5 Voice codecs standardized in GSM:
- Full-Rate (FR) Codec
- Half-Rate (HR) Codec
- Enhanced Full-Rate (EFR) Codec
- Adaptive Multi-Rate (AMR) Codec
- Adaptive Multi-Rate Wideband(AMR-WB) Codec
All Voice codecs include speech coding (source coding), channel coding (error protection and bad frame detection), Voice Activity Detection (VAD), lost speech frame substitution and muting and comfort noise insertion.
Speech coding is the application of data compression of digital audio signals containing speech.
Channel Coding is Forward Error Checking and Bit Interleaving.
The voice codecs either operate in GSM full-rate TCH at the gross rate of 22.8 kbps (FR, EFR, AMR-WB) or in half-rate TCH at gross rate of 11.4 kbps (HR), or in both (AMR). Encoding process is performed on a 20 ms speech frame at a time.
FR operates on speech coding rate of 13 kbps and channel coding rate of 9.8 kbps. The coding scheme used is RPE-LTP (Regular Pulse Excitation with Long-Term Prediction).
HR operates on speech coding rate of 5.6 kbps and channel coding rate of 5.8 kbps. The coding scheme used is VSELP (Vector Sum Excited Linear Prediction). The VSELP algorithm is an analysis-by-synthesis coding technique and belongs to the class of speech coding algorithms known as CELP (Code Excited Linear Prediction).
EFR operates on speech coding rate of 12.2 kbps. The coding scheme used is ACELP (Algebraic Code Excited Linear Prediction).
AMR-HR operates on speech coding rate of one of 6 modes (4.75/5.15/5.9/6.7/7.4/7.95 kbps). The coding scheme used is ACELP (Algebraic Code Excited Linear Prediction).
AMR-FR operates on speech coding rate of one of 8 modes (4.75/5.15/5.9/6.7/7.4/7.95/10.2/12.2 kbps). The coding scheme used is ACELP (Algebraic Code Excited Linear Prediction).
AMR-WB operates on speech coding rate of one of 9 modes (6.6/8.85/12.65/14.25/15.85/18.25/19.85/23.05/23.85 kbps). The coding scheme is ACELP.
HR, FR, EFR and AMR operates on 3.4 kHz band (narrow band) and sampling rate of 8kHz (8000 samples/sec) while AMR-WB operates on 7 kHz band (wide band) and sampling rate of 16kHz (16000 samples/sec).
Brief advantages of different codecs
Speech coding is the application of data compression of digital audio signals containing speech.
Channel Coding is Forward Error Checking and Bit Interleaving.
The voice codecs either operate in GSM full-rate TCH at the gross rate of 22.8 kbps (FR, EFR, AMR-WB) or in half-rate TCH at gross rate of 11.4 kbps (HR), or in both (AMR). Encoding process is performed on a 20 ms speech frame at a time.
FR operates on speech coding rate of 13 kbps and channel coding rate of 9.8 kbps. The coding scheme used is RPE-LTP (Regular Pulse Excitation with Long-Term Prediction).
HR operates on speech coding rate of 5.6 kbps and channel coding rate of 5.8 kbps. The coding scheme used is VSELP (Vector Sum Excited Linear Prediction). The VSELP algorithm is an analysis-by-synthesis coding technique and belongs to the class of speech coding algorithms known as CELP (Code Excited Linear Prediction).
EFR operates on speech coding rate of 12.2 kbps. The coding scheme used is ACELP (Algebraic Code Excited Linear Prediction).
AMR-HR operates on speech coding rate of one of 6 modes (4.75/5.15/5.9/6.7/7.4/7.95 kbps). The coding scheme used is ACELP (Algebraic Code Excited Linear Prediction).
AMR-FR operates on speech coding rate of one of 8 modes (4.75/5.15/5.9/6.7/7.4/7.95/10.2/12.2 kbps). The coding scheme used is ACELP (Algebraic Code Excited Linear Prediction).
AMR-WB operates on speech coding rate of one of 9 modes (6.6/8.85/12.65/14.25/15.85/18.25/19.85/23.05/23.85 kbps). The coding scheme is ACELP.
HR, FR, EFR and AMR operates on 3.4 kHz band (narrow band) and sampling rate of 8kHz (8000 samples/sec) while AMR-WB operates on 7 kHz band (wide band) and sampling rate of 16kHz (16000 samples/sec).
Brief advantages of different codecs
- HR effectively doubles network capacity as compared to FR.
- EFR imporves speech quality and is higly robust to network impairments.
- AMR is specifically designed to improve link robustness. AMR supports dynamic adaptation to network conditions, using lower bit rates during network congestion or degradation while preserving audio quality. By trading off the speech bit rate to channel coding, AMR maximizes the likelihood of receiving the signal at the far end.
- AMR-WB provides excellent speech quality due to wider speech bandwidth.
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