Browse Prior Art Database

SIMULCAST DELIVERED AUDIO QUALITY IMPROVEMENT METHOD VIA SUBAUDIBLE SIGNALLING PARITY

IP.com Disclosure Number: IPCOM000006305D
Original Publication Date: 1991-Dec-01
Included in the Prior Art Database: 2001-Dec-21
Document File: 2 page(s) / 89K

Publishing Venue

Motorola

Related People

Paul Cizek: AUTHOR [+2]

Abstract

FM simulcast systems are typically designed such that regions exist in which signals from 2 or more remote sites overlap. In these regions, to recover an intelligible signal, it is critical to keep the signals in phase by care- fully setting the delays and equating the frequency mod- ulation constants ofthe transmitters to reduce simulcast distortion. This is especially true in the overlap regions of equal signal strength where in the case of a two-site system. I80 degrees of phase difference will cause com- plete cancellation of the received signal. Such occurrences result in spikes in the demodulated signal that produce audible "pops:' These pops reduce the overall audio qual- ity of the received signal.

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MOTOROLA INC. Technical Developments Volume 14 December 1991

SIMULCAST DELIVERED AUDIO QUALITY IMPROVEMENT METHOD VIA SUBAUDIBLE SIGNALLING PARITY

by Paul Cizek and Mark Nowak

   FM simulcast systems are typically designed such that regions exist in which signals from 2 or more remote sites overlap. In these regions, to recover an intelligible signal, it is critical to keep the signals in phase by care- fully setting the delays and equating the frequency mod- ulation constants ofthe transmitters to reduce simulcast distortion. This is especially true in the overlap regions of equal signal strength where in the case of a two-site system. I80 degrees of phase difference will cause com- plete cancellation of the received signal. Such occurrences result in spikes in the demodulated signal that produce audible "pops:' These pops reduce the overall audio qual- ity of the received signal.

  Maintaining constant frequency modulation constants of the FM carriers becomes increasingly difticult when the modulating signal decreases in frequency Therefore, trunked simulcast, FM communication systems with subaudible signalling require special consideration of the control of the subaudible modulation. The purpose of this control is to insure a DC balanced signal. Figure I shows the effect of a non-balanced (DC) subaudible sig- nal received from 2 transmitter sites. The phase differ- ence passes through 180 degrees causing simulcast distortion. Because the signal is not balanced, the phase difference will continue to pass through I80 degrees over time.

  To further clarify the potential difference in the mod- ulating signal, the difference can be ekpressed in dB by the following expression:

tinue to ramp in the direction determined by the previ- ous bit. As more consecutive and identical bits are sent, the phase difference continues to change in the same direction increasing the likelihood of passing thr...