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multi rate speech coding used to reduce the size of high quality speech
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04/08/2023 Department of electronics and communication
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WELCOME
04/08/2023 Department of electronics and communication
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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