Streaming compressed video over communication channels that introduce errors, losses and excessive delay requires countermeasures to preserve the quality of the viewing experience.

We refer to the ability of the system to minimize and to quickly recover from the damage as its robustness. VSofts achieves robustness through two separate and complementary techniques: error resilience and error concealment.

Error resilience refers to mechanisms in the VSofts Encoder, that enhance the capability of the compressed bitstream to resist channel errors. When errors and losses arise in the stream, error concealment tools in the VSofts decoder analyze the losses and conceal them in the displayed video by minimizing the visual artifacts.

Error resilience functionality in the encoder produces a bitstream that supports error recovery at the decoder.

The following methods are used in VSofts Encoder:

• Selective Intra Coding (including intra macroblocks update based on motion tracking, row based distribution, random distribution,
• Frame segmentation into Slices
• Constrained Texture Prediction
• Flexible Macroblock Order (FMO)

Error concealment refers to the actions taken by the decoder to restore in some way the texture coded in lost packets, by concealing their artifacts. The concealment schemes can be spatial, temporal or combined. In spatial interpolation, the values of missing pixels are estimated from the surrounding pixels of the same frame, without using the temporal information. On the other hand, the temporal interpolation is based on the corresponding regions of the reference frames. If a motion vector is missing, it can be calculated based on the motion vectors of the surrounding regions. Combined scheme use some adaptive mechanism to choose best concealment for each lost macroblock.

The VSofts decoder uses the following schemes. They render the best results in combination with error resilience implementations, but they can also be useful on their own.

• Spatial Texture Interpolation
• Motion Compensation
• Severe Loss Compensation