The document describes a study that developed a system for encoding speech at low bit rates using Linear Predictive Coding (LPC). LPC is an efficient speech analysis technique that provides accurate estimation of speech parameters. The system encodes male and female speech signals. In the encoding process, LPC determines parameters to model the vocal tract during speech production. Decoding uses these parameters to synthesize a version of the original speech signal. Testing found that the system was successful in encoding speech at relatively low bit rates while maintaining good quality.

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Speech Compression using Voiced Excited Loosy Predictive Coding
International Journal of Emerging Technology and Advanced
Engineering
Abstract:
The aim of the project to develop a system for encoding good quality speech at a low bit rate .To
implement this we have used very efficient speech analysis technique Linear Predictive Coding (LPC). It
provides accurate estimation of speech parameters. An alternate explanation is that the linear
prediction filters attempt to predict future values of the input signal based on past signals. The speech
signals of males and females were coded. The encoding process of LPC involves determining a set of
accurate parameters for modeling the vocal tract during the production of a given speech signal.
Decoding involves using the parameters acquired in the encoding and analysis to build a synthesized
version of the original speech signal. The conclusion indicates that project was successful in coding the
speech signal at relatively low bit rates with good quality.

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(a) Path of Human Speech Production

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(b) Human vs. Voice Coder Speech Production
(c) Block Diagram of Voice Excited LPC Vocoder

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(d) Block Diagram of LPC Vocoder

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A: - Waveforms
B: - Spectrograms

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Conclusion:
We can see that the voice excited waveform looks closer to the original sound than the plain LPC
reconstructed one. Voice –discrimination tests indicate that voice identity is well preserved. Crucial
factors influencing the remade speech quality are accuracy of spectral flattening and the impulse
response of the analyzer low pass filters. Though there is an improvement in quality when we use voice
excited method, the bits per sample increase causing an increase in Bandwidth of signal. But at same
time when SNR for both cases were compared it was observed that the sound due to Plain LPC was
found to be noisier having a negative SNR. The voice excited LPC having a positive SNR.
References:
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NJ,1978
*2+ Speech coding overview‖ Jason Woodard, http://www-mobile.ecs.soton.ac.uk/speech_codecs/
*3+ Richard V.Cox ―Speech Coding(1999)‖,chapter9,1-9
[4] Mark Nelson and Jean-loup Gailly,‖Speech compression‖.
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*6+ Jerry. D. Gibson. ―Media Signal Processing‖.
*7+ Study of Linear Prediction Model for Audio Synthesis‖, Anjini Shukla ,
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[9] en.wikipedia.org/wiki/linear_predictive_coding
*10+ Roger M. Golden, ―Digital Computer Simulation of a Sampled-Data Voice-Excited Vocoder
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Sask., conference Publications
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