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Automatic Gain Control System

IP.com Disclosure Number: IPCOM000082442D
Original Publication Date: 1974-Dec-01
Included in the Prior Art Database: 2005-Feb-28
Document File: 3 page(s) / 46K

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Buckley, F: AUTHOR


The system illustrated in the drawing provides an improved automatic gain control (AGC) function for optical character recognition systems.

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Automatic Gain Control System

The system illustrated in the drawing provides an improved automatic gain control (AGC) function for optical character recognition systems.

In some character recognition systems employing cathode-ray tubes as flying spot scanners, two photoresponsive devices respond to the scanning. The first is responsive to changes in the light reflected from the document and the second is responsive to the incident light on the document. If the signal resulting from the reflected light is divided by the incident light signal, variations due to phosphor aging, phosphor voids, beam displacement, etc., can be eliminated. Signal division is obtained by deriving the log of each of these two signals, subtracting the incident light log signal from the reflected light log signal, and then deriving an antilog signal from this difference.

In the drawing, the signals from the photoresponsive devices are supplied to terminals 3 and 5, and the log signals are generated therefrom by log circuits 7 and 9. The outputs are supplied to a circuit node G and therefrom to the input of antilog circuit 11, with the output of circuit 9 being inverted by inverter 13. The output of antilog circuit 11 is supplied to an output terminal R via a suitable amplifier 15.

The operation of this portion of the system is such that wide-range current changes in the photoresponsive devices would be present at node G, and hence would result in undesirable fluctuations in the output at point R when the document background is being scanned, i.e., a "no black" condition.

The output signal at R is fed back to an operational amplifier 18 via resistor 17 and node S. Amplifier 18 has a gain of 1/2 to the signal from R. Amplifier 18 controls a negative current source including transistor T9. This current source will vary from 0 to some value such as 4ma and current will flow out of node W. A constant positive current source of 2ma, for example, is produced by transistor T7 and this current flows into node W. Node W now can have a change of + or - 2...