A digital video communication system includes, at a source site, an embedding circuit for embedding a digital watermark in a digital video stream to produce a watermarked digital video stream, and a multiplexer for multiplexing a text data stream with the watermarked digital video stream to produce a multiplexed signal. The least significant bits of a sample of the reference signal are stored in a comparison register. The clamp assembly also includes a pressing plate having upper and lower surfaces, a front nose portion, a rear end portion, and is provided with a keyhole shaped groove having an enlarged inner cavity open toward the rear end by a narrow neck extending thicknesswise.