Video Codec Simplified

Video Codec Basics

Algorithms

YUV

Resolution

Let us consider a small video frame of size of 352x288 pixels. For the ease of processing, the frame will be broken into smaller chunks. For many codecs, square blocks of 16 x 16 pixels are used, as shown figure.

The choice of an 16 x 16 block size is a good compromise between the number of unique pieces of information coded and the amount of commonality between pixels within a block. This blocks are termed as MacroBlocks.

Macroblocks are the basic building blocks of the standard for which the encoding/decoding process is carried out.

The Macroblocks can be again break into smaller pieces of 8x8 and 4x4 pixels. While most of the processing happens in 16x16 pixels, the smaller blocks are used wherever finer precision is required.

Figure: Macroblock, Block

YUV

Human eyes are sensing the the color and brightness by different set of sensors. The brightness sensors are comparatively more sensitive than the color sensors. Image compression algorithms are taking advantage of this phenomenon.

The compression algorithms first transforms the image from RGB to the luminance/chrominance (Y-Cb-Cr) color space. Here Y called as luma represents the brightness/grayscale and Cb-Cr are the two color components represents the extent to which the color deviates from gray toward blue and red, respectively.

Since human visual system is more sensitive to luma than chroma, we will use one fourth of the number of samples the chroma component has, than the luma component. This is done by down sampling half the number of samples in both the horizontal and vertical dimensions. This is called 4:2:0 sampling with 8 bits of precision persample.

Mostly used YUV format is 4:2:0

Resolution The choice of resolution mainly depends upon the processing power available for the given equipment. Higher the resolution, higher processing power is required and hence higher energy is also required. In the plain words, smaller the equipment, lesser the resolution supported. The common resolutions used are QCIF, CIF, SD and HD is detailed in the following sections.		Teleconference Resolution of 174 x 144 known as QCIF. Generally 5-10 Frames Per Second (fps) is used. Uncompressed size with the above mentioned resolution and fps some where around 1.6-3Mbps. After compression with general codecs such as MPEG2/H.263, the expected file size is around 32-64Kbps
		Multimedia Resolution of 352 x 288 known as CIF. Generally 30 Frames Per Second is used Uncompressed size 36Mbps Expected Compress size 200-300Kbps
		Standard definition Resolution of 720 x 486 known as SD. Generally 30 Frames Per Second is used Uncompressed size 168Mbps Expected Compress size 4-6Mbps
		High Definition Resolution of 1920 x 1080 known as HD. Generally 30 Frames Per Second is used Uncompressed size 1.2Gbps Expected Compress size 20Mbps

		Digital Cinema Resolution of 4096 x 2160 known as DC. Generally 24 Frames Per Second is used Uncompressed size 7.6Gbps Expected Compress size 100Mbps