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DSP Voice Frequency Compander

DSP Voice Frequency Compander

This project involves implementing the voice frequency companding part of Harris and Cleveland’s NBVM on Texas Instrument’s new DSP Starter Kit (DSK).
With the radio frequency (RF) spectrum getting more and more crowded all the time there is a profound need to make more efficient use of these bands. In 1978, Harris and Cleveland [1] developed a unique method of voice frequency and amplitude companding called narrow band voice modulation (NBVM). This technique offered significant improvement in bandwidth conservation, which in turn improved signal to noise ratio (SNR) and decreased co-channel interference. However, their technique required cumbersome analog hardware. With new DSP hardware it is now possible to develop these techniques in a flexible and fairly inexpensive way.

Simply put, frequency companding is a method of removing the most unused frequency components from a given source and then shifting the remaining components together. The resulting signal may be sent over nearly any analog or digital transmission system. On the receiving end, the frequency components are expanded back out before the signal is used. With the technique described in this article, the bandwidth necessary for intelligible speech can be reduced from about 3 kHz down to about 1.7 kHz, thus conserving over 40% in RF bandwidth usage. In addition, the SNR is improved by about 2.4 dB and co-channel interference is reduced.

In normal speech Harris and Cleveland found that intelligible speech can be maintained with certain bands of frequencies removed. A spectrogram of a voice sample shows frequency components at different times throughout a sampling period. The frequency components are calculated over small time intervals using a Fast Fourier Transform (FFT). Darker areas indicate higher energy. It is obvious that frequencies below 100 Hz and above 2500 Hz are used very little in human speech. Additionally, notice the frequencies from about 600 Hz to 1200 Hz. Harris and Cleveland found that after removing these frequency components, normal speech was still intelligible! Part of their companding technique was to simply remove this band of frequency components as well as those below 100 Hz and above 2500 Hz. All the remaining frequency components were then shifted together, ready for transmission.

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