A data processing apparatus constitutes a low-cost audio/video data transmission and reception system. A reception buffer monitoring circuit 21 monitors the size of receiver data being accumulated in a reception buffer 6. When the size of accumulated data is found to become higher than a high threshold, the circuit 21 causes a reception clock generation circuit 8 to generate a reception clock with a higher frequency. When the accumulated data size becomes lower than a low threshold, the reception clock is generated with a lower frequency. Based on the reception clock fed from the reception clock generation circuit 8, an audio/video decoder 7 decodes the audio/video data coming from the reception buffer 6. The data processing apparatus applies advantageously to a television transmission and reception system for transmitting and receiving TV broadcast signals.