JPS59189756A - Digital am modulating circuit - Google Patents

Abstract

PURPOSE:To obtain a digital AM modulating circuit suitable for IC formation by switching the first digital data, which is obtained by level shift of data of an input digital information signal, and the second digital data, which is complementary to the input digital information signal, with a pulse signal having a carrier frequency. CONSTITUTION:Eight bits b0-b7 of each sample of a digital color video signal calculated from a frame memory 4 are supplied to one input terminals of eight exclusive X-OR gates E0-E7. Data ''1'' is always supplied to one input terminal of an X-OR gate E8, and a pulse signal SC is supplied from a terminal 25 to the other input terminals of X-OR gates E0-E8. This pulse signal SC has 50% duty ratio and has the carrier frequency. Outputs of X-OR gates E0-E8 are supplied to the input terminal of a 9-bit D/A converter 9, and the output is supplied to a low-pass filter 10, and an analog AM modulation signal is obtained as the output of the low-pass filter 10.

Description

[Detailed Description of the Invention] "Industrial Application Field" This invention provides a
This invention relates to a digital AM modulation circuit that performs M modulation.

"Background Art and its Problems" As a conventional AM modulation circuit suitable for IC implementation, a double-balanced modulation circuit using a plurality of differential amplifiers has been used. This requires that the DC level of the modulation signal be specified for analog modulation circuits, and when the modulation signal is transmitted through multiple stages of circuits by DC coupling, each circuit:1. - The DC level of the modulation signal fluctuates due to fluctuations in the power source, changes over time, temperature fluctuations, etc., and there was a drawback that a predetermined ΔM modulation signal could not be obtained. No. 1, it requires an adjustment circuit for adjusting the target flow balance, which requires modest adjustment, and has the drawback of increasing the number of end-r-pins when integrated into an IC.

Further, when performing DSB modulation using a double-balanced modulation circuit, it is necessary to operate the circuit by destroying the DC balance. At this time, not only the carrier component but also the baseband (modulation signal) leaks to the output, and if this baseband leakage is close in frequency to the AM modulation output, it is difficult to remove it with a filter. Therefore,
Leakage of the bass band could cause negative effects such as beats. However, since the degree of modulation is determined by how the DC balance is set, there is a problem in that the degree of modulation cannot be stabilized due to the instability of the DC balance.

1-Object of the Invention The present invention is constructed with a digital circuit, has a stable modulation degree, does not require an adjustment circuit, and has high density packaging.
The object of the present invention is to provide a digital AM modulation circuit suitable for IC implementation.

1-Summary of the invention-1 The present invention provides a data conversion circuit that level-shifts the data of an input digital information signal to form first digital data, and a data conversion circuit that forms second digital data complementary to the input digital information signal. and a switch circuit to which first and second digital data are supplied and which is switched by a carrier frequency pulse signal 1.
, ``Embodiment'' An embodiment of the present invention is one in which the present invention is applied to a ΔM modulation circuit used in a transmission system that transmits a color video signal obtained from a still video camera through a telephone line. .

Regarding this still image transmission system, Figures 1 and 2
This will be explained with reference to the figures. In FIG. 1, reference numeral 1 denotes a video playback device that produces both still color video signals. This still color video signal is sent to a disc-shaped magnetic notebook.
Each field or frame is recorded as concentric tracks. This magnetic sheet is stored in a cassette, and this cassette is attached to a video camera.The subject light is converted into an electric signal by an image sensor such as a CCD, and the magnetic sheet is recorded in This color video source has a band of, for example, 6 MH2, as shown in FIG.

The color video signal reproduced by the video reproducing device 1 is supplied to the A/D converter 3 via the clamp circuit 2, where it is converted into a digital color video signal of 8 samples and written into the frame memory 4. A/1
] As a write clock for the converter 3 and the frame memory 4, a clock having a frequency of, for example, 14M (z) is supplied from the terminal 5.

The frame memory 4 performs time axis expansion of the color video signal, and a read clock having a frequency of -6400- of the write clock is supplied from a terminal 6. Through this time axis expansion processing, the band of the color video signal is compressed to, for example, 800H7 as shown in Figure 2B, making it possible to transmit it over a telephone line and to reproduce the image on the receiving side (generally slow operation speed). .

The digital color video signal read from the frame memory 4 is subjected to DSB modulation by the digital AM modulation circuit 7. This digital AM modulation circuit 7 is supplied with a carrier signal having a frequency of 1700H2, for example, from a terminal 8. A D/A converter 9 is connected to the digital AM modulation circuit 7, and an analog AM modulation output is taken out from its output.

Further, low pass filter 10. Amplifier 11. An AM modulated color video signal is sent to a telephone line via a line transformer 12. As shown in FIG. 2C, this signal d2 has a frequency spectrum distributed in a band of 800 FIZ above and below the carrier frequency of 1700 Hz.

The present invention is applied to the digital AM modulation circuit 7 described above. This invention provides a modulation signal S1 shown in FIG.
The modulated signal shown in FIG. 3C is formed by alternately selecting the phase-inverted signal S2 shown in FIG. This is done using a circuit. .

If one sample is 8 bits, the data will have a level range of 0 to 255. Therefore, the phase-inverted data as shown in FIG. 3B has a level range of -255 to O. For example, corresponding to the level of input data from 0 to 255, the signal S1 shown in FIG. 3A becomes a level of 0 to 255 as shown by the straight line 15 in FIG. The phase-inverted signal S2 shown in FIG. 4 has a level of 0 to 255, as shown by a straight line 16 in FIG. FIG. 4 shows the digital data values converted into analog data.

