Capturing the Moment With Digital Video Recorders
This session, the fifth chapter in our special six-part series on digital CCTV, will help both dealers and system integrators understand digital video basics, and properly select and install digital recording equipment.
Digital recorders were first introduced to the CCTV industry in the early 1990s. Today, more than 135 companies offer some form of digital recording device. Why the change to digital?
Analog recorders are still the primary means of providing permanent records for the surveillance market. However, tape maintenance, video playback quality and the excessive manpower required to view the tapes have caused many dealers and integrators, as well as end users, to explore the possibilities of digital recording.
For most, the first question that comes to mind is, “What are the major differences between a standard analog video recorder and the new digital video recorders (DVRs)?”
The answer is fairly simple. An analog recorder incorporates a magnetic field to align particles located on the surface of a standard VHS tape to correspond to the video signal. A digital recorder converts the signal into 1s and 0s, compresses this information, and then stores that information either on a digital audio tape (DAT), digital versatile disc (DVD) or hard disk drive (HDD).
Selecting analog recorders was a simple procedure in the past. One had only to select a video recorder with the total hours required for recording plus a few features such as alarm inputs, RS-232 interface, and, maybe, self-cleaning.
In digital recording, a simple block diagram can help explain the operation and selection of a digital video recorder.
The first conversion is usually a simple analog to digital converter (A/D) located in the recording device. However, this is where the basic changes end and the magic begins.
Once you make the decision to change from analog to digital, a number of questions must be asked including: What are compression methods? How long can I record? What is the resolution? What about the authenticity of digital recording?
Compression Minimizes the Image Size
There are many differing opinions about which compression method or methods are the best, and which one will become the CCTV standard. The most common methods seen in today’s recording usually consist of JPEG, MPEG, H.263, Wavelet or a combination of these methods.
JPEG: JPEG compression was established in 1974 by the Joint Photographic Expert Group and uses Discrete Cosine Transform (DCT) compression. Basically, it is compression based on a video stream of 8 X 8 pixels. For the most part, this is the type of compression used for loading and downloading images over the Internet.
MPEG: The Moving Picture Experts Group established MPEG compression and uses the same DCT compression theory found in JPEG. MPEG compression is based on motion-compensated block-base transform coding techniques with bit-rates of 1.5Mbits/second. A number of different techniques are used to achieve higher compression ratios.
MPEG comes in three basic forms, MPEG1, MPEG2 and MPEG4. The compression ratio for the different forms is: MPEG1 = 25 to 100:1; MPEG2 = 30 to 100:1; MPEG4 = 50 to 100:1
H.263: Because of the large file size in MPEG compression, many of today’s digital manufacturers have modified this standard to meet the needs of the CCTV industry. This modified standard is known as H.263. H.263 is a video compression algorithm designed for low bit-rate communications.
Wavelet: This commonly used compression method is based on full-frame information, and compression is based on frequency, not 8 X 8 pixel blocks. In theory, Wavelet compresses the entire image with multiple filtering; that is, it filters both the high and low frequencies and repeats this procedure several times. This compression method can offer compression ratios up to 350:1.
Extend Compression With Redundant Reduction
There are many other forms of compression technology, as well as extended compression ratios, incorporated in today’s digital recorders.
In order to help reduce the file size for each video image that is to be recorded, many manufacturers have incorporated two additional components of compression. The first is redundancy reduction and is accomplished by removing duplication from the signal source before it is compressed and stored.
The second form of reduction is called irrelevancy reduction. The method omits parts of the signal that will not be noticed by the signal receiver. The main two areas, as described by the Human Visual System or HVS organization, are in low frequency visual response and color recognition.
All of these compression methods and reduction schemes are used to reduce the file storage size, while maintaining a high-quality playback image.
But how long can you record? The answer to this question combines many different factors. These factors include compression ratios, image file size, number of recorded images per second, and the number of video inputs.
Storage Dictates How Much You Can Record
In analog recording, the storage media was easy, magnetic tape. The more tape available and the fewer recorded video images per second, the longer the recording time. In digital recorders, this storage changes.Some recorder manufacturers use only computer hard drives as their major form, some use DAT or DSS tape, some use digital discs, while others use a combination of both hard drives and digital tape. Which one is better? Well, like everything else, there are pros and cons for each.
Being Able to See Detail Demands High Resolution
This term is often misused and misunderstood in the security industry. In analog recording the image always filled the entire monitor screen. Digital resolution refers to spatial resolution (number of pixels per line) and the number of lines or rows per image. This not only changes the overall resolution of the system, it also affects the overall size of the displayed image.
Here are the different image formats and their size in relation to the monitor screen: 720 X 480 pixels = full image frame; 640 X 480 pixels = 1/2 image frame; 320 X 240 pixels = 1/4 image frame
Digital Video Might Not Be Accepted in Court
The acceptance of digital video evidence has not yet been proven in a U.S. court of law. However, other countries, such as the United Kingdom, have tested—and in some cases—approved or recommended standards for authentication. The requirements include:
Images must be original
Images should be recorded in WORM (This format allows for the operator to review the video information as many times as he or she likes.)
Images should have a check sum (a method by which the system records the number of levels and pixels per recorded line and stores this information somewhere in the recorder’s program.)
Images should have a digital signature
Authentication should also include a date/time stamp or digital signature inserted on to all recorded video images. The time should be clocked down to 1/100 of a second. All of these measures are being incorporated into today’s systems to ensure their acceptance by the court system when that time arises.
Multiplexed Recorders vs. Multichannel Recorders
Once a video signal is in digital format, many other forms of digital equipment can be incorporated into today’s DVRs. One of the more popular additions is that of video multiplexing. The main purpose for any multiplexer is to provide a simple and cost-effective method of viewing recorded video images using high-speed switching technology. This form of video recording not only eliminates the normal video gaps created by conventional sequential switchers, but can offer a system that can display multiple images on a single monitor screen, reducing system equipment costs.
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