Conversion using a variable reference based on image density

• US 5,027,117 A
• Filed: 03/24/1989
• Issued: 06/25/1991
• Est. Priority Date: 03/31/1988
• Status: Expired due to Fees

• Current Assignee: Ricoh Company Limited
• Original Assignee: Ricoh Company Limited
• Inventors: Yoshida, Yoshiki, Nagasawa, Kiyoto, Kagami, Yoshinobu
• Primary Examiner(s): Shoop, Jr., William M.
• Assistant Examiner(s): WILLIAMS, HOWARD L
• Application Number:
  US07/328,454
• Time in Patent Office: 823 Days
• Field of Search: 341/132, 341/139, 341/155, 341/156, 341/158, 341/159, 341/161, 341/162, 341/163, 358/461, 358/464, 358/465, 358/466
• US Class Current: 341/132
• CPC Class Codes: H03M 1/182 the feedback signal controlling the reference levels of the analogue/digital converter
• CPC Class Codes: H04N 1/40 Picture signal circuits H04N1/387 takes precedence
• CPC Class Codes: H04N 1/401 Compensating positionally unequal response of the pick-up or reproducing head H04N1/403 takes precedence

Abstract

An apparatus for converting an analog image signal to a digital image signal based on a reference voltage, includes a counter having a control terminal, for counting a first signal and outputting a counted value in digital form, a D/A converter for converting the counted value to an analog voltage signal served as the reference voltage, an A/D converter for converting the analog image signal to the digital image signal based on the reference voltage generated by the D/A converter, and a comparator for comparing in magnitude the digital image signal and a threshold level used for limiting the magnitude of the digital image signal, thereby outputting a control signal based on the results of comparison. The counter continues to count the first signal until the control signal derived from the comparator is supplied to the control terminal of the counter.

Claims

1. An apparatus for converting an analog image signal to a digital image signal based on a reference voltage, comprising:

counter means, having a control terminal, for counting a first signal and outputting a counted value in digital form;

digital-to-analog (D/A) converting means, connected to said counter means, for converting said counted value to an analog voltage signal served as said reference voltage;

analog-to-digital (A/D) converting means, connected to said D/A converter means, for converting said analog image signal to said digital image signal based on said reference voltage generated by said D/A converting means; and

comparing means, connected to said A/D converter means, for comparing in magnitude said digital image signal and a threshold level used for limiting the magnitude of said digital image signal, thereby outputting a control signal based on the results of comparison.

said counter means continuing to count said first signal until said control signal derived from said comparing means is supplied to said control terminal of said counter means,

wherein said comparing means comprises a plurality of comparators, each of which is supplied with corresponding threshold levels each used for limiting the magnitude of said digital image signal, and selection means for selecting one of the plurality of comparators and coupling the selected one of said comparators to said control terminal of said counter means.

2. An apparatus as claimed in claim 1, wherein said counter means comprises a down counter having a predetermined preset value and said control terminal, and said down counter counts down from said predetermined preset value with said first signal until said control signal is supplied to said control terminal.

3. An apparatus as claimed in claim 2, wherein said control terminal of said down counter is an enable terminal.

4. An apparatus as claimed in claim 1, wherein said first signal is a line synchronizing signal supplied from an external circuit each time said analog image signal amounting to one line is supplied to said A/D converter means.

5. An apparatus as claimed in claim 1, wherein said selector means selects one of the plurality of comparators, depending on a desired density of an image formed by said digital image signal.

6. An apparatus as claimed in claim 1, wherein when said counter means counts said first signal in a state where said analog input signal is a white signal supplied from an external system in which a white document is scanned.

7. An apparatus as claimed in claim 1, wherein said reference voltage generated by said D/A converter defines the full scale of said digital image signal.

8. An apparatus as claimed in claim 1, wherein said threshold level corresponds to a desired density of an image formed by said digital image signal.

9. An apparatus as claimed in claim 1, wherein said digital image signal is represented as the following formula,

10. An apparatus as claimed in claim 1, wherein said A/D converting means comprises an A/D converter having an input terminal to be supplied with said analog image signal, an output terminal through which said digital image signal is output, a first reference terminal for setting an upper-limit voltage which is a base on A/D conversion, and a second reference terminal for setting a lower-limit voltage which is a based on A/D conversion, and wherein said reference voltage supplied from said D/A converting means is supplied to said first reference terminal, and a predetermined voltage lower than said reference voltage is supplied to said second reference terminal.

11. An apparatus as claimed in claim 10, wherein said logic gate means comprises an AND gate and an inverter, and said predetermined time is a time prior to a time when said analog image signal represents real image data which follows a reference while signal.

12. An apparatus as claimed in claim 1, wherein said comparing means comprises a comparator for comparing said digital image signal supplied from said A/D converter and said threshold level, thereby generating said control signal, latch means for latching said control signal generated by said comparator, and logic gate means for passing said control signal during only a predetermined time and supplying the same to said counter means.

13. An apparatus as claimed in claim 12, wherein said latch means is reset and cleared each time when said analog image signal including image data amounting to a sheet of document is applied to said A/D converting means.

14. An apparatus as claimed in claim 1, wherein said analog image signal is a signal which has been subjected to a shading correction process.