The Digital Network Camera Surveillance School:

Chapter 3

Image compression - the key to fast transmission

Why do you need to compress images and video? Wouldn’t it be better just to record all information as it is? Indeed, but let’s see what this would mean:
According to IEC-601 (the standard for digital video) uncompressed TV is 165 Megabits per second (165Mbits/s), for just one minute we would need 9.900Mbits – the same as 9,9 Gigabits. One hour of video requires 594Gbits and 24 hours equals 14.256Gbits.
And just how much is 14.256Gbits? Well, if you are using a 28K modem it would take you about sixteen years to download 24 hours of video.
And that’s why we need to compress.

So, we do need to compress, but what standards are available? What’s the difference between them and which should I go for? Today there is a large number of compression standards to choose from, but before we go into detail about them we need to establish the differences between still image and video compression.

Cameras using still image standards send single images over the network. Cameras using video standards send still images mixed with data containing the changes. This way, non-changing data, like the background, is not sent in every image. Video standards often include audio in the data stream. The image refresh rate is referred to as frames per second, or fps for short.


Main differences between still image and video compression:
• Still image compression is simple and easy to work with
• It’s difficult to obtain a single image from a video stream using video compression
• Less data is used to store /transmit video in compressed form compared to still image
• It’s not possible to reduce the frame rate when using video compression
• Still image compression is more suitable when using a modem, or other media that only offers limited bandwidth.

With the exception of Motion-JPEG, all video compression standards mix still images with partially complete images. By storing only the changes from one full image to another, these ‘partially complete’ images reduce the file size of the compressed video sequence. Scenes containing little or no variation can be compressed quite dramatically.


Compression standards for still images

JPEG
This is short for Joint Photographic Experts Group international – a good and very popular standard for still images that is supported by most modern software. Conforming to ISO/IEC 10918, this is the preferred compression standard for many network cameras.
When using the JPEG-standard, each image is divided into 8x8 pixels. Each block is then individually compressed using the Digital Cosine Transform (DCT). If a very high compression ratio is used, the 8x8 pixel blocks can actually be seen in the image.

Wavelet
A Popular standard optimised for images containing small amounts of data. Wavelet is not standardised and requires special software for viewing. However since many manufacture are using it, it can be seen as a de facto standard.


JPEG 2000
Based on Wavelet (not JPEG) technology, this is a new but still relatively little-used standard.

GIF
This is short for Graphics Interchange Format -a bit-mapped graphics file format used widely on the Internet. Limited to 256 colors, GIF is a good standard for images that are not too complex, e.g. scanned images and logotypes. It is not recommended for network cameras, as the compression ratio is too limited.



Compression Standards for video

Motion-JPEG
With motion-JPEG, each frame within the video is stored as a complete image in the JPEG format. The still images are displayed at a high frame rate to produce very high-quality video - but at the cost of producing comparatively large file sizes. For more information about JPEG please look at www.jpeg.org

H.261, 263, 321, 324 etc.
This set of acronyms refers to the standards recommended by the International Telecommunications Union (ITU). Designed for video conferencing, but sometimes used for network cameras, these standards offer a high frame rate. However, the quality of the images is low. These standards usually provide image resolutions up to 352x288 pixels. As the available resolution is quite limiting, newer products tend not to use this standard.

MPEG 1
Stands for Moving Picture Encoding Group international standard ISO/IEC 11172. There are numerous possible variations of this format, but normally the Constraint Parameters Bitstream, CPB, limitation is used, giving “VCR quality”. This gives a performance of up to 352x288 pixel, 30 frames per second (fps) at a maximum of 1.86Mbit/s. Audio according to MPEG 1 layer 1, 2 and 3 are also included in the standard. (MPEG 1 layer 3 audio is commonly known as MP3). As the available resolution is quite limiting, newer products tend to use MPEG 2 instead.

MPEG 2
Moving Picture Encoding Group international standard ISO/IEC 13818 is a popular standard that offers high-quality video suitable for installations where TV-quality is needed.
There are numerous possible variations of this format but normally the Main Profile at Main Level (MP@ML) limitation is used to provide a resolution of 720x480 pixels at 30 fps (NTSC) or 720x576, 25 fps (PAL) An MPEG 2 player can display either MPEG 1 or MPEG 2 formats. DVD-films and digital cable TV use MPEG 2 with a bit rate of approx. 5Mbit/s.

All MPEG formats use a “Differential method”, the first frame is a complete picture similar to a JPEG frame, the next frames are only an up-date of the difference between the first frame and how it looks now. Depending on how the GOP (Group of Pictures) is set, you can have different amounts of up-dated frames (B and I-frames) between the frames containing a complete picture (P-frame) for more information about MPEG please look at www.mpeg.org

MPEG 3
A cancelled standard aimed at HDTV (High Definition TV).

MPEG 4
Moving Picture Encoding Group international standard ISO/IEC 14496. This standard covers a wide variety of applications ranging from the video displayed in cellular phones, to full feature-length movies shown in a cinema. MPEG 4 is generally considered to be the video standard of the future but it is still relatively new and is not widely used.
This format contains many enhancements to all the earlier MPEG standards; for example, it consumes less bandwidth and can mix video with text, graphics and 2-D and 3-D animation layers. Although these new enhancements pack the video in a very efficient manner, a lot of computing power is needed to use them in real time. And it therefore requires a substantial capital investment in more powerful computers.
Several network cameras use lowquality compression technique (e.g. H.263) packed as MPEG 4. Study the video quality, and evaluate the available MPEG 4 applications carefully before choosing this standard.

Proprietary standards
In addition to the recognised official standards, numerous other proprietary standards have been developed and are used by different video equipment manufacturers. In the interests of future maintenance and integration, it is highly recommended that you avoid these and only use documented and widely used open standards.