- Adaptive Multi-Rate (AMR) speech coding allows variable bit rates depending on channel conditions between 4.75 kbit/s and 12.2 kbit/s over full-rate channels and between 4.75 kbit/s and 7.95 kbit/s over half-rate channels. It uses source coding, channel coding, and rate adaptation based on channel estimation to optimize quality and efficiency. - AMR utilizes algebraic code excited linear prediction speech coding, unequal error protection, recursive systematic convolutional channel coding, and discontinuous transmission with voice activity detection and comfort noise generation. This allows it to save bandwidth during silence and adapt to changing channel conditions.

1. 8/5/2014 Department of electronics and
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2. Adaptive Multi Rate
Speech Coding
ABHIRAM.S
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3. What is speech signal?
• Speech is the primary method of human
communication. Or it is the vocal method
of communication.
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4. 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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5. 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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6. 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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7. 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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8. 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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9. 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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10. Basic concept of AMR
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11. • 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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12. 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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13. 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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14. 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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15. 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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16. 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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17. 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).
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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

18. Features
• It is the 4th speech compression algorithm
standardized by ETSI.
The first 3 are
i)Full Rate
ii)Half Rate
iii)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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19. 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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20. 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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21. • 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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22. 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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23. 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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24. Questions?
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25. Thank you
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