Abstract: The digital signal is decomposed in intrafield blocks each comprising a set number of adjacent image elements, and each intrafield block, or an intrafield block or an interframe block derived therein, is subjected to a Discrete Cosine Transform and the transformed signal is applied to quantizer to generate a quantized signal which is encoded at variable length and loaded to a transmission buffer, the quantization resolution being inversely proportional to the buffer fill rate. According to the invention, a digital low-pass filter is connected to filter the block to be subjected to Discrete Cosine Transform, and is subjected to the buffer so that its upper-frequency cutoff lowers as the buffer fill rate increases.