AMR Full Form ( Adaptive Multi-Rate Codec)

Definition and Explanation of Adaptive Multi-Rate (AMR)

AMR (Adaptive Multi-Rate) is a speech coding algorithm used in digital communication systems to compress speech audio signals, primarily in mobile networks. The codec was developed by Ericsson and is widely used in mobile communication systems worldwide.

AMR adapts the bit rate and codec complexity to match the characteristics of the speech signal and available network resources, ensuring high-quality speech transmission while minimizing bandwidth usage. It uses variable bit-rate coding to compress audio signals, allowing for more efficient use of network bandwidth and storage resources.

The codec supports eight different bit rates ranging from 4.75 kbps to 12.2 kbps, with the bit rate changing dynamically based on the signal quality and network conditions. This makes it ideal for mobile networks, where the signal quality and available bandwidth can vary widely. AMR provides high-quality speech transmission with low bandwidth requirements, making it a popular choice for mobile communication systems.

History and Evolution of AMR

The Adaptive Multi-Rate (AMR) codec was first introduced in 1999 by Ericsson as an extension to the previous Full Rate (FR) codec, which was used in Global System for Mobile Communications (GSM) networks.

The AMR codec evolved over time, with improvements made to its compression algorithms and bit rate capabilities. In 2003, AMR-WB (Adaptive Multi-Rate Wideband) was introduced, which expanded the codec's bit rate range to 6.60 kbps to 23.85 kbps and increased the speech bandwidth from 200 Hz to 7000 Hz. This improvement enabled higher-quality voice calls, even in noisy environments, making it popular for voice over IP (VoIP) and video conferencing applications.

In 2007, AMR-WB+ (Adaptive Multi-Rate Wideband Plus) was introduced as an enhancement to AMR-WB, providing even higher speech quality at lower bit rates. It introduced new features such as packet loss concealment and stereo coding, making it more suitable for multimedia applications.

In 2011, AMR-WB+ was standardized by the Third Generation Partnership Project (3GPP) as the Enhanced Voice Services (EVS) codec, which further improved the speech quality and compression efficiency of the codec.

Working Principle of AMR

The Adaptive Multi-Rate (AMR) codec uses a variable bit-rate coding scheme to compress audio signals efficiently while ensuring high-quality speech transmission. The codec works by analyzing the characteristics of the speech signal and adapting the bit rate and codec complexity to match the signal quality and available network resources.

The AMR codec uses a codebook-based encoding approach, where it first divides the speech signal into small segments called frames. Each frame is then analyzed to determine its spectral and pitch characteristics, and the codec selects the best codebook entry to represent the frame based on its characteristics.

The codec then encodes the selected codebook entry using a variable bit-rate coding scheme, where the bit rate is adapted dynamically based on the signal quality and network conditions. The codec uses a lower bit rate for frames with less complex spectral and pitch characteristics and a higher bit rate for frames with more complex characteristics.

This variable bit-rate coding scheme allows the codec to efficiently compress the speech signal while maintaining high-quality speech transmission. The AMR codec supports eight different bit rates ranging from 4.75 kbps to 12.2 kbps, with the bit rate changing dynamically based on the signal quality and network conditions.

Applications of AMR

Some of the applications of AMR are −

• Voice calls − AMR is widely used in mobile networks for voice calls, as it provides high-quality speech transmission with low bandwidth requirements.

• Voice over IP (VoIP) − AMR is used in VoIP applications to compress audio signals efficiently while maintaining high-quality speech transmission. It is compatible with many VoIP systems and provides clear audio with low latency.

• Video conferencing − AMR is used in video conferencing applications to compress audio signals for transmission over the internet. It provides clear audio with low bandwidth requirements.

• Voice recording − AMR is used in voice recording applications, as it provides high-quality audio compression with low storage requirements. It is commonly used in mobile devices for recording voice memos and other audio content.

• Mobile multimedia − AMR is used in mobile multimedia applications, such as music players and video players, to provide high-quality audio with low bandwidth requirements.

Conclusion

AMR, a speech coding method, is frequently used in mobile networks and other digital communication systems to compress audio signals while preserving high-quality voice transmission. Its variable bit-rate coding scheme and codebook-based encoding approach allow for efficient compression and bandwidth usage, making it an ideal codec for mobile communication systems. With its continued evolution and standardization, AMR has become a reliable and efficient codec for various applications, including voice calls, VoIP, video conferencing, voice recording, and mobile multimedia.

FAQs

Q1. How is AMR used in GSM?

AMR (Adaptive Multi-Rate) is the standard codec used for voice communication in GSM (Global System for Mobile Communications) networks.

In GSM networks, AMR operates at bit rates ranging from 4.75 kbps to 12.2 kbps, depending on the signal quality and available bandwidth. The codec uses a variable bit-rate coding scheme to adapt to changing network conditions, allowing it to provide high-quality speech transmission even in low-bandwidth environments.

During a call on a GSM network, the AMR codec continuously monitors the signal quality and adjusts the bit rate accordingly. If the signal quality is poor, the codec will use a lower bit rate to maintain the call quality, and if the signal quality improves, it will increase the bit rate to provide better call quality.

Q2. Is AMR used in other networks besides GSM?

While AMR was initially developed for use in GSM (Global System for Mobile Communications) networks, it is now used in various other mobile networks, including WCDMA (Wideband Code Division Multiple Access), CDMA 2000 (Code Division Multiple Access 2000), and LTE (Long-Term Evolution) networks. It is also used in various applications outside of mobile networks, such as voice over IP (VoIP) and video conferencing.

Q3. What is the impact of AMR on call quality?

AMR is designed to provide high-quality speech transmission with low bandwidth requirements, making it ideal for mobile networks where bandwidth is limited. The codec uses a variable bit-rate coding scheme to adapt to the available network resources and signal quality, ensuring high-quality speech transmission even in noisy environments. Overall, AMR has a positive impact on call quality, providing clear and reliable voice communication for mobile users.