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Asterisk: The Future of Telephony by Leif Madsen, Jared Smith, Jim Van Meggelen

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Codecs

Codecs are generally understood to be various mathematical models used to digitally encode (and compress) analog audio information. Many of these models take into account the human brain’s ability to form an impression from incomplete information. We’ve all seen optical illusions; likewise, voice-compression algorithms take advantage of our tendency to interpret what we believe we should hear, rather than what we actually hear.[60] The purpose of the various encoding algorithms is to strike a balance between efficiency and quality.[61]

Originally, the term CODEC referred to a COder/DECoder: a device that converts between analog and digital. Now, the term seems to relate more to COmpression/DECompression.

Before we dig into the individual codecs, take a look at Table 8-1—it’s a quick reference that you may want to refer back to.

Table 8-1. Codec quick reference

Codec

Data bitrate (kbps)

Licence required?

G.711

64 kbps

No

G.726

16, 24, or 32 kbps

No

G.723.1

5.3 or 6.3 kbps

Yes (no for passthrough)

G.729A

8 kbps

Yes (no for passthrough)

GSM

13 kbps

No

iLBC

13.3 kbps (30-ms frames) or 15.2 kbps (20-ms frames)

No

Speex

Variable (between 2.15 and 22.4 kbps)

No

G.711

G.711 is the fundamental codec of the PSTN. In fact, if someone refers to PCM (discussed in the previous chapter) with respect to a telephone network, you are allowed to think of G.711. Two companding methods are used: μ-law in North America and A-law in the rest of the world. Either one delivers an 8-bit word transmitted 8,000 times per second. ...

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