One of the premises of the novel's universe is that computational power is generally absurdly plentiful, but communications bandwidth over interstellar distances is not. Most communications are in plain text (modeled after USENET) but in some cases, "evocations" are used to extrapolate video and audio from an ultra-compressed data stream.

The trouble, of course, is that it's not very obvious what aspects of the image you're seeing are real, and what aspects were dreamed up by the system doing the extrapolating.