tuning the B receiver.
Automatic tuning method. (2) In the SSB receiver, the 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_0
), then calculate the amplitude spectrum consisting of harmonics of f_0 and the cross-correlation function of log|G(f)|, and obtain the frequency hp of the difference between both spectra from the frequency showing the maximum value of the cross-correlation function, An SSB automatic tuning method characterized in that the receiving frequency of the receiver is controlled by HP to automatically tune the SSB receiver. (3) Obtain the frequency hp that is the difference between the transmitting frequency and the receiving frequency, and use hp to control the receiving frequency of the receiver. In the SSB receiver, change the receiving frequency by k·hp (here, 0<k<1) Then, when the next HP obtained is smaller than the previous HP, control is continued as it is, and when it is larger, the sign of HP is reversed and control is performed automatically without considering the upper and lower sides of the sideband of the SSB signal. The SSB automatic tuning method is characterized by synchronization.