Abstract
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.
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