Speech compression and tone detection in a real-time system

During the night, encrypted spoken newspapers are broadcasted over the terrestrial FM radio network. These papers can be subscribed to by persons with reading disabilities. The subscribers presently have a special receiver that decrypts and stores the newspaper on a cassette tape. A project aiming to design a new receiver, using digital technology, was started during the year 2002. This report describes the parts of the project involving speech compression and tone detection. An overview of different compression techniques, with emphasis on waveform coding, is given. A detailed description of Adaptive Differential Pulse Code Modulation (ADPCM), the compression technique chosen for implementation on a Digital Signal Processor (DSP) is also included. ADPCM was first implemented on the ADSP-2181 DSP, with a good result. In the final version of the digital receiver the ADSP-2191 DSP will be used and hence the code was converted to fit this DSP. Due to some problems this implementation could not be completed within the time frame of this thesis.

The final part of this thesis consists of finding a method for detecting a tone inserted between articles in the spoken newspaper. The tone detection is composed of two parts, the first part is reducing the amplitude of the speech while maintaining the amplitude of the tone. For this part a digital resonator was chosen and implemented both in Matlab and on the ADSP-2191 DSP. The second part of the tone detection consists of deciding whether the tone is present or not, this part was implemented only in Matlab.

Author: Berglund, Kristina

Source: Lulea University of Technology

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