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A METHOD OF EMBEDDED LOW SPEED SIGNALING IN BAND LIMITED VOCODER APPLICATIONS

IP.com Disclosure Number: IPCOM000007124D
Original Publication Date: 1994-Feb-01
Included in the Prior Art Database: 2002-Feb-27
Document File: 3 page(s) / 185K

Publishing Venue

Motorola

Related People

Michael Bushman: AUTHOR [+2]

Abstract

Presented is a method of embedding low speed digital information in a band limited digital vocoder data stream. Advantages of the method are 1) an orthogonality between the vocoder data and the embedded signal which allows the both signals to occupy the data channel simultaneously without increasing the signal bandwidth, and 2) recovery of the vocoder stream without degradation.

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MOlDROLA Technical Developments Volume 21 February 1994

A METHOD OF EMBEDDED LOW SPEED SIGNALING IN BAND LIMITED VOCODER APPLICATIONS

by Michael Bushman and Kenneth Fuchs

  Presented is a method of embedding low speed digital information in a band limited digital vocoder data stream. Advantages of the method are 1) an orthogonality between the vocoder data and the embedded signal which allows the both signals to occupy the data channel simultaneously without increasing the signal bandwidth, and 2) recovery of the vocoder stream without degradation.

  Many vocoding techniques are bandwidth inef- ticient in the sense they do not posses uniform power spectral densities for the entire allotted channel bandwidth over time. When such inelliciencies exist, the signaling scheme allows for the introduction of a low speed embedded signal without degradation in the vocoder signal. An example of such a signal- ing system is digitally encrypted voice signaling using Continuously Variable Slope Delta (CVSD) modu- lation. With the energy in speech rolling off significantly below 300 Hz, there is little voice infor- mation in the digitized voice signal below this fre- quency. The digitized voice is to be digitally encrypted, which is the equivalent of adding (modulo-2) a pseudo-random digital signal to the digitized voice, and the required channel bandwidth will extend virtually to DC. The encrypted signal's channel requires greater bandwidth than the voice information, therefore the additional bandwidth can be utilized for low speed data applications.

  In an analog application one could simply add a low speed signal to a band limited voice signal at the transmitter and decimate the result into the respective signals at the receiver by selectively filtering the channel information. In fact, such a method is theoretically possible with the encryption system above if the low speed data is added to the voice before it is modulated by the CVSD and later recovered by filtering after demodulation by the CVSD. However CVSD modulation does exhibit a small amount of energy at low frequencies related to step size information. Further, very high Q filters would be required to optimize the bandwidth ofthe

0 Mowrola. 1°C. 1994

embedded signal, A better approach is to perform the embedding and recovery of the low speed data in the digital environment. This approach allows the serial binary low speed signal to be embedded by adding it modulo-2 to the CVSD modulated voice (also a serial binary signal) at the transmitter. At the receiver, the recovered low speed signal can then be subtracted modulo-2 from the channel informa- tion resulting in uncorrupted voice information for demodulation.

  It is proposed that the low speed data be added modulo-2 to the CVSD modulated voice, this is not the same as the analog addition of the two signals. Consider two binary signals whose logic low is a -1 and whose logic high is a +l, it can readily be seen that the analog equivalent...