Browse Prior Art Database

Stored Waveform Technique for the DPSK Modem

IP.com Disclosure Number: IPCOM000083405D
Original Publication Date: 1975-May-01
Included in the Prior Art Database: 2005-Mar-01
Document File: 3 page(s) / 42K

Publishing Venue

IBM

Related People

Dubil, JF: AUTHOR [+3]

Abstract

Described is a microcoded modem transmitter for transmitting a 4-phase informational waveform by selectively outputting time domain elements of the waveform, whose full amplitude was stored in the system in a pulse code modulated form.

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Stored Waveform Technique for the DPSK Modem

Described is a microcoded modem transmitter for transmitting a 4-phase informational waveform by selectively outputting time domain elements of the waveform, whose full amplitude was stored in the system in a pulse code modulated form.

M. F. Choquet, et al, "Microcoded Modem Transmitters", IBM Journal of Research and Development, July 1974, pages 338 to 351 discloses various microcoded designs for modem transmitters, wherein time domain signal elements of the waveform desired to be transmitted, are stored in a digital memory as pulse code modulated samples. The system operates on the dibit signal synthesis principal and, therefore, the amplitude components of the signal elements are actually stored by the system. Selection logic selects the signal element which corresponds to the dibit configuration desired and fetches at regular intervals, the various samples that correspond to this signal element. The amplitude components of the signal elements selected, must be passed through a digital adder so that the output signal to be transmitted has an amplitude, which is the sum of the amplitudes of the stored component signals. Experience has shown that the use of adders introduces long-term drift problems for the system.

Fig. 1 improves upon existing microcoded modem transmitters, by eliminating the necessity for a digital adder at the output of the signal storage. The system block diagram consists of an address generator 1, a read-only storage (ROS) 2, an inverter 3, a digital-to-analog (D/A) converter 4, and an output filter 5. The ROS 2 contains eight waveforms shown in Fig. 2. each consisting of twenty-four n-bit words stored in pulse code modulated form. It has been found that there are only right unique elementary waveforms in the time domain in a 4-phase differential phase shift keying (DPSK) info...