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any one side of the connection are sent at similar frequencies as in the 300 bit/s systems, but slightly out of phase.

In early 1973, Vadic introduced the VA3400 which performed full-duplex at 1,200 bit/s over a normal phone line.

In November 1976, AT&T introduced the 212A modem, similar in design, but using the lower frequency set for transmission. It was not compatible with the VA3400, but it would operate with 103A modem at 300 bit/s.

In 1977, Vadic responded with the VA3467

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