JPS59200548A - Frame synchronization system - Google Patents

Abstract

PURPOSE:To economize the constitution of a frame synchronizing circuit by applying the reverse Fourier transformation to signals of two waves different in frequency and sending out the resulted waves alternately, frame by frame, and calculating the shifted amount at reception time from the amplitude ratio of two waves received on a reception side. CONSTITUTION:A transmission side generates a clock by a clock generator 1 to generate frame pulses F and an encoding start pulse (a) after a specific numbers of pulses F are generated by a counter 2 by using said clock. A high speed inverse Fourier transformation FFT<-1> processing circuit 3 performs the inverse Fourier transformation of data 4 of f1 and f2 to send pulses F of the f1 and f2 alternately, frame by frame, to a transission line L through an interface IF circuit 5. The reception side detects 7 the head f1 of a signal received through an IF circuit 6 and actuates an FET processing circuit 8. The circuit 8 extracts spectrum amplitudes of the f1 and f2 and a logK arithmetic circuit 9 calculates the logarithm of the amplitude ratio from the extracted amplitudes to fine the amount DELTAS of shifting at the reception time, which is inputted to a shifting circuit 10. A shifted amount checking circuit 11 controls the circuit 8 and sends out the encoding start pulses when the DELTAS attains to 0.

Description

Detailed Description of the Invention (1) Technical Field of the Invention The present invention relates to an encrypted transmission method for analog signals such as audio information, and particularly to encryption and decoding of band-split analog signals using Fourier transform. This relates to frame synchronization methods.

(2) Technical background Encryption of analog signals is used as a security measure for communications over communication channels with limited bandwidth.

There are methods for performing a scrambler on the time, amplitude, or frequency of a signal, and among these methods, a frequency scrambler method has been conventionally used as a method with a wider band and less signal delay.

(3) Prior Art and Problems Generally, a transmission delay occurs in the transmission path between the sending side and the receiving side, so it is necessary to achieve frame synchronization of the transmitted information even in the above-mentioned encrypted transmission.

Conventionally, the technology for achieving this synchronization was on the transmitting side.

The synchronization code is converted to PSK (Phase Shiftl (e
ying) or FSK (FrequencyShi
ft Keying), etc., and on the receiving side, a PSK, FSX, etc. demodulator is used to detect the synchronization code and establish frame synchronization. However, there is a drawback that the device configuration for transmitting and receiving the synchronization coat becomes large.

(4) Purpose of the invention The purpose of the present invention is to solve the above-mentioned conventional problems.
The object of the present invention is to realize a frame synchronization method for encrypting and decoding one piece of communication information using fast Fourier transform with a simple configuration, and to enable common use of hardware.

(5) Structure of the Invention To achieve the second object, the present invention relates to a band-divided analog signal encrypted transmission system.

The transmitting side is equipped with means for generating two waves of signals with different frequencies using inverse Fourier transform, and the two waves of signals are sent out alternately every frame, and the receiving side receives them using Fourier transform. means for calculating the spectral amplitude ratio of the two-wave signals; and means for calculating the shift amount of the reception time using the amplitude ratio (+ifi), and determining the synchronization position by shifting the reception time using the shift I- amount. It is characterized by

(6) Examples of the invention The present invention will be explained in detail below with reference to Examples.

In the frequency scrambler method, fast Fourier transform (hereinafter simply FFT) is used to increase the number of signal band divisions.
A method using the following methods is being considered. This r"F
The basic principle of a frequency scrambler using T (FFT scrambler) is to convert a sampled sound waveform of a fixed interval (one frame) into a frequency string using FFT, and then randomly recombine this frequency string (commonly known as; (referred to as transliteration), then converted back to a time waveform by inverse FFT and transmitted through the transmission path.
Sending out.

On the receiving side, a demodulated waveform is obtained using a similar method.

FIG. 5 shows the basic configuration of this frequency scrambler system. In the transmitting side device SE, audio information, etc.
Convert it into a frequency spectrum by .

In the reference circuit 52, the data is sent to the transmission path (2) via a scrambler (transfer) circuit and an inverse FFT (FFT') circuit 53 using the encryption key KEY. At the receiving side device RE, the receiving I
The # signal is converted into a 3-frequency spectrum by the FFT circuit 54, descrambled by the key KEY in the decoding device 55, and the original audio information is obtained via the 7FFT (FFT-1) circuit 56.

The problem here is that due to delays in the transmission path I7, it becomes necessary to synchronize the scrambler and descrambler.

The present invention performs this synchronization using FFT and inverse FFT, and FIG. 1 shows a block diagram of the frame synchronization method of the present invention. In the figure, SE indicates the transmitting side and RE indicates the receiving side. In particular, only the parts related to synchronization according to the present invention are shown, so 1 is a clock generator for synchronization signals, 2
3 is an inverse FFT (FFT-1) processing circuit, 4 is a sending blind data, 5 and 6 are interface circuits, and 7 is a frequency (fl) detector ( 8 is an FFT processing circuit; 9 is an arithmetic circuit; 10 is a shift amount check circuit; and 11 is a shift amount check circuit. In addition, L is a transmission line.

fl and f2 are two frequency signals, (b) an encryption start pulse, and (c) a decryption start pulse.

