Adaptive Multirate (AMR)

Adaptive Multi-Rate is a codec based on an audio compression format which is used for speech coding. It is considered a hybrid of speech codes and is capable of transmitting both waveform signals and speech parameters. For most third-generation (3G) cellular systems it is a mandatory speech codec and is a preferred codec for the Global System for Mobile Communications (GSM) standard. Adaptive Multi-Rate provides superior audio performance, better coverage and quality, and is easier to implement than previous formats.

There are two different versions of AMR. The basic version, also called AMR-Narrowband (AMR-NB), which is mainly intended for use by GSM and AMR-Wideband (AMR-WB), which is mainly intended for use by UMTS. Wideband AMR uses a speech bandwidth of 50 - 7000 Hz, whereas the bandwidth of narrowband AMR is 300 - 3400 Hz. This gives wideband AMR a more natural speech quality.

• AMR-Narrowband: The narrowband AMR speech codec is actually a combination of eight speech codecs with bit rates of 12.2, 10.2, 7.95, 7.4, 6.7, 5.9, 5.15 and 4.75 kbps. The Narrowband AMR speech codec utilises the algebraic code excitation linear prediction (ACELP) algorithm that is employed also in GSM EFR and D-AMPS EFR codecs. The six lowest bit rates can be used in the half-rate channel mode. The speech codec modes perform also error correction and bad-frame detection. This is done by a combination of convolutional coding for the error correction and a cyclic redundancy check (CRC) for the detection of bad frames.
• AMR-Wideband: AMR-Wideband has nine different codec rates, ranging from 6.60 to 23.85 kbps. The codec rates from 6.60 to 19.85 kbps can be supported by GSM as well. AMR-Wideband uses the same algorithm as AMR-Narrowband.

Adaptive multirate (AMR) - Features

• Sampling frequency 8 kHz/13-bit (160 samples for 20 ms frames), filtered to 200–3400 Hz.
• The AMR codec uses eight source codecs with bit-rates of 12.2, 10.2, 7.95, 7.40, 6.70, 5.90, 5.15 and 4.75 kbit/s.
• Generates frame length of 95, 103, 118, 134, 148, 159, 204, or 244 bits for AMR FR bit rates 4.75, 5.15, 5.90, 6.70, 7.40, 7.95, 10.2, or 12.2 kbit/s, respectively. AMR HR frame lengths are different.
• AMR utilizes Discontinuous Transmission (DTX), with Voice Activity Detection (VAD) and Comfort Noise Generation (CNG) to reduce bandwidth usage during silence periods
• Algorithmic delay is 20 ms per frame. For bit-rates of 12.2, there is no "algorithm" look-ahead delay. For other rates, look-ahead delay is 5 ms. Note that there is 5 ms "dummy" look-ahead delay, to allow seamless frame-wise mode switching with the rest of rates.
• AMR is a hybrid speech coder, and as such transmits both speech parameters and a waveform signal
  • Linear predictive coding (LPC) is used to synthesize the speech from a residual waveform. The LPC parameters are encoded as Line spectral pairs (LSP).
  • The residual waveform is coded using Algebraic Code Excited Linear Prediction (ACELP).
• The complexity of the algorithm is rated at 5, using a relative scale where G.711 is 1 and G.729a is 15.
• PSQM testing under ideal conditions yields Mean Opinion Scores of 4.14 for AMR (12.2 kbit/s), compared to 4.45 for G.711 (µ-law)
• PSQM testing under network stress yields Mean Opinion Scores of 3.79 for AMR (12.2 kbit/s), compared to 4.13 for G.711 (µ-law)

Adaptive multirate (AMR) - Applications

• Digital telephony – GSM and 2.5G/3G circuit switched telephony, VoIP, Wi-Fi, satellite telephony, Push to talk over Cellular, video telephony
• Multimedia – Audio and videoconferencing (H.324 video), chat and virtual reality immersion environments, streaming, content downloads, Multimedia Messaging Services, digital radio broadcasting, audio books, real-time collaboration tools, PDA/handset-hosted applications, ringtones, content creation tools, multimedia transcoders, multimedia gateways, interactive gaming
• Voicemail/Unified Messaging
• Network analysis and simulation tools
• Test and measurement equipment
• Static answering machines
• Media players
• Media servers
• Media gateways
• Content creation tools
• Consumer electronics
• Toys

External References

• The Adaptive Multirate Speech Decoder E. Ekudden, R. Hagen, I. Johansson, and J. Svedberg
• Universal Mobile Telecommunications System (UMTS); AMR speech Codec; General description Etsi
• Standardization of the Adaptive Multirate Codec Kari Järvinen