Two layer video signal coding

An apparatus for encoding a video signal for transmission, comprising:

• means for coding data representing the video signal by a base layer coding operation which includes a base layer quantizer having a base layer quantization step size to provide coded video data for transmission;
• means for deriving inverse-coded video data by carrying out an inverse base layer coding operation on the coded video data;
• means for deriving difference data from the data representing the video signal and the inverse-coded video data;
• and
• means for coding the difference data by an enhancement layer coding operation;
• characterised in that the means for coding the difference data operates only when the energy of the difference data exceeds a variable threshold, the threshold being inversely proportional to the base layer quantization step size.