JPS5816663B2 - AGC Cairo - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention is applied to a facsimile transmitter that scans a transmission document having a document width wider than 172 times the width of the scanning area, and controls the gain of the image signal obtained by scanning. Regarding AGC circuit.

In facsimile transmitters, it is common to use auto gain control, auto slice, or a combination of both in order to obtain good image signal output for transmitted documents of various paper qualities and contrasts.

Since the automatic gain control (hereinafter abbreviated as AGC) circuit determines the gain based on the difference in high and low levels of the signal, a problem occurs when the transmitted document is narrower than the width of the scanning area.

In other words, the reflectance of the material forming the scanning area in a facsimile transmitter and the base reflectance of the transmission original paper are generally different.

Therefore, if the scanning area is made black, both edges of the reproduced document will be black on the recording surface.

On the other hand, if the scanning area is made white, the recording surface will be in a state in which both ends are pure white, a base signal of intermediate density is reproduced in the center, and a character signal is reproduced in the black direction in the meantime.

In this state, since the normal AGC circuit determines the gain based on the difference between the white level at both ends and the blanking (absolute point) level, the base of the transmitted document is not involved in the AGC operation at all.

This problem will not be improved even if the scanning area is set to an intermediate color between white and black, and will instead have an adverse effect on the slice circuit and the like.

In order to solve the above-mentioned drawbacks, it is possible to automatically detect the document width and always perform gain control only for image signals within the document width, but in order to achieve this, a complicated device configuration is required. It is expected that this will lead to an increase in costs.

Therefore, it is an object of the present invention to provide an AGC circuit that does not require means for automatically detecting the document width and can perform optimal gain control to the same extent as when such means are used.

An embodiment of the present invention will be described below with reference to the drawings.

In FIG. 1, 1 is an AG that controls the gain of the original image signal A.
C amplifier, 2 is an AC amplifier that amplifies the output B of AGC amplifier 1.
Or DC amplifier, 3 is the absolute black level of signal B OV
4 is a blanking circuit that processes the output C of the clamp circuit 3 based on a blanking signal F generated by the blanking pulse generation circuit 7 in accordance with the conditions described later; 5 is a blanking circuit; a rectifier circuit that rectifies the output of the ranking circuit 4 and returns the output E to the AGC amplifier 1 as an AGC control voltage; 6 a scanning circuit that generates necessary signals such as synchronization signals and clock signals for scanning; 7; is composed of a monomulti, for example, and as shown in FIG.
This is a blanking pulse generation circuit that generates a blanking pulse with -T as the blanking level.

t=A and t=B of this blanking pulse F are set as follows.

1=0 to T corresponds to the entire scanning area of each scanning period, whereas the document width of the transmitted original within the scanning area does not always cover the entire scanning area.

Therefore, assuming that the facsimile transmitter handles a transmission document having the desired minimum document width l, even if the transmission document is set at any position in the scanning area, the signal C corresponding to both ends of the transmission document will always be generated. are set to be included in t=0 to A and t=B to T, respectively.

In other words, when the width of the scanning area 8 is the same as shown in FIG. 2, and the transmission original with the minimum original width l placed in this scanning area 8 is set at the leftmost edge, the transmission original 9 is at the rightmost edge. Assuming that the transmission original document 9' is set in 1 position and l
The times t=A and t=B are set so as to extract only the signal C corresponding to the portion between the position of .

The operation of the AGC circuit configured as described above will be explained with reference to the time chart of FIG.

Here, it is assumed that the original to be transmitted is set approximately in the middle of the scanning area 8, as shown at 9'' in FIG.

Then, as shown in FIG. 3A, the original image signal obtained by scanning has a character signal A' on the original base (level L2) with the scanning surface level L3 at the bottom between the blanking level L1 synchronization signals. Appears in a superimposed form.

This signal A passes through the AGC amplifier 1 and is converted into the gain-controlled signal B, but this process is completed by the clamp circuit 3.
This is done using the following feedback loop.

That is, the signal B is branched and input to the clamp circuit 3, converted by the clamp circuit 3 so that the absolute black level becomes Ov, and is inputted as the signal C to the blanking circuit 4.

The blanking circuit 4 performs a blanking operation to extract only the signal C existing between t2A and B based on this blanking pulse F to obtain a signal.

This signal no longer has information corresponding to the level L3 of the scanning plane.

Therefore, if the DC signal E obtained by rectifying this signal by the rectifier circuit 5 is used as the AGC control voltage, the AGC operation can be performed based on the level of the difference between the document base and the blanking level.

For example, if the level of the image signal decreases during a given main scanning period due to some reason, that is, the level of the image signal approaches the blanking level L1, and with this decrease, the output D level of the blanking circuit 4 increases. changes toward Ov, and as a result, the level of the DC signal E, which is rectified by means such as peak hold in the rectifier circuit 5 and is output, changes in the direction of the Ov force.

Then, gain control can be performed by applying this DC signal E to the AGC amplifier 1 as a bias voltage or the like for a transistor amplifier circuit and acting in a direction to increase the level of the image signal (in the direction of the white level).

As a result, the level difference B at the output B of the AGC amplifier 1
' always appears constant.

As described above, according to the present invention, the facsimile I
Assuming that a transmitted document has the desired minimum document width handled by the J transmitter, no matter where the transmitted document is located, an image corresponding to a part of the transmitted document will always be selected from among the image signals obtained by scanning. Since the signal is extracted and the AGC operation is performed based on this signal, it is possible to automatically detect the document width with a simple configuration and always perform gain control only for image signals within the document width. Gain control can be achieved.

Similarly, the present invention is not limited to the above-mentioned embodiments, but can be implemented with various modifications.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an embodiment of the present invention, FIG. 2 is a schematic diagram showing the relationship between a scanning area and a transmitted document, and FIG. 3 is a signal waveform diagram of each part in the above embodiment. 1...AGC amplifier, 2...Amplifier,
3... Clamp circuit, 4... Blanking circuit, 5... Rectifier circuit, 6... Scanning circuit, 7... Blanking Pulse generation circuit.

Claims (1)

[Claims] 1. In an AGC circuit in a facsimile transmitter that scans a document to be transmitted whose width is wider than 1/2 of the width of the set scanning area, an AGC amplifier and an AGC amplifier that perform gain control on the image signal obtained by scanning are used. , with a desired minimum document width as l, extract only the image signal corresponding to a portion of the scanning area that exists between the position L-7 and the position l with one end of the scanning difference area as a reference. a circuit that generates a blanking signal whose time is set as shown in FIG. An AGC circuit comprising: