.amr. There also exists another storage format for AMR that is suitable for applications with more advanced demands on the storage format, like random access or synchronization with video. This format is the 3GPP-specified 3GP container format (digital), container format based on ISO base media file format.
Usage
The frames contain 160 samples and are 20 milliseconds long. AMR uses various techniques, such as algebraic code-excited linear prediction, ACELP, discontinuous transmission, DTX, voice activity detection, VAD and comfort noise, 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, eight are available in a Full Rate, full rate channel (FR) and six on a Half Rate, half rate channel (HR).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, 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)Licensing and patent issues
AMR codecs incorporate several patents of Nokia, Ericsson, Nippon Telegraph and Telephone, NTT and VoiceAge, the last one being the ''License Administrator'' for the AMR patent pools. VoiceAge also accepts submission of patents for determination of their possible essentiality to these standards. However, it's very difficult to determine if there were actually any patents in existence for the so-called inventions related to AMR/AMR-WB codecs, since inventors (and their lawyers) do everything they can to hide patents related to AMR/AMR-WB technology. Apparently, all these patents are hidden from all other researches and general audience that could perhaps spot prior art in the claimed "inventions" patented by the patent holders of the AMR/AMR-WB codecs. The initial fee for professional content creation tools and "real-time channel" products is US$6,500. The minimum annual royalty is $10,000, which, in the first year, excludes the initial fee. Per-channel license fees fall from $0.99 to $0.50 with volume, up to a maximum of $2 million annually. In the category of personal computer products, e.g., media players, the AMR decoder is licensed for free. The license fee for a sold encoder falls from $0.40 to $0.30 with volume, up to a maximum of $300,000 annually. The minimum annual royalty is not applied to licensed products that fall under the category of personal computer products and use only the free decoder. More information:Software support
* 3GPP TS 26.073AMR speech Codec (C source code)reference implementation * Audacity (audio editor), Audacity (beta version 1.3) via the FFmpeg integration librariesRetrieved on 2010-02-28 (both input and output format) * FFmpeg with OpenCORE AMR librariesFFmpeg General Documentation - AMR external librarySee also
* Adaptive Multi-Rate Wideband (AMR-WB) * Extended Adaptive Multi-Rate – Wideband (AMR-WB+) * Half Rate * Full Rate * Enhanced Full Rate (EFR) * Sampling rate * IS-641 * 3GP * Comparison of audio coding formats * RTP audio video profileReferences
External links