Optimum Pre-Emphasis and De-Emphasis Networks for Transmission of Television by PCM

The advantages attendant to the use of pre-emphasis and de-emphasis networks in a PCM television system are examined. The optimum pre-emphasis network is obtained by maximizing the ratio of peak-peak signal to rms weighted noise with the coder input constrained to prevent overload. The results show that for a system using a 12-Mc sampling rate the weighted quantizing noise in the video band can be reduced about 3.7 dB, which corresponds to adding half a digit. This particular figure is based on the assumptions that quantizing noise is subjectively equivalent to thermal noise and that we have a reasonable estimate of the video signal spectrum. Networks approximating the optimum pre-emphasis and de-emphasis networks have been constructed and the performance with these networks compared to that without pre-emphasis (and de-emphasis). A visual comparison of the received pictures shows the improvement, particularly in breaking up contours.