3GPP AMR Audio Codec

The 3GPP AMR codec is a narrowband, low-bitrate speech codec specifically designed for low-latency voice communications such as Push-to-Talk (PoC) and Mission-Critical Push-to-Talk (MCPTT). It's not widely used in consumer apps, but important in specialized telecom and emergency communication systems.

The main differences between AMR-NB (Adaptive Multi-Rate Narrowband) and AMR-WB (Adaptive Multi-Rate Wideband) lie in audio bandwidth, quality, sampling rate, and use cases. Here's a breakdown:

Bandwidth and Audio Quality

• AMR-NB (Narrowband):

  • Optimized for speech in the 300 Hz to 3400 Hz range.
  • Suitable for traditional telephony.
  • Lower audio quality due to narrower frequency range.

• AMR-WB (Wideband):

  • Covers a broader range: 50 Hz to 7000 Hz.
  • Offers better voice clarity, especially for consonants and natural tone.
  • Used in HD Voice services.

Sampling Rate

• AMR-NB: 8 kHz (samples per second).
• AMR-WB: 16 kHz — doubles the temporal resolution, improving fidelity.

Bitrates

• AMR-NB: Supports 8 bitrates from 4.75 kbps to 12.2 kbps.
• AMR-WB: Supports 9 bitrates from 6.6 kbps to 23.85 kbps.

Codec Complexity

AMR-WB is more complex and requires more processing power and bandwidth, but yields significantly better sound quality.

Use Cases

Codec	Typical Use Cases
AMR-NB	2G/3G voice calls, legacy phones
AMR-WB	VoLTE, HD Voice, modern phones

Summary

Codec	PCM Bitrate	AMR Bitrate	Compression Ratio
AMR-NB	128 kbps	12.2 kbps	~10.5:1
AMR-WB	256 kbps	23.85 kbps	~10.7:1
AMR-WB	-	12.65 kbps	~20.2:1
AMR-NB	-	4.75 kbps	~27:1

Class Documention

I am providing a simple C++ API:

• AMRNB
• ARMWB

However, I recommend to use the high level Codec API from the Arduino Audio Tools project.

Installation Arduino

You can download the library as zip and call include Library -> zip library. Or you can git clone this project into the Arduino libraries folder e.g. with

cd  ~/Documents/Arduino/libraries
git clone https://github.com/pschatzmann/codec-amr.git

I recommend to use git because you can easily update to the latest version just by executing the git pull command in the project folder.

Licence

This code implements both an AMR-NarrowBand (AMR-NB) and an AMR-WideBand (AMR-WB) audio encoder/decoder through external reference code from http://www.3gpp.org/.

The license of the code from 3gpp is unclear!