Posts Tagged ‘ security dvr’



Security DVR Compression Methods

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Friday, March 19th, 2010

Security camera methods used to capture, transmit, and store data have greatly improved in a relatively short period of time. In order to discuss the security camera compression methods used with digital video recorders (DVRs), we need to have an understanding about how and what compression is and what methods are used to store security data.

Security cameras basically take digital pictures just like a standard digital camera. Although cameras vary by construction, the concept is basically the same for each type. A sensor takes a “picture” and instead of using film, creates a binary or digital file of the picture and transmits it to the storage media. A “still” picture is basically one picture or frame while motion-video is the same as a still picture, but there are several pictures taken in rapid succession, generally up to 30 pictures per second. The higher the quality (the more “fluid”) of motion video, the greater the frame rate up to 30 frames per second (FPS).

The problem with digital video is the incredibly large size of the files. A good quality “snapshot” may take up to 500 Kilobytes (KB) to 1 Megabyte (MB) of file space. Therefore, a motion video at a frame rate of 30 FPS could take up to 30 MB for just one second of monitoring.

Up until just a few years ago, the digital file was usually stored on digital tape media, such as a standard VHS tape. Some units used continuous loop tapes that just recorded over previous material after a period of time. Other units may have used standard VHS tapes and automatically rewound the tape to re-record over again, or owners may have had to insert a new tape each time the current tape reached the end.

Technological advances in the computer industry, especially those pertaining to storage technology and file compression have had tremendous benefits for the security camera industry. Now, the most common unit for storing is the DVR. Instead of saving image files to video or digital tape, images are stored as computer files on a hard disk drive, similar to the ones found in personal computers.

Computer (and likewise DVR) hard drives have various storage capacities. Less than 10 years ago, 250 GB hard drives were extremely expensive and rare. Based on a 30 FPS uncompressed video using 1 MG per frame, the hard drive would have a capacity of approximately 8333 seconds or 2 1/3 hours. This type of storage and technology made DVR storage prohibitive for security camera use.

However there was an exception. If the file size of the frame could be reduced in size, then more frames could be stored per unit of storage space on a hard disk. Reducing the size of the digital file is called compression, and is usually accomplished by some sort of mathematical computation called an algorithm. Typical compression file formats for still pictures include JPEG, GIF, and PNG. Each has a special set of mathematical instructions that determine how the file can be compressed in size. Typical compression file formats for digital video include JPEG2000, MPEG, and the newer H.264. Video compression formats are called CODECs for COmpression/DECompression which refers to the process the file goes through to be stored and when retrieved to be viewed. Unfortunately, hard drive storage capacity and file compression formats did not develop overnight or at a synchronous rate.

By current standards it is not unusual for personal computers to have 750 GB hard drives or larger and H.264 compression technology wasn’t really used in the security camera industry until about 2007-2008. The following list should give you an idea of the capabilities of the three popular compression formats used most often in surveillance cameras.

Using a DVR with a hard drive capacity of 250 GBs, at a frame rate of 30 fps with a screen resolution or size of 720 x 480 the following compression rates yield:

• JPEG2000 can record for approximately 20 hours
• MPEG4 can record for approximately 68 hours
• H.264 can record for approximately 120 hours

These formats are available in most DVRs today with the H.264 being the newest and the yielding the highest performance.

There is one other item that needs to be mentioned concerning DVR compression methods. A DVR not only stores the digital video files but it generally has a central processing unit or CPU, like a personal computer, which processes the CODEC to create the file. High frame rates and high compression rates places a heavy load on the processor, and can actually inhibit performance if the processor’s speed is not fast enough. To compensate for this, separate processing boards may be purchased that are added to the DVR to alleviate the load on the CPU to ultimately provide high performance surveillance video.

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What is a Substream?

Written By:
Tuesday, September 15th, 2009

Some of the newer DVRs on the market offer a feature called either a substream or a dual stream. A sub stream is used for viewing your DVR over the LAN or WAN when the resolution or frame rate of the recorded video is too high for the available bandwidth. For example, if your DVR was recording at 4CIF (or D1) resolution which is 704×480 in realtime (30fps), that would provide you with some exceptional recorded footage that would be both 4 times larger than the average CIF (360×240) recording and in realtime. Unfortunately, in most cases you would not be able to remotely view the video with fluid motion over the internet because there would not be enough available up bandwidth coming from the DVR location. In this case you would benefit from a sub stream that would be set at a lower resolution like CIF (360×240). This would allow you to have a high quality recording, but still be able to view fluid video over the internet.

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