Here, since 255 to -255 is difficult to imagine, as shown in Figure 4, if this value is in the range of 511 to 0, it becomes 256 when the input data is 0, and 511 when the input data is 255. It is sufficient to generate 0 (inverted version). Since 0 to 511 can be represented by 9 bits, the input data is shifted by 1 bit to generate data 256 to 511, and the 1 and 0 of this are inverted to create data 256 to 511.
Obtain 0 data.

FIG. 5 shows the basic configuration of the present invention based on such processing, in which 21 indicates input data (for example,
22 is an inversion circuit that inverts the output of the conversion circuit 21 between 0 and 1; 23 is the output of the conversion circuit 21 and the inversion circuit; This is a switch circuit to which the output of circuit 22 is supplied.

This switch circuit 23 is supplied with a switching pulse having a carrier frequency and a duty ratio of 50% from a terminal 24.

During the H (Jf3 level) period of this switching pulse, the output of the conversion circuit 21 is selected, and during the L (low level) period, the output of the inversion circuit 22 is selected. The output of this switch circuit 23 is supplied to the D/Δ converter 9, and an AM modulated output as shown in FIG. 3C is obtained as the output.

FIG. 6 shows an embodiment of the invention. IE(1-E7
Eight issuflusive OR gates (hereinafter referred to as X-
It is abbreviated as 0R gate. ) are supplied with each bit of the 8 bits (bo ``b7) of each sample of the digital color video signal read out from the frame memory 4.Bit bo is the lowest pinot). b7 is the most significant bit. Also, data of 1 is always supplied to one of the X-OR gates indicated by E8. A pulse signal S. is supplied to the input terminal from the terminal 25. This pulse signal s.

is at the gear rear frequency with a duty ratio of 50%. (X-OR game) The output of Eo-E8 is supplied to the input terminal of a 9-bit D/A converter 9. The output of the analog D/A converter 9 is supplied to the low pass filter 10, and the output of the low pass filter 10 is analog AM.
A modulated signal is obtained.

Pulse signal S. In the period when is O, X -OR gate E8
The output becomes 1, and each bit of data from bo to b
7 is the same as X-OR gate E. ~ Appears in the output of E7. This is an operation that converts input data into a level range of 256-511. On the other hand, the pulse signal S. is l
During the period , the output of the X-0R gate E8 becomes 0, and each bit b of the data. ~b7 is inverted and becomes X −O
Appears at the outputs of R gates Eo to E7. This is an operation that converts the input data to the 255-00 level range. By performing these two conversion operations on the carrier frequency, digital AM modulation can be performed.

Oh, pulse signal S. D/A in synchronization with the level 1 change of
The output of converter 9 causes a level change of 1/29.

However, this level of level 1 fluctuation is r+J: /-1
, (It is hardly noticeable in the image and does not pose a problem.

Still, it is possible to reproduce both still images with sufficient image quality even if one sample is not 8 bits but 7 bits. Therefore, as shown in FIG. 7', when the original data is 8 bits, the least significant bit bo is omitted and the upper 7 bits 1]l to b7 are converted to X is supplied as one input of Eo, and 10 manpower is always available.
-OR gate E7 is provided, and these X -OR gates 1
- Pulse signal S from terminal 25 as the other human power of Eo-E7. It may also be configured to supply the following. In this case, an 8-bit I)/A=i inverter 9' can be used. In fact, an 8-bit D/Δ converter 9' is easier to obtain than a 9-bit one.

"Application example" X -OR all bits of 1 the/pull of dizotal data
If the carrier frequency is inverted by the gate, balanced modulation can be performed.

In the example of the child series, an X-OR gate is used, but unlike this, two data conversion tables as shown by lines 15 and 16 in FIG. The input data may be used as the address input of this memory, and the two data conversion tables may be alternately switched by a carrier frequency pulse signal. When this memory is used, it becomes possible to perform not only linear transformation but also nonlinear transformation.

Furthermore, the present invention is applicable not only to the still and dual image transmission system as shown in FIG. 1, but also to modulation circuits for AM broadcasting.

[-Effects of the Invention] According to the present invention, since it can be configured as a digital IC circuit, the degree of modulation can be made stable without being affected by fluctuations in the DC level due to temperature fluctuations, and the adjustment circuit can be It is possible to implement a digital AM modulation circuit that is unnecessary and also does not cause baseband leakage.

[Brief explanation of drawings]

FIGS. 1 and 2 are block diagrams and explanations of the transmitting side of the MET-11 image transmission system in which the digital/LAM modulation circuit according to the present invention is used.
:j wave number spectrum diagram, Figures 3 and 4 are waveform diagrams and route diagrams used to explain this emission, Figure 5 is a block diagram showing the basic configuration of this invention, and No. 61 No. 1 is a diagram of this A block diagram of one embodiment of the invention, FIG. 7 is a block diagram of another embodiment of the invention. 4... Frame memory, 7...
...D) IJ i AM modulation circuit, 9,9'・
・・・・・・-D/A CON KU-TA, 23・・・・・・
Switch circuit, EO to E8... Exclusive OR gate. Agent Masato Sugiura

Claims (2)

[Claims] (1) a data conversion circuit that levels-shifts the data of an input digital information signal to form first digital data; a circuit that forms second digital data complementary to the input digital information signal; and a switch circuit to which the second digital data is supplied and which is switched by a pulse signal of a gear rear frequency.
M modulation circuit. (2) (11-1) 2-lucive OR gates in which each bit of the same sample of the input digital information signal is supplied as one input, and an input always at a predetermined level is supplied as one input; and one exclusive OR gate, and by supplying a carrier frequency pulse signal to the other input of the two exclusive OR gates, the first and second digital data are 2. The digital AM modulation circuit according to claim 1, wherein the digital AM modulation circuit performs the processing of forming the signal and switching between the two using a carrier frequency.