Describe is a video stream mixer (60) for generating a mixed encoded video stream from an encoded primary video stream associated with a primary window, and at least one encoded secondary video stream, each associated with a corresponding secondary window that is smaller than the primary window. The mixer includes the following elements: A video stream splitter (20) configured to split the encoded primary video stream into an encoded unmixed sub-stream associated with a sub-window in the primary window and not involved in mixing, and an encoded complementary sub-stream associated with a sub-window complementary to the other sub-window in the primary window and involved in mixing; A video stream decoder (22) configured to decode the complementary sub-stream and each secondary video stream; A video signal mixer (24) configured to mix the decoded complementary sub-stream with each decoded secondary video stream to form a mixed video signal; A video encoder (26) configured to encode the mixed video signal into an encoded mixed sub-stream; A video stream combiner (28) configured to combine the encoded unmixed sub-stream with the encoded mixed sub-stream.