Header consists of many parts and may include quantization tables and 2048 bits of user data. Also each frame has two GUIDs and timestamp. Frame header is packed into big-endian dwords.
Actual frame data consists of packed macroblocks with technique almost identical to JPEG: DC prediction and variable-length codes with run length encoding for other 63 coefficients.
DC coefficient is not quantized.
SMPTE standardized it as VC3.