VIDEO SIGNAL NOISE-LIMITING APPARATUS

• US 3,715,477 A
• Filed: 03/11/1971
• Issued: 02/06/1973
• Est. Priority Date: 03/11/1971
• Status: Expired due to Term

• Current Assignee: Charles Langdon Olson, John Francis Monahan, Robert Adams Dischert
• Original Assignee: Charles Langdon Olson, John Francis Monahan, Robert Adams Dischert
• Inventors: Dischert, Robert Adams, Olson, Charles Langdon, Monahan, John Francis
• Primary Examiner(s): Richardson, Robert L.
• Application Number:
  US05/123,233
• Time in Patent Office: 698 Days
• Field of Search: 178/5.4R,7.3R,7.5R,7.35,7.55,DIG.12 325/65,473,474,477 328/165,167
• US Class Current: 348/623
• CPC Class Codes: H04N 5/21 Circuitry for suppressing or minimising disturbance, e.g. moiré or halo
• CPC Class Codes: H04N 9/646 for image enhancement, e.g. vertical detail restoration, cross-colour elimination, contour correction, chrominance trapping filters

Abstract

The signal is separated into high and low frequency constituents by a filter, the high frequency signal constituent is cored to remove low amplitude excursions therefrom, and the cored high frequency signal constituent is combined with the low frequency signal constituent in suitable proportions to produce a resultant signal having an improved signal-to-noise ratio. In one form of the apparatus the video signal is the luminance component. In another form the noise-limiting is applied to each of a plurality of component color signals. A composite color television video signal is noise-limited by further forms of the apparatus.

Claims

1. Video signal noise-limiting apparatus comprising:

means providing video signals;

filtering means comprising at least one comb filter for separating a video signal into its low and high frequency constituents, said comb filter producing peak and null points of said high frequency signal constituent alternately with null and peak points respectively of said low frequency signal constituent;

means for effectively removing low amplitude excursions from said high frequency signal constituent, thereby reducing noise effects in said high frequency signal constituent; and

adding means for combining said noise reduced high frequency signal constituent with said low frequency signal constituent in suitable proportions to produce an output signal with an improved signal-to-noise ratio.

2. Video signal noise-limiting apparatus as defined in claim 1, wherein:

said one comb filter includes a delay line and a pair of output terminals and provides a frequency-response at each of said output terminals having peak and null points separated by a frequency corresponding to one-half the delay time of said delay line, wherein respective peak and null points at one of said pair of output terminals corresponds in frequency to respective null and peak points at the other of said pair of output terminals.

3. Video signal noise-limiting apparatus as defined in claim 2, wherein:

said video signals comprise a composite color television signal having an amplitude varying luminance component representative of the brightness of a colored subject and a phase- and amplitude-modulated color subcarrier chrominance component representative of the color of said subject, both of said luminance and chrominance components including combinations of a plurality of component color signals representative of selected colors of said subject; and

at least one of said filter delay lines has a delay time equal substantially to the period corresponding to twice the frequency of said color subcarrier.

4. Video signal noise-limiting apparatus as defined in claim 3, wherein:

all of said filter delay lines have respective delay times corresponding to one-half the period of said color subcarrier.

5. Video signal noise-limiting apparatus as defineD in claim 3, wherein:

at least another of said filter delay lines has a delay time equal substantially to a horizontal line period.

6. Video signal noise-limiting apparatus comprising:

means providing a composite color television signal having an amplitude varying luminance component representative of the brightness of a colored subject and a phase- and amplitude-modulated color subcarrier chrominance component representative of the color of said subject;

filter means including a first comb filter having a delay line with a delay time equal substantially to a horizontal line period, for producing said luminance component and said color subcarrier chrominance component at separate output terminals, a second comb filter having a delay line with a time delay equal substantially to one-half the period corresponding to the frequency of said color subcarrier for producing low and high frequency portions of said luminance component, a third comb filter having a delay line with a time delay equal substantially to one-half the period corresponding to the frequency of said color subcarrier for producing low and high frequency portions of said color subcarrier chrominance component;

a first means for removing low amplitude excursions from said high frequency portions of said luminance component thereby providing a first noise reduced signal and a second means for removing low amplitude excursions from said low frequency portions of said color subcarrier chrominance component thereby providing a second noise reduced signal; and

adding means combining said first and said second noise reduced signals with said low frequency portion of said luminance component and said high frequency portion of said color subcarrier chrominance component to form a single combined output signal.

7. Video signal noise-limiting apparatus comprising:

a source of composite color television signals having an amplitude varying luminance component representative of the brightness of a colored subject and a a phase-and amplitude-modulated color subcarrier chrominance component representative of the color of said subject;

a comb filter having first and second delay lines each with a delay time equal substantially to a horizontal line period and a third delay line with a delay time equal substantially to one-half the period corresponding to the frequency of said color subcarrier for producing in response to said composite signal a low frequency signal constituent having all of the brightness and color information of said subject and a high frequency signal constituent having only information relating to transitions between light and dark areas of said subject;

means for removing low amplitude excursions from said high frequency signal constituent supplied thereto to provide a noise-reduced high frequency signal constituent;

means for combining said noise-reduced high frequency signal constituent and said low frequency signal constituent to produce an output signal.

8. Video signal noise-limiting apparatus comprising:

means providing video signals;

filtering means for separating at least a selected one of said video signals into its low and high frequency constituents;

a delay line included in said filtering means and terminated in its characteristic impedance at its input end and unterminated at its output end;

means for removing low amplitude signal excursions from said high frequency constituent to provide a noise-reduced high frequency constituent;

adding means for combining said noise-reduced high frequency constituent with its respective low frequency constituent to provide an output signal;

a signal-forwarding transistor included in said filtering means and coupled to transmit to said adding means a first subdivision of said selected video signal unmodified in amplitude and time and a second subdivision of said selected video signal unmodified in amplitude but delayed in time for twice the delay time of said delaY line and produced by reflection from the unterminated end of said delay line;

a signal-amplifying transistor included in said filtering means and coupled to transmit to said adding means a third subdivision of said selected video signal doubled in amplitude and delayed in time for the delay time of said delay line; and

a differential amplifier transistor included in said filtering means and coupled to said signal-forwarding transistor and to said signal-amplifying transistor to transmit to said adding means a fourth subdivision of said selected video signal doubled in amplitude and delayed in time for the delay time of said delay line.

9. Video signal noise-limiting apparatus as defined in claim 8, wherein:

said selected video signal is one of a plurality of component color signals in a television system employing a color subcarrier modulated by combinations of said component color signals; and

said filtering means delay line has a delay time equal substantially to one-half the period corresponding to the frequency of said color subcarrier.

10. Video signal noise-limiting apparatus as defined in claim 9, wherein:

said means for removing low amplitude signal excursions is coupled between said differential amplifier transistor and said adding means.

11. Video signal noise-limiting apparatus as defined in claim 10, wherein:

the coupling between said means for removing low amplitude signal excursions and said differential amplifier transistor includes a feedback pair of signal-amplifing transistors.

12. Video signal noise-limiting apparatus as defined in claim 11, wherein:

each of the low and high frequency constituents of said selected video signal comprise first, second and third units; and

said adding means includes, a first resistor to receive said first and second subdivisions of said selected video signal to produce said first and second units of said low and high frequency signal constituents, a second resistor to receive said third video signal subdivision to produce said third unit of said low frequency signal constituent, and a third resistor to receive said fourth video signal subdivision to produce said third unit of said high frequency signal constituent.