Asterisk: The Future of Telephony

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. ...