Modern Speech Compression
Much of the later work in speech compression was motivated by military research into digital communications for secure military radios, where very low data rates were required to allow effective operation in a hostile radio environment. At the same time, far more processing power was available, in the form of VLSI integrated circuits, than was available for earlier compression techniques. As a result, modern speech compression algorithms could use far more complex techniques than were available in the 1960s to achieve far higher compression ratios.
These techniques were available through the open research literature to be used for civilian applications, allowing the creation of digital mobile phone networks with substantially higher channel capacities than the analog systems that preceded them.
The most common speech coding scheme is Code Excited Linear Prediction (CELP) coding, which is used for example in the GSM standard. In CELP, the modelling is divided in two stages, a linear predictive stage that models the spectral envelope and code-book based model of the residual of the linear predictive model.
In addition to the actual speech coding of the signal, it is often necessary to use channel coding for transmission, to avoid losses due to transmission errors. Usually, speech coding and channel coding methods have to be chosen in pairs, with the more important bits in the speech data stream protected by more robust channel coding, in order to get the best overall coding results.
The Speex project is an attempt to create a free software speech coder, unencumbered by patent restrictions.
Codec2 is another free software speech coder, unencumbered by patent restrictions, which manages to achieve very good compression, as low as 1200 bps.
Major subfields:
- Wide-band speech coding
- AMR-WB for WCDMA networks
- VMR-WB for CDMA2000 networks
- G.722, G.722.1, Speex, IP-MR and others for VoIP and videoconferencing
- Narrow-band speech coding
- FNBDT for military applications
- SMV for CDMA networks
- Full Rate, Half Rate, EFR, AMR for GSM networks
- G.723.1, G.726, G.728, G.729, iLBC and others for VoIP or videoconferencing
Read more about this topic: Speech Coding
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