Pulse or digital communications – Bandwidth reduction or expansion
Reexamination Certificate
1998-03-30
2001-11-06
Kelley, Chris (Department: 2713)
Pulse or digital communications
Bandwidth reduction or expansion
C375S240010
Reexamination Certificate
active
06314137
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a video data compression system, and more particularly to a compression encoding system for use in a video recording unit which sequentially switches video data obtained from a plurality of video cameras such as those used in a video surveillance system.
2. Description of the Related Art
Conventionally, a video surveillance system which records video from multiple video cameras (hereafter simply called cameras) onto a long-time recording time-lapse VTR has been used. This system uses a switcher to switch video from multiple cameras in a time-dividing manner.
Earlier patent disclosures dealing with this type of video surveillance systems are found in Japanese Patent Laid-Open Application No. Sho 64-65989 and Japanese Patent Laid-Open Application No. Hei 5-73312.
The system disclosed in Japanese Patent Laid-Open Application No. Sho 64-65989 synchronizes multiple cameras with the reference signal to eliminate the discontinuity of video signals at switching time.
It also synchronizes the VTR recording timing with the video signal switching signal to minimize unstable operation time. This results in a shorter video switching period, making high-density recording possible.
In addition, detecting at the time of playback, the camera identification signal superimposed on the video signal allows only the desired video to be selectively monitored.
The system disclosed in Japanese Patent Laid-Open Application No. Hei 5-73312 switches timely between a 2-system video selection means and a 2-system video signal synchronization means to enable a video surveillance system, which has cameras not under control of external synchronization signals or each controlled by its own synchronization method, to synchronize video signals for proper switching.
A system which switches multiple cameras in a time-division manner to combine their video signals into one mixed video signal is called a frame switcher. Many such systems are commercially available.
Today, digital video recording and playback systems which record and play back digital image data have become popular.
In general, image data to be recorded digitally is compressed to prevent the amount of data from increasing and therefore to allow the recording medium to be used for recording for a longer period of time.
Well-known image data compression techniques include JPEG (Joint Photographic Experts Group) and MPEG (Moving Pictures Experts Group).
Because these techniques are described in detail in many books, the following briefly gives the summary of their algorithms.
First, JPEG will be described
JPEG divides an image into multiple small blocks and converts each block into two-dimensional frequency components through DCT (Discrete Cosine Transform). It then reduces the amount of data through non-linear quantization and entropy encoding.
A high intra-frame correlation in the image means closer two-dimensional frequency components, thus increasing the encoding efficiency and reducing the amount of data while ensuring the picture image quality.
However, an image composed of fine patterns results in a lower intra-frame correlation and therefore decreases the compression efficiency. Thus, a compression ratio of up to 10:1 is required to ensure a reasonable JPEG image.
Next, MPEG will be described.
In addition to the technique used by JPEG described above, MPEG uses an inter-frame correlation to reduces the amount of data. More specifically, it finds changes between every two frames and performs DCT on the changes. This is called inter-frame predictive encoding.
For slow-moving video with little or no change between frames, MPEG produces a very small amount of data.
Even for fast-moving video, MPEG provides an extremely high predictive encoding efficiency because it detects motion vector between frames and motion compensation for them.
As a result, the image quality is not degraded at all even when the compression ratio is several scores to one and, therefore, a recording/playback system using the MPEG compression algorithm can record video data longer in time than JPEG.
SUMMARY OF THE INVENTION
Time lapse VTRs, which record analog video signals on magnetic tapes, have been used in most conventional video surveillance systems. It is also possible to configure a video surveillance system using digital recording system explained above.
That is, a system with only one camera digitizes the output video signals, reduces the amount of data by compressing data with MPEG, and records compressed data on a disk or tape.
A system with multiple cameras has an MPEG compression unit and a recording/playback unit installed for each camera to record digitized video signals sent from each camera.
The problem is that a video surveillance system with multiple MPEG compression encoding units is very expensive because the MPEG compression encoding unit is very expensive. It is therefore preferable that, as with the conventional video surveillance system, the system has only one compression encoding unit which compresses video signals sent from multiple cameras for recording.
However, compressing digitized mixed video signals generated by a frame switcher with MPEG poses the following problem.
In most cases, video generated by separate cameras has no correlation. This means that the amount of changes among video obtained by separate cameras is much larger than the amount of changes among video obtained by the same camera.
Therefore, a frequent switch among multiple cameras in the time divided manner affects inter-frame predictive encoding. Video data cannot be compressed efficiently for recording.
This invention seeks to solve the problems associated with the prior art described above. It is an object of this invention is to provide a video data compression system, video recording/playback system, and video data compression encoding method which efficiently compress multiple video data items through predictive encoding.
To solve the above problems, a first embodiment of this invention is a video data compression system which compresses mixed video data generated by mixing a plurality of video data items in a time-divided manner, wherein the mixed video data is compressed by performing inter-frame or inter-field predictive encoding for each of the plurality of video data items.
For example, such a system comprises first storage means for storing the mixed video data; first control means for controlling writing and reading the mixed video data to and from the first storage means; and predictive encoding means for performing predictive encoding among a plurality of images of the mixed video data.
The first control means writes the mixed video data into the first storage means, the mixed video data being divided into the plurality of video data pieces, and sequentially reads the mixed video data, a plurality of images at a time, for each of the plurality of video data items from the first storage means and wherein
the predictive encoding means performs predictive encoding among a plurality of images of video data sequentially read, a plurality of images at a time, from the first storage means.
In addition, when the mixed video data is composed of the plurality of video data items, each of the plurality of video data items sequentially appearing for a predetermined number of images, the predictive encoding means performs predictive encoding on a plurality of images appearing at an interval of a number of images, the number being a product of the predetermined number of images, the number of the plurality of video data items, and a natural number.
The first embodiment of this invention compresses mixed video data, composed of a plurality of video data items, through predictive encoding for each of the plurality of video data items, thus efficiently compressing the mixed video data.
A second embodiment of this invention is a video data compression system which compresses a plurality of video data items, comprising video data acquisition means for sequentially acquiring a predetermined number of vide
Horiuchi Tadasu
Kanesaki Takayuki
Nishijima Hideo
Ono Koichi
Tsukiji Nobuyoshi
Hitachi , Ltd.
Kelley Chris
Mattingly Stanger & Malur, P.C.
Wong Allen
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