System information
Codec Translators
The codec translators (Table 2-6) allow Asterisk to convert audio stream formats be-
tween calls. So if a call comes in on a PRI circuit (using G.711) and needs to be passed
out a compressed SIP channel (e.g., using G.729, one of many codecs that SIP can
handle), the relevant codec translator would perform the conversion.
†
If a codec (such as G.729) uses a complex encoding algorithm, heavy
use of transcoding can place a massive burden on the CPU. Specialized
hardware for the decoding/encoding of G.729 is available from hard-
ware manufacturers such as Sangoma and Digium (and likely others).
Table 2-6. Codec translators
Name Purpose Popularity/Status
codec_adpcm Adaptive Differential Pulse Coded Modulation codec Insignificant
codec_alaw A-law PCM codec used all over the world (except Canada/USA) on the PSTN Essential
codec_a_mu A-law to mu-law direct converter Useful
codec_dahdi Utilizes proprietary Digium hardware transcoding card Essential
a
codec_g722 Wideband audio codec Useful
codec_g726 Flavor of ADPCM Insignificant
codec_gsm Global System for Mobile Communications (GSM) codec Useful
codec_ilbc Internet Low Bitrate Codec Insignificant
codec_lpc10 Linear Predictive Coding vocoder (extremely low bandwidth) Insignificant
codec_resample Resamples between 8-bit and 16-bit signed linear Usable
codec_speex Speex codec Usable
codec_ulaw Mu-law PCM codec used in Canada/USA on PSTN Essential
a
If you are using a Digium codec transcoder card.
Format Interpreters
Format interpreters (Table 2-7) perform the function of codec translators, but they do
their work on files rather than channels. If you have a recording on a menu that has
been stored as GSM, a format interpreter would need to be used to play that recording
to any channels not using the GSM codec.
‡
† More information about what codecs are and how they work is available in “Codecs” on page 625.
‡ It is partly for this reason that we do not recommend the default GSM format for system recordings. WAV
recordings will sound better and use less CPU.
18 | Chapter 2: Asterisk Architecture