Abstract: A dual-speed, dual format full-duplex two-wire voiceband data transmission system provides automatic speed selection at the answering terminal responsive to a handshaking sequence which is compatible with existing systems operating at telegraph speeds. Existing systems provide asynchronous full-duplex serial data transmission in the speed range of zero to 300 bits per second using frequency-shift keying of tones in split frequency bands dedicated to the respective transmisson directions. Alternative full-duplex serial data transmission at 1200 bits per second using phase-shift keying of carrier waves in these same split frequency bands can now be provided from a common line protocol.

Abstract: A modulator for differentially encoded phase-shift keyed digital data is implemented entirely by digital means. Baseband serial binary data are illustratively transformed into dibit pairs which are assigned discrete quaternary carrier-wave phase shifts. A "staircasing" shift register whose weighted outputs are combined to form a quantized phase-modulated carrier wave is advanced by a relatively high-speed counter through a frequency divider network. The division rate of the divider is selectively accelerated in each baud interval in accordance with the assigned dibit coding. The resultant quantized carrier wave is then lowpass filtered prior to application to a telephone transmission line. By controlling the division rate of the divider rather than the advancement rate of the shift register, gradual transitions are imparted to the output carrier wave with minimal harmonic distortion.

Abstract: Asynchronous character-oriented (start-stop) data are transmitted through a synchronous transmission channel at a rate which can exceed the synchronous rate. The excess data rate is accomplished in an input buffer at the transmitter by suppressing occasional stop bits in proportion to the difference between synchronous and asynchronous rates. An output buffer at the receiver detects start bits and thereafter monitors the presence or absence of stop bits. When a stop bit is absent, the output buffer restores it before delivering the character to the data user.Special control signals not organized into characters, such as, all-space signals are also monitored at both transmitter and receiver to insure that stop bits are not spuriously inserted.

Abstract: Sample timing recovery in a differentially coherent phase-shift keyed data transmission system is accomplished through the use of digital techniques. Samples of received phase-modulated signal waves taken at a rate high with respect to the carrier frequency are applied to a delay medium advantageously implemented by a shift register. Stored samples spaced by known carrier phase differences are compared for the purpose of detecting phase changes. Digital indications of such phase changes resulting from differentiation of comparator outputs are then effective to control a phase-locked oscillator and provide a substantially jitter-free timing recovery wave without using excess bandwidth for the transmitted wave.