Abstract— GSM Voice Channel used hybrid based on Linear Predictive Coding Codec. It also used some technical in speech processing as VAD (Voice Activity Detectors), AGC (Automatic Gain Control). As a result, audio signals that are not voice - like are greatly distorted because they violate speech signal properties assumed by the underlying codecs. In this paper, discuss about GSM voice channel and presents the approach for transmit normal data over that channel with: Constant - Differential frequency shift keying technique.
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