Video communication has advanced significantly over the last decade. Low bit rate video coding and low cost packet switching network access have made video communication practical and cost-effective. CCITT has recommended a compression standard (H.261) with a rate of px 64 Kb/s for p=1 to 30. The key elements of H.261 are: (1) interframe compensation, (2) motion compensation, (3) discrete cosine transform (DCT), (4) quantization, and (5) coding. For interframe compensation, only the difference of two consecutive frames is transmitted. In motion compensation, a spatial displacement vector is derived. DCT is used to convert spatial data into spatial frequency coefficients. All transformed coefficients are quantized with uniform quantizer for which step size is adjusted according to the buffer occupancy. Quantized coefficients are encoded using both fixed and variable length coding. At the decoder, the inverse operation of compression is performed. In this thesis, a detailed description of H.261 and its implementation in software are provided.
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