On the transmitting side, a clock pulse generated by a clock CLOCK generator 1 is used by a counter 2 to generate a frame pulse (F) and an encryption start pulse (B). The encryption start pulse (b) is the (
As shown in (b), after a predetermined synchronizing frame pulse (F) is sent out, it is generated after being counted down by the counter 1 up to 9, for example, 6 frames. Frame pulse (F) is synchronized UN number f1. Used to create f2 delimiters. When the inverse FFT (FFT) processing circuit 3 receives data fj and f2, it performs inverse Fourier transform on these data and sends them out alternately every frame via the transmission line L interface circuit 5 as a sine wave on the time axis. It will be done. This time chart is shown in the diagram for explaining the frame synchronization timing process in FIG. 2 for the transmission signal. Furthermore, after the encryption start pulse (b) is generated, the encrypted data 4 (Fig. 2) is subjected to an inverse FFT (FFT) as explained in Fig. 5.
) Processing circuit 3--Input and converted as a 2-hour waveform.

sent to the transmission line.

On the receiving side, on the other hand, the beginning of the signal sent from the transmission path I is detected by the FL wave detector 7 via the interface circuit 6. This FL wave detector 7 is usually composed of a narrow band filter or the like. The output of the fl wave detector 7 is further input as a starting signal 5TART to the FFT processing circuit 8, and the FFT processing circuit 8 extracts the spectral amplitudes of fl and f2 from the received signal. Further, these spectral amplitude information is processed by a logK calculation circuit 9 to calculate the amplitude of fl.

Letting the amplitude of f2 be a2, it is processed as =al/a2, and the logarithm 1ogK of is calculated.

This calculation result is input to the shift circuit IO as a shift amount ΔS (described later). Shift amount check circuit to shift 1] In step 1, this shift amount ΔS is checked, and the FF is turned on again.
Sends a control signal to the T processing circuit 111188.

The frame timing position is shifted and the secondary fl and f2 signals are received. When the shift stitch check circuit 11 becomes "0", a decoding start pulse is generated as synchronization is established. This reception control is explained with reception signal B in FIG.

That is, two synchronization frame signals f1. When f2 is delayed by the transmission delay Δ and the signal is detected (a), the spectrum amplitude of fl and f2 is obtained by Fourier transform.

Extract a2 and calculate a1/a2. If this spectral amplitude ratio is used, as shown in the synchronization characteristic in Figure 3 (the horizontal axis is the delay Δt), I o gK = o, that is, al/a2 = 1 (in this example, Δt is in the range of 120 to 150 and wide). However, in this case, the error can be reduced by taking a more detailed predicted value of Δ), and synchronization is established by the sequentially obtained shift amount ΔS.

FIG. 4 shows a control flow for explaining the control of this shift amount ΔS. When data is transmitted with a frame length of F, as explained earlier, if the transmitting side sends two signals f I and f 2 that match the frame before starting encrypted communication, the receiving side Now let's detect the rising edge of fl, II)
, This is used as a trigger to operate the FFT processing circuit 8. In the logK calculation circuit 9, the logarithm value 1ogKi of the spectral amplitude ratio of the two waves is determined (42), and then the synchronization shift width Δti is determined from the relationship shown in FIG. 3 (43). That is,
What is logKi and Δti?

By converting the data into data such as Δtl = IogK1 Δt2 = IogK2 Δtn = IogKn and storing it in a memory etc., the human power f1. Δti can be determined by a L a 2 for f2.

The correct synchronization position is determined from the value of Δti.

Find Δ5i=2F−Δti ('44), use this shift amount ΔSi to shift the time axis of the received signal, and calculate the amount gKi+l of the 9th input signal46), and perform the above operation (Fig. 4). 4
Section 3 to 48)); Repeat until 1 amount gKi+l >logKi.

The correct synchronization position is determined by asymptotically approaching al = a2 and finally obtaining a shift amount ΔS = 2 F +1 that is shifted by one frame.

Incidentally, the clock synchronization signal on the receiving side shown in FIG.

The transmitting side inserts a pilot signal outside the band of the confidential voice to be transmitted, and the signal is sent to the receiving side.

On the receiving side, a clock can be obtained from this signal.

(7) As described in detail of the invention 9 According to the present invention, using FFT,
Two waves of signals fl and f2 are generated as synchronization signals, and the amount of shift 1 is determined on the receiving side based on the spectral amplitude ratio of these signals. This makes it possible to share the FF7 circuit and FFT circuit used in encryption, and The synchronization circuit configuration can be made more economical, and the 11η encoding and decoupling devices can be made smaller.

[Brief explanation of drawings]

FIG. 1 is a block diagram of the frame synchronization method of the present invention. FIG. 2 is a diagram explaining timing processing of frame synchronization,
FIG. 3 is a diagram showing the relationship between spectrum amplitude and delay amount, FIG. 4 is a control flow diagram for determining the shift i amount, and FIG. 5 is a diagram showing the basic configuration of the frequency scrambler system. 3; Inverse Fourier transform processing circuit 7; 4 Figure

Claims (1)

[Claims] In the encrypted transmission method for band-split analog signals,
The transmitting side is equipped with means for generating two waves of signals with different frequencies using inverse Fourier transform, and the two waves of signals are sent out alternately every frame, and the receiving side receives them using Fourier transform. It is characterized by comprising means for calculating a spectral amplitude ratio of the two waves of signals, and means for calculating a shift it of reception time based on the amplitude ratio, and determining a synchronization position by shifting the reception time according to the shift amount. Frame synchronization method.