HDTV and the vestigial sideband syndrome

Advanced TV (ATV) standards can only be made directly compatible with the 162 million existing TV receivers in the US by delivering the signals to the receiver on a vestigial sideband, amplitude modulated radio frequency carrier. However, low frequency group delays (due to phase error in the vestigial sideband filters required for modulated and demodulation) often generate a delayed edge effect that can obscure the crispness and freedom from artifacts in ATV pictures. Because of difficulties in understanding, quantifying, and controlling low-frequency group delay, vestigial sideband technology may be too fragile to support any form of high-quality ATV. The only alternative appears to be a dual system in which programs are simulcast on different channels to conventional NTSC and new ATV standards. In Europe, terrestrial broadcasting to PAL and SECAM standards will apparently continue indefinitely, while incompatible ATV standards will be used for direct broadcasting from satellites.<
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