Adaptive multi-rate compression

From Wikipedia, the free encyclopedia

(Redirected from Adaptive
Multi-Rate)

Jump to: navigation,
search

Adaptive Multi-Rate Narrow
Band (AMR-NB)

File
extension:

.amr

MIME type:

audio/amr

Type of format:

Audio

Adaptive Multi-Rate (AMR) is an audio data
compression scheme optimized for speech coding. AMR was
adopted as the standard speech codec by 3GPP in October 1998 and is now
widely used in GSM. It
uses link adaptation to
select from one of eight different bit rates based on link conditions.

AMR is also a file format for storing spoken audio using the AMR codec. Many
modern mobile telephone handsets will allow you to store short recordings in the
AMR format, and some commercial programs exist to convert between this and other
formats such as MP3,
although it should be remembered that AMR is a speech format and is unlikely to
give ideal results for other audio. The common filename extension is
.amr.

Contents

1 Usage

2 Features

3 Licensing and patent
issues

4 See also

5 External
references

[edit]
Usage

The frames contain 160 samples and are 20 milliseconds long. AMR uses
different techniques, such as Algebraic
Code Excited Linear Prediction (ACELP), Discontinuous
Transmission (DTX), Voice activity
detection (VAD) and Comfort Noise Generation
(CNG). The usage of AMR requires optimized link adaptation that selects the best
codec mode to meet the local radio channel and capacity requirements. If the
radio conditions are bad, source coding is reduced
and channel coding is
increased. This improves the quality and robustness of the network connection
while sacrificing some voice clarity. In the particular case of AMR this
improvement is somewhere around S/N = 4-6 dB for usable communication. The new
intelligent system allows the network operator to prioritize capacity or quality
per base station.

There are a total of 14 modes of the AMR codec, 8 are available in a full
rate channel and 6 on a half rate channel.

Mode

Bit-rate

Channel

Compatible with

AMR_12.20

12.20 kbit/s

FR

ETSI GSM Enhanced
Full Rate

AMR_10.20

10.20 kbit/s

FR

AMR_7.95

7.95 kbit/s

FR/HR

AMR_7.40

7.40 kbit/s

FR/HR

TIA/EIA
IS-641 TDMA Enhanced Full Rate

AMR_6.70

6.70 kbit/s

FR/HR

ARIB 6.7
kbit/s Enhanced Full Rate

AMR_5.90

5.90 kbit/s

FR/HR

AMR_5.15

5.15 kbit/s

FR/HR

AMR_4.75

4.75 kbit/s

FR/HR

AMR_SID

1.80 kbit/s

FR/HR

[edit]
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 bit rates 4.75, 5.15, 5.90, 6.70, 7.40, 7.95, 10.2, or 12.2 kbit/s,
respectively

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 which uses 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 (u-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 (u-law)

[edit]
Licensing and patent issues

AMR codecs incorporate several patents. For more information about this,
please refer to:

VoiceAge licensing
information, including pricing to license the AMR codecs

3GPP legal issues

The 3G Patent
Platform and its licensing policy

AMR Codecs as Shared
Libraries - amrnb and amrwb libraries development site. These libraries
are based on the reference implementation and were created to prevent ugly
embedding of possibly patented source code into many Open Source projects.

[edit]
See also

AMR-WB

AMR-WB+

Half
Rate

Full
Rate

Sampling rate

IS-641

[edit]
External references

3G TS 26.071 - Mandatory Speech Codec speech processing functions; AMR
Speech Codec; General Description

Retrieved from "http://en.wikipedia.org/wiki/Adaptive_multi-rate_compression"

Categories: Sound
technology | Speech
codecs | Audio
engineering

Views

Article

Discussion

Edit
this page

History

Personal tools

Sign
in / create account

Navigation

Main
page

Contents

Featured
content

Current events

Random article

interaction

About Wikipedia

Community
portal

Recent
changes

Contact Wikipedia

Donate to Wikipedia

Help

Search

Toolbox

What
links here

Related
changes

Upload file

Special pages

Printable
version

Permanent
link

Cite
this article

In other languages

Česky

Deutsch

Español

Français

日本語

Polski

Русский

Suomi

Türkçe

This page was last modified 01:42, 27 August 2007.

All text is available under the terms of the GNU
Free Documentation License. (See Copyrights
for details.)
Wikipedia® is a registered trademark of the Wikimedia Foundation, Inc., a
U.S. registered 501(c)(3) tax-deductible
nonprofit charity.

Privacy policy

About Wikipedia

Disclaimers