Dynamic video focus control

Details Dynamic video focus control Dynamic video focus control

1995-12-28

2001-11-06

Rao, Andy (Department: 2613)

Pulse or digital communications

Bandwidth reduction or expansion

Television or motion video signal

C375S240250

Reexamination Certificate

active

06314140

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to dynamic video focusing controls for video cameras, and particularly to accommodating rapid changes in scenes being taken by video-telephone cameras operating on transmission lines with predetermined channel capacity.
BACKGROUND OF THE INVENTION
Video-telephone cameras have substantial capacity to capture the illumination over a number of pixel sites, and to divide the analog voltage representing the illumination amplitude at each pixel into a multiplicity of values represented digitally by a number of bits. They can also produce a large number of frames representing each image captured. The total number of bits per unit time then equals at least to the total number of bits per pixel times the number of pixels per frame times the number of frames per unit time.
The degree to which the amplitude of the analog signal appearing at each pixel site may be subdivided determines the pixel definition. For example, the amplitude may be divided into 128 values with 7 bits. This produces a fine gray scale gradation which a viewer may perceive as high definition and “fine” focus. On the other hand, dividing the analog amplitude at a pixel site into only two values with a single on off bit produces a high contrast that may be perceived as poor or “soft” focus.
Similarly, the frame rate can also be changed. A high frame rate assures smooth motion while a low frame rate may produce flicker or perception of jerky motion.
The capacity of a video-telephone camera to generate high bit rates from the product of the number of bits per pixel site, the number of pixels per frame, and the frame rate, exceeds the capacity of most transmission channels to carry the bit rate. Accordingly, a coder is used to reduce the number of pixel levels which the amplitude at any pixel site is divided, i.e. the number of bits per pixel sites, and/or the frame rate. The number of bits per pixel site is generally referred to herein as the pixel definition or focus. The coder selects a suitable pixel definition or focus relative to a particular frame rate on the basis of the channel capacity.
A typical coder operates according to a conventional CCITT standard. In that environment, the coder uses a predetermined relationship between pixel definition and frame rate. It is possible for the camera user to preset the relationship between the pixel definition or focus and the frame rate on the basis of the user's perception. However, such arrangement cannot accommodate rapid changes in scenes being taken without overloading the channel capacity or producing undesirable picture quality.
SUMMARY OF THE INVENTION
According to an embodiment of the invention video images are digitally transmitted by setting a pixel definition and a frame rate to a data stream of video images on the basis of available channel capacity, sensing changes in the content between successive image frames, determining whether the changes lie within a predetermined range, and adjusting the pixel definition if the changes lie outside the range.
The various features of novelty which characterize the invention are pointed out in the claims. Objects and advantages of the invention will become evident following the detail description when read in light of the accompanying drawings.


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