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A User's Guide to Digital Video Surveillance

Bitrate

Many customers specify a standard for video quality, which in fact specifies no quality at all. Huh??? Ever heard the terms CIF, 2CIF, 4CIF or D1?

CIF (Common Intermediate Format) is used to standardize the horizontal and vertical resolutions in pixels of YCbCr sequences in video signals. A CIF is commonly defined as one-quarter of the 'full' resolution of the video system it is intended for (listed below as 4CIF). Note that this full resolution does not match what is currently referred to as D1 video (based upon Sony's D1 format).

Video resolutions (in pixels) x NTSC-based x PAL-based

CIF (NTSC 352 × 240) -  (PAL 352 × 288)
4CIF (NTSC 704 × 480) -  (PAL 704 × 576)
D1 (NTSC 720 × 480) -  (PAL 720 × 576)

NTSC is the video system or standard used in North America and most of South America. In NTSC, 30 frames are transmitted each second. Each frame is made up of 525 individual scan lines.

PAL is the predominant video system or standard mostly used overseas. In PAL, 25 frames are transmitted each second. Each frame is made up of 625 individual scan lines.

The problem with these standards is that these “resolutions” speak nothing of the quality of the video. 4CIF simply means that in the NTSC format there are 704 pixels that will be filled with some amount of data across each of 480 lines. CIF means, there are 352 pixels which will be filled with some amount of data across each of 240 lines. How much data has yet to be determined, which is the bitrate.

While a 4CIF image “can” produce a better quality image then a CIF image because the more pixels used to represent an image, the closer the result can resemble the original; it does not necessarily have to be the case. 4CIF does not mention the amount or quality of the data to be displayed in the pixels. These are “empty” pixels. Pixels need to be filled with data. Data is referred to in terms of “bits.”  Accordingly, a CIF image filled with more bits then a 4CIF image can produce better visual and audible results.

In telecommunications and computing, bitrate (sometimes written bit rate, or as a variable Rbit) is the number of bits that are conveyed or processed per unit of time. In digital multimedia, bitrate is the number of bits used per unit of time to represent a continuous medium such as audio or video. It is quantified using the bit per second (bit/s) unit or some derivative such as Mbit/s.

While often referred to as "speed", bitrate does not measure distance/time but quantity/time, and thus should be distinguished from the "propagation speed" (which depends on the transmission medium and has the usual physical meaning).

In digital video, bitrate represents the amount of information, or detail, which is stored per unit of time of a recording. The bitrate depends on several factors:

  • the original material may be sampled at different frequencies
  • the samples may use different numbers of bits
  • the data may be encoded by different schemes
  • the information may be digitally compressed by different algorithms or to different degrees

Generally, choices are made about the above factors in order to achieve the desired trade-off between minimizing the bitrate and maximizing the quality of the material when it is recorded or played.

In digital video to reduce the bitrate a “lossy” compression is utilized. A lossy compression is a compression method that discards some of the information or data to make a video or audio program occupy less storage space or less transmission bandwidth.

If lossy data compression is used on audio or visual data, differences from the original signal will be introduced since data has been discarded. If the compression is substantial, or lossy data is decompressed and recompressed, this may become noticeable in the form of compression artifacts, which appear in the form of blocky mosaic images. Whether, these affect the perceived quality, and, if so how much; depend on the compression scheme, encoder power, and the characteristics of the input data, the viewer’s perceptions, and the viewer's familiarity with artifacts.

Ostensibly what we are saying is quoting a specification as 4CIF doesn’t mean a thing. Further, even quoting a specified bitrate at a particular resolution, also means nothing, because of the amount of variables as described above.

Many times the encoding power or compression scheme is insufficient regardless of the bitrate to generate “usable” video, because of it being applied with outdated technology.

So what is a customer to do? We will discuss some overall general rules of thumb and concepts; but, they are by no means the Holy Grail.

In order to view good quality 4CIF recorded video the following bitrate estimates may apply using the specified codecs:

H.264:  1 – 2 Mbps
MPEG4 (Part 2): 2 – 4 Mbps   
MPEG2:  3 – 6 Mbps

Just because you use the above bitrates does not necessarily mean you will have acceptable quality video.  These are general numbers for the particular codec and do not account for all the other potential issues we have discussed in this guide.

The higher the bitrate the more storage it requires and the slower the download and transmission time. This comes back to the truck that needs to pull 10,000 pounds. The bigger file sizes created by the high bitrates require a lot more resources to move them along.

What does this mean in terms of how much hard drive space is required? Again, it’s an estimate based upon our evaluation and testing of various systems in the marketplace and speaking with professionals who have also tested the same equipment. There could be significant variations depending upon multiple factors, but for the highest possible quality this is a good rule of thumb.

H.264:  ½ – 1 GB per hour
MPEG4 (Part 2): 1 – 2 GB per hour
MPEG2:  1.5 – 3 GB per hour

This is assuming the bitrates mentioned above, with “acceptable” video quality at 25/30 frames per second (PAL/NTSC). Remember this is the highest quality, which may not be required in all instances.

There are several other things that would skew in the favor of the newer codecs and make them even more attractive. H.264 uses what is known as a Variable bit-rate (VBR). VBR allows a codec to change its bit-rate dynamically to adapt to the ``difficulty'' of the audio and video being encoded. In the example of a swinging PTZ or other rapid movement, a higher bit-rate to achieve good quality is required, while less active scenes can be coded adequately with fewer bits. For this reason, VBR can achieve lower bit-rate for the same quality, or a better quality for a certain bit-rate. Hard drive capacity can be substantially increased. The older codecs use a Constant bit-rate (CBR). Therefore, there are no efficiencies regardless of the scene activity; the bitrate is constant to whatever it has been set to in the firmware.

As we cover all these topics it becomes more evident why the move to digital surveillance has been slow. Fact is, it’s not so easy to comprehend, plus the underlying technology required to provide video to a high standard was just not there until the last couple of years. It was a combination of the lack of powerful enough components and efficient compression technology.

 

 

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