H.261 Video Coding

H.261 is video coding standard published by the ITU (International Telecom Union) in 1990. It was designed for datarates which are multiples of 64Kbit/s, and is sometimes called p x 64Kbit/s (p is in the range 1-30). These datarates suit ISDN lines, for which this video codec was designed for.

The coding algorithm is a hybrid of inter-picture prediction, transform coding, and motion compensation. The datarate of the coding algorithm was designed to be able to be set to between 40 Kbits/s and 2 Mbits/s. The inter-picture prediction removes temporal redundancy. The transform coding removes the spatial redundancy. Motion vectors are used to help the codec compensate for motion. To remove any further redundancy in the transmitted bitstream, variable length coding is used.

H.261 supports two resolutions, QCIF (Quarter Common Interchange format) and CIF (Common Interchange format).

***Picture Formats Supported***
Picture format	Luminance pixels	Luminance lines	H.261 support	Uncompressed bitrate (Mbit/s)
				10 frames/s		30 frames/s
				Grey	Colour	Grey	Colour
QCIF	176	144	Yes	2.0	3.0	6.1	9.1
CIF	352	288	Optional	8.1	12.2	24.3	36.5

Some example video sequences I have coded with the H.261 codec are shown below. Also some graphs of Image quality versus bitrate and compression ratio as shown below. The Image quality measure used is the peak signal to noise ratio (PSNR).

Some Example H.261 coded Video sequences

Here are some h261 video sequences. They were H261 coded then recoded with MPEG at high quality to make them viewable. A MPEG player is available from Berkeley university however this also only compiles for UNIX, but DOS/Windoze versions are available. To download and display the video just click on the movie icon in the tables below.

Miss America (QCIF)

Lab Sequence

Description	Average PSNR(dB)	Compression Ratio
Original	n/a	1:1
High Quality	38	25:1
Low Quality	34.7	104:1
Low Quality with motion vectors	35	113:1

Description	Average PSNR(dB)	Compression Ratio
Original	n/a	1:1
High Quality	35.4	13:1
Low Quality	32	75:1
Low Quality with motion vectors	32	76:1

Comparison of Quality Versus Bitrate/Compression-ratio

The graph below shows the image quality (PSNR) versus bitrate for the Miss America video sequence (QCIF resolution). Click on the graph below for the full size version. The graph shows the results of sequences coded at 30 frame/s and 10 frame/s. Also results without motion vectors, with motion vectors (+MV), and with motion vectors and loop filtering (+MV+LF) are plotted.

The graph below is the same results but the image quality (PSNR) is plotted against the compression ratio. Again click on the graph to get the full size version.