A fast macroblock mode selection method in downscaling transcoder based on H.264 is proposed in this paper

which the total non-zero coefficients of responding four macroblocks in pre-coded frame is utilized as the criterion to select macroblock mode.We propose an exponent function model between threshold and re-quantization parameter (

Th-Q

r

model).This model is converted into a linear regression model

and least square method is adopted to estimate parameters in the model.In the intra mode selection part

Th-Q

r

model is used to classify I4×4 or I16×16;In the inter mode selection part

Th-Q

r

model is used to classify the motion activity in downscaled frame

and different candidate macroblock modes are used according to classi

fied motion activity areas.We compared the PSNR performance and computational complexity of proposed method with the full search algorithm.For the intra mode selection part

we can speed up about 4~5 times

and for the inter mode selection part

we can speed up about 35 times.