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Filtered multitone (FMT) modulation is a filter bank based modulation scheme that is a candidate for the next generation of digital subscriber line (DSL) technology. The high level of spectral containment provided by FMT schemes offers the potential for larger achievable bit rates than more conventional discrete multitone (DMT) schemes in typical DSL environments. In this paper we provide an effective design method for the prototype filter in the FMT filter bank. Our method allows efficient evaluation of the inherent trade-off between the subchannel spectral containment provided by the prototype filter and the intersymbol interference that the filter generates. An appropriate operating point on this trade-off curve is then identified by computing the achievable bit rate for FMT systems with prototype filters which lie on the curve. Our numerical results indicate that careful exploration of the filter design trade-off results in a significant gain in the achievable bit rate.