For the transmission of a standard CEPT, i.e. 2.048 Mbit/s rate digital transmission, in a 240 kHz supergroup band, a 1024-QAM (quadrature amplitude modulated) system employing raised cosine filters with an excess bandwidth of 17% is required. In the paper, the performance of a 1024-QAM system in the presence of different filter imperfections is studied by means of computer simulation. To assess the effects of different channel imperfections, such as filter distortion and/or selective fades caused by residual distortion which are not cancelled by adaptive equalisers, computer simulated results are given, presenting the degradation of the system performance to linear, parabolic, sinusoidal amplitude and group delay distortions. It is shown that sinusoidal amplitude and linear group delay cause the most significant performance degradations. For comparison purposes we also include the results of a same baud rate 512-QAM system.