Effect of sharp-cut-off video-frequency filters on chrominance signal response in PAL colour television

In television System I, the nominal bandwidth is 5.5 MHz but in most practical transmission systems the actual bandwidth is greater. There is a likelihood that video lowpass filters will be used in the future, for example in association with digital codecs, and it has been found that even when such filters have a passband of 5.5 MHz, critical signals such as colour bars and electronically generated captions exhibit ringing. To investigate the effect, the responses of existing PAL coder/decoders have been measured with and without a low-pass filter to define the transmission channel bandwidth. The filter currently proposed by the CCIR to shape the chrominance signal in the coder has a substantial response well above 5.5 MHz. In this situation it will have to be accepted that critical signals may be distorted at intermediate points along future transmission channels containing low-pass filters.