04/08/2023 Department of electronics and communication

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WELCOME

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Adaptive Multi Rate Speech Coding

ABHIRAM.S

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What is speech signal?

• Speech is the primary method of human communication. Or it is the vocal method of communication.

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What is speech coding?

• Speech coding means the conversion of speech signal which already has been digitalized, into another digital signal featuring a lower bit rate than original.

• To transmit/store a speech waveform using as few bits as possible while retaining high quality

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Why coding necessary • Speech coding systems is to transmit

speech with the highest possible quality using the least possible channel capacity.

• To save bandwidth in telecoms applications and to reduce memory storage requirements.

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Limitations of fixed coding rate

• If the source coding and channel coding rates are independent of channel quality; fading occur

• Unnecessary coding for a good channel, wastage of channel

• Information loss due to bad channel, fading

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Why Multi Rate Coding?

• Multi rate coding helps to provide different coding rate for different bits.

• Signal can be coded by different channel coding rate according to their significance.

• Audio data compression can be done by reducing bit rate.

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Rate adaptation in speech coding

• The source/channel coding rate can be adapted according to some conditions.

• Adaptation provides better flexibility of channel

• Improved utilization of channels• Channel/Source/Receiver condition are

the main factors affecting rate adaptation.

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Adaptive multi rate coding of speech

• The ratio between source bit rate and error protecting redundancy is adapted according to channel conditions.

• The AMR concept solves the source channel rate allocation problem in a more intelligent way.

• That is when the channel is bad, source encoder operate at low bit rate. When the channel is good, highest rate of speech encoding is used.

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Basic concept of AMR

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• a speech codec with variable bit rate (SP-Enc,

• SP-Dec),• a channel codec with variable error

protection rate, matching to the bit rate of the speech codec (CH-Enc, CH-Dec),

• a Channel estimation entity (CH-Est),• a control unit for the rate adaptation.

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Source coding

• Information source is coded to reduce the bit rates.

• That is to reduce the redundancy of message

• According to channel conditions there are 14 modes of operations with different bit rate.

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Channel coding of source encoded bit

• Unequal error protection(UEP)• Recursive systematic convolutional

codes(RSCC)• Transmission of mode bits• Decoding an RSC code with a standard

NSC decoder.

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Unequal error protection(UEP)

• Information is classified based on their sensitivity and protected.

• More error protection is provided for most significant bit.I.e.; Redundancy for MSB is greater than that of LSB

• A high SNR is provided for most significant bit

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Recursive systematic convolutional codes (RSCC)

• BER is lower than Non Systematic Convolutional Codes (NSCC)

• SNR is higher than NSCC• Systematic bits can be collected before

channel decoding

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Transmission of mode bits

• Mode bits are the most significant bits.• Error detection techniques such as ‘cyclic

redundancy check’ , ‘quality estimation’ are used to detect error in mode bit.

• Mode bits are assigned at the beginning of a block of codes.

• Many sub classes are provided.

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There are a total of 14 modes of the AMR codec, 8 are available in a full rate channel (FR) and 6 on a half rate channel (HR).

Mode Bitrate (kbit/s) Channel

AMR_12.20 12.20 FR

AMR_10.20 10.20 FR

AMR_7.95 7.95 FR/HR

AMR_7.40 7.40 FR/HR

AMR_6.70 6.70 FR/HR

AMR_5.90 5.90 FR/HR

AMR_5.15 5.15 FR/HR

AMR_4.75 4.75 FR/HR

AMR_SID 1.80 FR/HR

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Features• It is the 4th speech compression algorithm

standardized by ETSI.The first 3 are i)Full Rateii)Half Rateiii)Enhanced Full Rate

• The AMR codec uses eight source coders with different bit-rates

• AMR is a hybrid speech coder which uses Algebraic Code Excited Linear Prediction (ACELP)

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Cntnd…

• Sampling frequency 8 kHz or 160 samples for 20 ms frames

• The AMR codec uses eight source codecs with bit-rates of 12.2 kbit/s, 10.2 kbit/s, 7.95 kbit/s, 7.40 kbit/s, 6.70 kbit/s, 5.90 kbit/s, 5.15 kbit/s, 4.75 kbit/s.

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It Generates

frame length of bit rates

95bits 4.75bits

103bits 5.15bits

118bits 5.90bits

134bits 6.70bits

148bits 7.40bits

159bits 7.95 bits

204bits 10.2bits

244bits 12.2bits

Respectively.

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• AMR utilizes Discontinuous transmission (DTX), with Voice Activity Detection (VAD) and Comfort Noise Generation (CNG) to reduce bandwidth usage during silence periods.

• The complexity of the algorithm is rated at 5 based on the standard scale

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Applications

• In GSM mobile communication• In 3G mobile systems.• Low bit rate high quality speech

communication systems.• Digital radio systems• Used in UTMS

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Reference

• A. Antoniou: Digital Filters, Analysis, Design and Applications, 2nd edition, New York: McGraw-Hill, 1993

• T. Fingscheidt, 0. Scheufen, “Robust GSM Speech Decoding• Using the Channel Decoders Soft Output,” in Proc. Of

EUROSPEECH’97, Rhodos, Greece, pp. 1315-1318, Sept.1997

• S. Heinen, M. Adrat, 0. Steil, P. Vary, W.Xu, “A 6.1 to 13.3 kb/s variable rate CELP codec (VR-VCELP) for

• AMR speech coding,” in Proc. of ICASSP’99, Phoenix, Arizona, vol. 1, pp. 9-12, March 1999

• J. Hagenauer, “Rate-compatible punctured convolutional codes (RCPC codes) and their applications,” in IEEE

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Questions?

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Thank you