1. Speech /Audio Coding Standard
LPC-10
By
Sonawane Swapnil R
511006
Sub.: Speech
DEP-E&TC
VIIT Pune
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2. What is Speech Coding?
• “Speech coding" = finding a representation of
speech which can be transmitted efficiently
through a digital channel.
• It is usually lossy coding, meaning that the
waveform can not be completely reproduced
by the decoder, instead, only the information
which is useful to a human listener is retained.
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3. Coding Algorithm
• ADPCM: Adaptive Differential Pulse Code
Modulation
• LPC-10: LPC Vocoder with 10 coefficients
• CELP: Code Excited LPC
• RPE-LTP: Regular Pulse Excited LPC with Long
Term Prediction
• VSELP: Vector Sum Excited LPC
• IMBE: Improved Multi-Band Excitation
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4. LPC – 10/ FS-1015 :-
• BRIEF HISTORY :-
– Is a secure telephony speech encoding standard developed by
the United States Department of Defense and later by NATO. The
standard was finished 1984.
– Algorithm uses linear predictive coding vocoder.
– The vocoder enables understandable speech, but the quality is
very unnatural and synthetic.
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7. Conti..
• Analysis process extract from the speech signal the parameters
required to model it.
• First parameter :- Type of speech signal (voiced or unvoiced).
• The result is a voicing indicator,
• When voiced, its period has to be estimated in order to reflect its
height. This period, called as pitch .
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1 Voiced segment
0 Unvoiced segment
8. Conti..
• The result of this analysis is a set of ten reflection coefficients
(hence the name LPC 10) which sufficiently & faithfully describe the
cross-sectional variations in the vocal tract.
• Finally, for each frame, the level of the speech signal is evaluated in
order to control the gain of the synthesizer on the synthesis side.
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10. Conti..
• The algorithms employed to synthesize the speech signal reflect the
assumed speech production model.
• They include, in succession:
– a noise generator, used for unvoiced sounds;
– a periodic signal generator, to which the pitch is provided, for voiced
sounds;
– a switch allowing selection of either generator according to the type of
speech signal to be produced in the current frame;
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11. Conti..
– a filter of order 10, which filters the excitation selected; it is at this
level that the distinction between the different vowels and the
different consonants is made;
– a gain control system, which gives the synthetic signal the right
volume;
– optionally, a “post-filtering” system, designed to mask certain
imperfections in the synthesizer and to make the synthesized signal
more pleasant to the human ear.
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12. VOCODER AT 2,400 BIT/S:-
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LP Coefficients Pitch Voicing Energy
0 41 48 53
- The remaining 1 bit is for synchronization
Linear predictive coding (LPC) is a tool used mostly in audio signal processing and speech processing for representing the spectral envelope of a digital signal of speech in compressed form, using the information of a linear predictive model.[1] It is one of the most powerful speech analysis techniques, and one of the most useful methods for encoding good quality speech at a low bit rate and provides extremely accurate estimates of speech parameters.
MOS (Mean Opinion Score
The most widely used measure of quality is the Mean Opinion Score
(MOS), which is the result of averaging opinion scores for a set of
between 20 and 60 untrained subjects.
Standards Organization
ISO: International Standards
Organization (http://www.iso.ch)
ITU: International Telecomm
unication Union (formerly CCITT)
(http://www.itu.ch)
