Speech /Audio Coding Standard LPC-10

By

Sonawane Swapnil R

511006

Sub.: Speech

DEP-E&TC

VIIT Pune

1

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.

2

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

3

LPC – 10/ FS-1015 :-

• BRIEF HISTORY :-

– Is a secure telephony speech encoding standard developed bythe United States Department of Defense and later by NATO. Thestandard was finished 1984.

– Algorithm uses linear predictive coding vocoder.

– The vocoder enables understandable speech, but the quality isvery unnatural and synthetic.

4

PROPERTIES :-

• 10 LP(linear predictor) coefficients are used.

• Bandwidth: 2.4kbps

• Samples/frame : 180 samples

• Bits/frame: 54 bits

• Frame Size: 22.5ms = 44.44 frames/sec

5

ANALYSIS :-

6

Conti..

• Analysis process extract from the speech signal the parametersrequired 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 itsheight. This period, called as pitch .

7

1 Voiced segment

0 Unvoiced segment

Conti..

• The result of this analysis is a set of ten reflection coefficients(hence the name LPC 10) which sufficiently & faithfully describe thecross-sectional variations in the vocal tract.

• Finally, for each frame, the level of the speech signal is evaluated inorder to control the gain of the synthesizer on the synthesis side.

8

SYNTHESIS:-

9

Conti..

• The algorithms employed to synthesize the speech signal reflect theassumed 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 voicedsounds;

– a switch allowing selection of either generator according to the type ofspeech signal to be produced in the current frame;

10

Conti..

– a filter of order 10, which filters the excitation selected; it is at thislevel that the distinction between the different vowels and thedifferent consonants is made;

– a gain control system, which gives the synthetic signal the rightvolume;

– optionally, a “post-filtering” system, designed to mask certainimperfections in the synthesizer and to make the synthesized signalmore pleasant to the human ear.

11

VOCODER AT 2,400 BIT/S:-

12

LP Coefficients Pitch Voicing Energy

0 41 48 53

- The remaining 1 bit is for synchronization

SPEECH CODER COMPARISON:-

13

APPLICATIONS

• Digital telephony

• Satellite bradcasting

• Radio communications with secure voice transmissions

14

THANKU

15