Speech coding std
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Speech coding is used to efficiently transmit speech through digital channels by retaining only the information useful to listeners. The LPC-10 standard uses linear predictive coding with 10 coefficients to analyze and synthesize speech. During analysis, it extracts parameters like voicing and pitch from the speech signal. The synthesis process uses these parameters to generate noise or periodic excitation, apply an LPC filter, and control gain. LPC-10 transmits speech at 2.4kbps by coding the 10 LPC coefficients, pitch, voicing, and energy into 54 bits per frame. It enables understandable but unnatural sounding speech and is used for secure voice transmissions.
Speech coding std
- 1. Speech /Audio Coding Standard LPC-10 By Sonawane Swapnil R 511006 Sub.: Speech DEP-E&TC VIIT Pune 1
- 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. 2
- 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 3
- 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. 4
- 5. 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
- 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 . 7 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. 8
- 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; 10
- 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. 11
- 12. VOCODER AT 2,400 BIT/S:- 12 LP Coefficients Pitch Voicing Energy 0 41 48 53 - The remaining 1 bit is for synchronization
- 14. APPLICATIONS • Digital telephony • Satellite bradcasting • Radio communications with secure voice transmissions 14
- 15. THANKU 15
Editor's Notes
- #4: Waveform Compression Coding ,, Parametric Compression Coding ,, Hybrid Compression Coding—Analysis-by-Synthesis
- #5: 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.
- #14: 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)