Video amplifier for optical scanners

Claim 2

A signal-handling apparatus, comprising: conversion means for receiving first and second variable input signals and for converting said input signals to first and second logarithmic representations; difference means responsive to said converting means for producing an output signal having a magnitude related to the difference between said logarithmic representations, and to a level-control signal; and level-control means for producing said level-control signal as a function of the difference between said representations.

Claim 3

The apparatus of claim 1, wherein said difference means comprises a differential amplifier having a gain which is substantially independent of the magnitude of said level-control signal.

Claim 4

The apparatus of claim 2, wherein said differential amplifier comprises: first and second transistors each having a base coupled to said converting means and having an emitter; a third transistor having a base coupled to said level-control means and having a collector and an emitter; and first and second resistors each coupled from the emitter of one of said first and second transistors to the collector of said third transistor, the values of said resistors being substantially greater than the internal base-emitter resistances of said first and second transistors.

Claim 6

The apparatus of claim 4, wherein said level-control means comprises: means for storing a voltage indicative of the magnitude of said level-control signal; charging means responsive to said output signal for varying said stored voltage; and means responsive to said inhibiting signal for disabling said charging means.

Claim 7

The apparatus of Claim 5, wherein said storage means and said charging means together comprise a capacitive low-pass filter having a frequency-response characteristic lower than that of said difference means.

Claim 9

The apparatus of claim 1 wherein said first input signal is responsive to the intensity of a radiant-energy source, and wherein said second input signal is responsive to the intensity of radiant energy reflected from an object illuminated by said source.

Claim 10

A signal-handling apparatus for a pair of variable input signals, comprising: means for producing a pair of signals proportional to the logarithms of respective ones of said input signals; amplifying means responsive to said logarithmic signals for producing a signal related to the quotient of said input signals; and feedback means responsive to said quotient signal for varying the operating point of said amplifying means without substantially changing its gain.

Claim 11

A signal-handling apparatus for a pair of variable input signals, comprising: means for producing a pair of signals proportional to the logarithms of respective ones of said input signals; amplifying means responsive to said logarithmic signals for producing a signal related to the quotient of said input signals; and feedback means responsive to said quotient signal for varying the operating point of said amplifying means without substantially changing its gain.

Claim 12

The apparatus of claim 9, further comprising: means responsive to said amplifying means for digitizing said quotient signal into a plurality of magnitudes; and means responsive to said digitizing means for inhibiting said feedback means when said digitized signal has a predetermined one of said magnitudes.

Claim 13

The apparatus of claim 10 wherein said feedback means comprises a low-pass filter having a corner frequency lower than the minimum frequency of either of said input signals.

Claim 14

The apparatus of claim 10, further comprising: means for generating a deflectable beam of radiant energy; a first photodetector directed toward said generating means for producing a first of said output signals in response to the intensity of said beam; means for transmitting said beam to a reflective object; and a second photodetector directed toward said object for producing a second of said output signals in response to the intensity of reflections of said beam from said object.

Claim 15

A signal-handling apparatus for an optical scanner having means for generating a movable light beam and means for imaging said beam on a document to be scanned so as to cause reflections from said document, said apparatus comprising: a first detector disposed with respect to said generating means so as to develop a first signal indicative of the intensity of said light beam; a second detector disposed with respect to said document so as to develop a second signal indicative of the intensity of said reflections; division means responsive to said detectors for producing a third signal related to the quotient of said first and second signals; thresholding means responsive to said division means for producing a substantially discontinuous fourth signal from said third signal; and control means responsive to said division means and to said thresholding means for varying the magnitude of said third signal when said fourth signal has a first value, and for inhibiting said variation when said fourth signal has a second value.

Claim 16

A signal-handling apparatus for an optical scanner having means for generating a movable light beam and means for imaging said beam on a document to be scanned so as to cause reflections from said document, said apparatus comprising: a first detector disposed with respect to said generating means so as to develop a first signal indicative of the intensity of said light beam; a second detector disposed with respect to said document so as to develop a second signal indicative of the intensity of said reflections; division means responsive to said detectors for producing a third signal related to the quotient of said first and second signals; thresholding means responsive to said division means for producing a substantially discontinuous fourth signal from said third signal; and control means responsive to said division means and to said thresholding means for varying the magnitude of said third signal when said fourth signal has a first value, and for inhibiting said variation when said fourth signal has a second value.

Claim 17

The apparatus of claim 13, wherein said division means comprises: means responsive to said detectors for converting said first and second signals to logarithmic representations; and an amplifier having first and second inputs for accepting said representations and a third input for accepting a fifth signal from said control means, said amplifier being adapted to produce said third signal as a substantially linear function of said representations and of said fifth signal.

Claim 18

The apparatus of claim 14, wherein said control means comprises: capacitive means for storing a voltage representative of said fifth signal; charging means responsive to said third signal for varying said stored voltage; and inhibiting means responsive to said second value of said fourth signal for preventing variation of said stored voltage in response to said third signal.

Claim 19

The apparatus of claim 2, wherein said level-control means is further responsive to an inhibiting signal for freezing the magnitude of said level-control signal when the magnitude of said output siganl bears a predetermined relationship to a threshold magnitude.

Claim 20

The apparatus of claim 16, wherein said level-control means comprises: means for storing a voltage indicative of the magnitude of said level-control signal; charging means responsive to said output signal for varying said stored voltage; and means responsive to said inhibiting signal for disabling said charging means.

Claim 21

The apparatus of claim 17, wherein said storage means and said charging means together comprise a capacitive low-pass filter having a frequency-response characteristic lower than that of said difference means.

Claim 22

The apparatus of claim 2 wherein said converting means comprises a pair of forward-biased diodes connected in shunt across said receiving means.