Magfan Pehara and Zydo have it right. The Sonic Holography circuitry took a band limited (mostly midrange) signal from each channel, delayed it slightly, and injected it in reverse phase in the opposite channel. Carver built the circuit into their preamps, and also offered it in several stand alone configurations. The C9 was a dedicated Sonic Hologram processor, and the ECS-U included Holography along with several other speaker control functions for the Amazing series speakers. Both units went between pre and power amp, or in the tape loop.
In my early years of audio (the 80's) I loved the Hologram generator. It required careful setup of speakers, room and listening position, but the effect was quite startling on most material. Images extended well beyond the speakers and often wrapped 180 degrees around the listener. You could often hear the acoustic space if the recording was live. Occasionally sounds would appear to come from behind.
As I grew in the hobby I realized how gritty and transitory both the C9 and ECS-U sounded and I weaned myself off them. They were built with crappy parts and cheap power supplies. Sometimes I wonder what a high quality implementation of the concept would sound like.
Crosstalk cancellation would be a natural for digital processor implementation, and a vaguely recall the some Meridian processor offered it as a side function.
As Magfan mentioned, Polk offered a series of speakers in the 80's that atured an extra midrange driver in each speaker that was fed a reverse phase signal from the opposite channel. Spacing provided the necessary acoustic delay to make the effect work.
Bottom line: The Carver gear was never great sounding, and updrading the pre would be a good place to start.
