Method and circuit apparatus for combining two television signals

• US 4,947,240 A
• Filed: 03/20/1989
• Issued: 08/07/1990
• Est. Priority Date: 03/26/1988
• Status: Expired due to Fees

• Current Assignee: Robert Bosch GmbH
• Original Assignee: Robert Bosch GmbH
• Inventors: Hausdorfer, Michael
• Primary Examiner(s): Britton, Howard W.
• Assistant Examiner(s): Zarabian, Amir
• Application Number:
  US07/325,777
• Time in Patent Office: 505 Days
• Field of Search: 358/22, 358/105, 358/69, 358/36, 358/138
• US Class Current: 348/592
• CPC Class Codes: H04N 9/75 Chroma key

Abstract

A decision signal for switching or fading between foreground and background television signals in accordance with the chroma-key method is subjected to vertical scan delay filtering for line signal averaging in television signals to avoid disturbance of the decision signal by results of the field interlace feature of a standard television signal. The delay for such filtering is provided by a random access digital memory and provides a delay of one period of the vertical scan frequency. Filtering is obtained by averaging delayed and undelayed output of the chromakey color selection signal or by recursively filtering that signal utilizing the same period and type of delay.

Claims

1. Method for combining a foreground television signal with a background television signal by the chromakey procedure comprising the steps of:

deriving a first decision signal from the foreground television signal by reference to a color selection signal thereof;

subjecting said first decision signal so derived to vertical scan period delay filtering for line signal averaging to produce a second decision signal free of disturbance by interlace pattern effects, and

utilizing said second decision signal for switching or fading said foreground television signal in and out of said background television signal.

2. Method according to claim 1, wherein said vertical scan period delay filtering is produced by additive combination of respective half-amplitude first decision signals of a current picture field and of an immediately preceding picture field.

3. Method according to claim 2, wherein said first decision signal is in digital form at least for the purpose of producing a delayed first decision signal of a preceding picture field.

4. Method according to claim 1, wherein the vertical scan period delay filtering of said first decision signal is performed by recursive filtering.

5. Method according to claim 1, wherein said first deciion signal is a signal in digital form.

6. Apparatus for combining a first color television signal with a second color television signal by the chromakey method, wherein color-selective means are provided for deriving a first decision signal from said first color television signal, comprising, in combination:

a first attenuation stage (11) to which said first decision signal is provided as an input, for reducing said first decision signal amplitude by half and providing a reduced decision signal at its output;

means for delaying said first decision signal for the period of vertical scan of said television signals (13) said means having said first decision signal as an input and providing a delayed decision signal at an output;

second attenuation means (12) having an input connected to said output of said delaying means for reducing by half the amplitude of said delayed first decision signal and providing a reduced delayed decision signal at its output, and

means for adding together said reduced first decision signal and said reduced delayed decision signal and producing therefrom a second decision signal freed of disturbances at least partly caused by picture field interlace.

7. Apparatus according to claim 6, wherein said delaying means is constituted as a random access memory for digital signals and, if needed for processing said first decision signal, is preceded by an analog to digital converter and followed by a digital to analog converter provided in said apparatus.

8. Apparatus for combining two color television signals by the chromakey method, wherein said television signals are a background signal and a foreground signal and wherein means are provided for deriving a first decision signal from said foreground signal, and comprising recursive filter means having an input and including delay means having a propagation time of a vertical scan period and means for multiplying a delayed decision signal obtained at the output of said delay means by a value which is complementary to the value of said first decision signal, to produce a complementary delayed decision signal and means for adding said first decision signal to said complementary delayed decision signal to produce a disturbance-free second decision signal for use in chromakey television signal combination which is also supplied to said input of said delay means.

9. Apparatus according to claim 8, wherein said delay means is constituted as a random access memory for digital signals.