Technical notes
Compression standards

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Introduction

Digital images and digital video are normally compressed in order to save space on hard disks and to speed up transmission. There are presently several compression standards used for network transmission of digital signals on a network. Datas sent by a camera using video standards contain still images mixed with data containing changes, so that unchanged datas (for instance the background) are not sent in every image. Consequently the frame rate measured in frames per second (fps) is much greater.

Still images and compressed video signals.

Still images are simple and easy to send. However it is difficult to obtain single images from a compressed video signal. The video signal uses a lesser data to send or store a video image and it is not possible to reduce the frame rate using video compression.
Sending single images is easier when using a modem connection or anyway with a narrow bandwidth.

Main compression standard for still imagesMain compression standards for video signal
JPEGM-JPEG (MotionJPEG)
WaveletH.261, 263 etc.
JPEG 2000MPEG1
GIFMPEG2
 MPEG3
 MPEG4
JPEG

(Joint Photographics Expert Group)
Popular compression standard used exclusively for still images. Each image is divided in 8 x 8 pixels; each block is then individually compressed. When using a very high compression the 8 x 8 blocks can be actually seen in the image. Due to the compression mechanism,  the decompressed image  is not the same image which has been compressed; this because this standard has been designed considering the performance limits of human eyes. The degree of detail losses can be varied by adjustin compression parameters. It can store up to 16 million colours.

Wavelet

Wavelets are functions used in representing data or other functions. They analize the signal at different frequencies with different resolutions. Optimized standard for images with low amount of data with sharp discontinuities. Wavelet compression transforms the entire image differently from JPEG and is more natural as it follows the shape of the objects in the picture. It is necessary to use a special software for viewing,  being this a non-standardized compression method.

JPEG2000

Based on Wavelet technology. Rarely used.

GIF 

(Graphic Interchange Format).
Graphic format used widely with Web images. It is limited to 256 colours and is a good standard for images which are not too complex. It is not recommended for netwok cameras being the compression ratio too limited.

M-JPEG (Motion-JPEG)

This is not a separate standard but rather a a rapid flow of JPEG images that can be viewed at a rate sufficient to give the illusion of motion. Each frame within the video is stored as a complete image in JPEG format. Single images do not interact among themselves. Images are then displayed sequentially at a high frame rate. This method produces a high quality video, but at a cost of large files.

H.261, H.263, ecc

Standards approved by ITU (International Telecommunications Union). They are designed for videoconference applications and produce images with a high compression rate, but with a low quality. The available resolution is limited (up to 352 x 288 pixels).

MPEG 1

Produces images of VCR quality and includes also audio signal. Initially it was designed for laser disk speeds, hard disk and the CD-ROMs. Being its resolution limited MPEG 2 is preferred. The transmission rate varies from 1.5Mbps and 3.5Mbps. The normally used variation gives a performance of 352x288 pixels resolution, 30fps with max rate of 1.86 Mbit/sec. From this standard come also the MP3 and VideoCD standards.

MPEG 2

It is not the next generation of MPEG 1, but rather another standard designed for high quality digital images and audio. A popular standard offers high TV quality, with a broad band. The transmission rate varies between 3.5 Mbps and 10 Mbps. The normally used variation offers images with a resolution of 720x576 pixels, 25fps (PAL) with typical rate of  1-10 Mbit/sec.

MPEG 3

The normally used variation has a resolution of 352x288 pixels, 30fps with max rate of 1.86 Mbit/sec.

MPEG 4

It is not widely used, though it is regarded as the standard of the future. This standard has many enhancements to any of the previous standards (for example it can be used with a narrower band and can mix video with text and graphics). However it is necessary to use a high computing capacity and requires an investment in more powerful computing machines.

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Last modified 10-05-2013
 
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