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AMR-NB (Adaptive Multi-Rate Narrowband)

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AMR-NB (Adaptive Multi-Rate Narrowband) is a type of audio codec that is designed to compress audio signals for transmission over digital networks. It is widely used in mobile phones and other portable devices for voice communication. In this article, we will discuss the various aspects of AMR-NB, including its features, working principle, and applications.

Features of AMR-NB

AMR-NB is a speech codec that is designed to operate at bitrates ranging from 4.75 kbps to 12.2 kbps. The codec uses a variable bitrate (VBR) scheme, which means that the bitrate can vary depending on the complexity of the audio signal. The codec supports eight different bitrates, which are selected dynamically based on the current conditions of the communication channel. The bitrates supported by AMR-NB are:

  • 4.75 kbps
  • 5.15 kbps
  • 5.9 kbps
  • 6.7 kbps
  • 7.4 kbps
  • 7.95 kbps
  • 10.2 kbps
  • 12.2 kbps

AMR-NB uses a complex algorithm to compress audio signals. The algorithm is based on the concept of parametric coding, which involves the extraction of the most important parameters of the audio signal and their subsequent transmission over the communication channel. The codec uses several different techniques to compress the audio signal, including:

  • Speech model-based coding: AMR-NB uses a model-based approach to speech coding, where the properties of the human vocal tract are modeled and used to compress the audio signal.
  • Variable bitrate encoding: AMR-NB uses a VBR scheme, where the bitrate of the codec is adapted dynamically based on the current conditions of the communication channel. This allows for better quality audio transmission under varying network conditions.
  • Error concealment: AMR-NB uses a sophisticated error concealment mechanism that allows the codec to recover from transmission errors in the received signal.

Working principle of AMR-NB

The working principle of AMR-NB involves several different steps. These steps are:

  • Analysis: The input speech signal is analyzed using a filter bank that splits the signal into several frequency bands. The filter bank used in AMR-NB consists of 10 sub-bands that cover a frequency range of 300 Hz to 3400 Hz.
  • Pitch analysis: The pitch of the input speech signal is analyzed to determine the fundamental frequency of the speech waveform. The pitch analysis is used to extract the pitch period and pitch gain of the signal.
  • Encoding: The most important parameters of the speech signal are encoded using a combination of various techniques such as waveform coding, algebraic codebook coding, and fixed codebook coding. The resulting encoded bitstream is transmitted over the communication channel.
  • Decoding: The received bitstream is decoded at the receiver end using the inverse operations of the encoding process. The decoder uses the received parameters to reconstruct the speech signal.
  • Post-processing: The decoded speech signal is subjected to several post-processing operations to improve the quality of the audio signal. These operations include noise reduction, spectral smoothing, and synthesis filtering.

Applications of AMR-NB

AMR-NB is widely used in mobile phones and other portable devices for voice communication. The codec is used in various communication technologies such as GSM (Global System for Mobile Communications), CDMA (Code Division Multiple Access), and 3G (Third Generation) networks. The codec is also used in applications such as video conferencing, voice over IP (VoIP), and multimedia messaging services (MMS).

Advantages of AMR-NB

AMR-NB offers several advantages over other speech codecs. These advantages include:

  • Low bitrate: AMR-NB operates at a low bitrate, which makes it ideal for use in low-bandwidth networks. The low bitrate also reduces the amount of data that needs to be transmitted, which reduces the overall cost of communication.
  • High compression efficiency: AMR-NB uses several different techniques to compress the audio signal, which results in high compression efficiency. This allows for better quality audio transmission over low-bandwidth networks.
  • Adaptive bitrate: AMR-NB uses a VBR scheme that allows the codec to adapt the bitrate dynamically based on the current conditions of the communication channel. This results in better quality audio transmission under varying network conditions.
  • Error resilience: AMR-NB uses a sophisticated error concealment mechanism that allows the codec to recover from transmission errors in the received signal. This improves the overall reliability of the communication system.

Disadvantages of AMR-NB

Despite its many advantages, AMR-NB also has some disadvantages. These include:

  • Limited frequency range: AMR-NB is designed to operate in a narrowband frequency range of 300 Hz to 3400 Hz. This limited frequency range means that the codec is not suitable for transmitting high-fidelity audio signals.
  • Limited dynamic range: AMR-NB is designed to compress speech signals, which have a relatively limited dynamic range. The codec may not perform as well when compressing other types of audio signals that have a wider dynamic range.
  • Delay: AMR-NB introduces some delay in the audio signal due to the encoding and decoding process. This delay can be noticeable in certain applications such as real-time communication.

Conclusion

AMR-NB is a widely used speech codec that is designed to compress audio signals for transmission over digital networks. The codec uses a variable bitrate scheme and a sophisticated algorithm to compress the audio signal. AMR-NB is widely used in mobile phones and other portable devices for voice communication, and it is also used in various other applications such as video conferencing and voice over IP. While AMR-NB offers several advantages over other speech codecs, it also has some disadvantages, such as a limited frequency range and a noticeable delay in the audio signal.