Voice Transmission At 6000 Bits Per Second Using Delta Modulation

Digital systems are of major importance in modern communications. This research investigates the minimum clock (bit) rate of a commercially available delta modulator/demodulator needed to transmit intelligible voice signals. Intelligible voice and music signals were recovered using a clock rate as low as 6 Kbits/sec. (Author).

A variable bit width delta modulator (VBWDM) demodulator was designed, built and tested to achieve voice and music communication using a bandlimited channel. Only baseband modulation is applied to the input signal. Since there is no 'clock' used during the digitizing process at the modulator, no bit synchronization is required for signal recovery in the receiver. The modulator is a hybrid design using 7 linear and 3 digital integrated circuits (IC), and the demodulator uses 2 linear ICs. A lowpass filter (LPF) is used to simulate the channel. The average number of bits sent over the channel is measured with a frequency counter at the output of the modulator. The minimum bandwidth required for the LPF is determined according to the intelligibility of the recovered message. Measurements indicate an average bit rate required for intelligible voice transmission is in the range of 2 to 4 kilobits per seconds (kbps) and between 2 to 5 kbps for music. The channel 3 dB bandwidth required is determined to be 1.5 kilohertzs. Besides the hardware simplicity, VBWDM provides an option for intelligible digitized voice transmission at very low bit rates without requiring synchronization. Another important feature of the modulator design is that no bits are sent when no signal is present at the input which saves transmitter power (important for mobile stations) and reduces probability of intercept and jamming in military applications. Keywords: Voice communication, Delta modulation, Analog-to-digital converter, Bit rate, Variable bit width, Hard limiter, DM, VBWDM, Radiofrequency wave propagation, Theses. (JG).

Masters Theses in the Pure and Applied Sciences was first conceived, published, and dis seminated by the Center for Information and Numerical Data Analysis and Synthesis (CINDAS) * at Purdue University in 1957, starting its coverage of theses with the academic year 1955. Beginning with Volume 13, the printing and dissemination phases of the ac tivity were transferred to University Microfilms/Xerox of Ann Arbor, Michigan, with the thought that such an arrangement would be more beneficial to the academic and general scientific and technical community. After five years of this joint undertaking we had concluded that it was in the interest of all concerned if the printing and distribution of the volume were handled by an international publishing. house to assure improved service and broader dissemination. Hence, starting with Volume 18, Masters Theses in the Pure and Applied Sciences has been disseminated on a worldwide basis by Plenum Publishing Corporation of New York, and in the same year the coverage was broadened to include Canadian universities. All back issues can also be ordered from Plenum. We have reported in Volume 25 (thesis year 1980) a total of 10,308 theses titles from 27 Canadian and 214 United States universities. We are sure that this broader base for theses titles reported will greatly enhance the value of this important annual reference work. While Volume 25 reports theses submitted in 1980, on occasion, certain universities do report theses submitted in previous years but not reported at the time.

Monograph on communications satellites and their future potential - discusses the outer space segment (incl. Satellite links and construction, orbit and inclination, transmission losses, etc.), and the ground segment (incl. Multiplexing, modulation, data transmission, packet radio terminals, data protection, trade-offs in systems design, interactive computing, etc.). Diagrams, graphs and illustrations.