Contents An Overview of Digital Video
First, we give a brief overview of digital video; the interested reader is referred to [17], [18] for a more detailed discussion. Let us start with an analog video signal generated by an analog video camera. The analog video signal consists of a sequence of video frames. The video frames are generated at a fixed frame rate (30 frames/s in the National Television Standards Committee, NTSC, format). For each video frame, the video camera scans the frame line by line (with 455 lines in NTSC). To obtain a digital video signal the analog video signal is passed to a digitizer. The digitizer samples and quantizes the analog video signal. Each sample corresponds to a picture element (pel). The most common digital frame formats are Common Intermediate Format (CIF) with 352 × 288 pels (i.e., 352 pels in the horizontal direction and 288 pels in the vertical direction), Source Intermediate Format (SIF) with 352 × 240 pels, and Quarter CIF (QCIF) with 176 × 144 pels. In all three frame formats, each video frame is divided into three components. These are the luminance component (Y), and the two chrominance components: hue (U) and intensity (saturation) (V). Since the human eye is less sensitive to the color information than to the luminance information, the chrominance components are sampled at a lower resolution. Typically, each chrominance component is sampled at half the resolution of the luminance component in both the horizontal and vertical directions. (This is referred to as 4:1:1 chroma subsampling.) In the QCIF frame format, for instance, there are 176 × 144 luminance samples, 88 × 72 hue samples, and 88 × 72 intensity samples in each video frame, when 4:1:1 chroma subsampling is used. Finally, each sample is quantized; typically, 8 bits are used per sample.
