Codecs are commonly comprehended to be different numerical models used to carefully encode (and pack) simple sound data. A large number of these models consider the human mind's capacity to shape an impression from deficient data. We've all observed optical dreams; in like manner, voice-compression calculations exploit our propensity to decipher what we accept we ought to hear, instead of what we really hear.117 The reason for the different encoding calculations is to find some kind of harmony among productivity and quality. On a sound CD, quality is unmistakably more significant than data transfer capacity, so the sound is quantized at 16 bits (times 2, as it's sound system), with a testing pace of 44,100 Hz. Taking into account that the CD was imagined in the late 1970s, this was very great stuff. The phone organize doesn't require this degree of value (and needs to streamline data transmission), so phone signals are encoded utilizing 8 bits, at an examining recurrence of 8,000 Hz.
Initially, the term CODEC alluded to a COder/DECoder: a gadget that changes over among simple and advanced. Presently, the term appears to relate more to COmpression/DEComprension.
G.711 is the key codec of the PSTN. Truth be told, in the event that somebody alludes to PCM as for a phone arrange, you are permitted to consider G.711. Two companding techniques are used: m-law in North America and A-law in the remainder of the world. It is possible that one conveys a 8-piece word transmitted 8,000 times each second. In the event that you crunch the numbers, you will see this requires 64,000 bits to be transmitted every second.
Numerous individuals will reveal to you that G.711 is an uncompressed codec. This isn't actually valid, as companding is viewed as a type of compression. What is genuine is that G.711 is the base codec from which the entirety of the others are determined.
This codec has been around for quite a while (it used to be G.721, which is presently out of date), and it is one of the first packed codecs. It is otherwise called Adaptive Differential Pulse-Code Modulation (ADPCM), and it can run at a few bitrates. The most widely recognized rates are 16 kbps, 24 kbps, and 32 kbps. As of this composition, Asterisk at present backings just the ADPCM-32 rate, which is by a wide margin the most famous rate for this codec.
G.726 offers quality almost indistinguishable from G.711, yet it uses just a large portion of the data transfer capacity. This is conceivable because as opposed to sending the aftereffect of the quantization estimation, it sends just enough data to depict the contrast between the present example and the past one. G.726 tumbled from favor during the 1990s because of its powerlessness to convey modem and fax signals, but since of its transmission capacity/CPU execution proportion it is presently making a rebound. G.726 is particularly appealing because it doesn't require a great deal of computational work from the framework.
Not to be confused with G.723 (which is another outdated variant of ADPCM), this codec is intended for low-bitrate discourse. It has two information bitrate settings: 5.3 kbps and 6.3 kbps. G.723.1 is one of the codecs required for consistence with the H.323 convention (albeit different codecs might be utilized by H.323). It is right now burdened by licenses and subsequently requires authorizing whenever used in business applications. This means while you can switch two G.723.1 calls through your Asterisk framework, you are not permitted to interpret them without a permit.
Taking into account how little transmission capacity it uses, G.729A conveys noteworthy sound quality. It does this using Conjugate-Structure Algebraic-Code-Excited Linear Prediction (CS-ACELP).117 Because of licenses, you can't use G729A without paying a permitting charge; in any case, it is amazingly mainstream and is therefore all around bolstered on a wide range of telephones and frameworks.
117 CELP is a well known technique for packing discourse. By scientifically demonstrating the different ways people make sounds, a codebook of sounds can be manufactured. As opposed to sending a genuine tested sound, a code relating to the sound is then sent. (Obviously, there is significantly more to it than that.) Jason Woodward's Speech Coding page (http://www-mobile.ecs.soton.ac.uk/speech_codecs/) is a wellspring of accommodating data for the non-scientifically slanted. This is genuinely substantial stuff, however, so wear your reasoning top.
To accomplish its noteworthy compression proportion, this codec requires a similarly amazing measure of exertion from the CPU. In an Asterisk framework, the use of intensely packed codecs will rapidly impede the CPU.
G.729A uses 8 kbps of data transmission.
GSM is the sweetheart codec of Asterisk. This codec doesn't come burdened with an authorizing necessity the way that G.723.1 and G.729A do, and it offers remarkable execution as for the interest it puts on the CPU. The sound quality is commonly viewed as of a lesser evaluation than that delivered by G.729A, however as quite a bit of this boils down to closely-held conviction, make certain to give it a shot.
GSM works at 13 kbps.
The Internet Low Bitrate Codec (iLBC) gives an appealing blend of low transfer speed use and quality, and it is particularly appropriate to continuing sensible quality on lossy system joins.
Normally, Asterisk underpins it (and backing somewhere else is developing), however it isn't as mainstream as the ITU codecs and in this way may not be perfect with basic IP phones and business VoIP frameworks. IETF RFCs 3951 and 3952 have been distributed on the side of iLBC, and iLBC is on the IETF models track.
Because iLBC uses complex calculations to accomplish its elevated levels of compression, it has a genuinely high CPU cost in Asterisk.
While you are permitted to use iLBC without paying sovereignty charges, the holder of the iLBC patent, Global IP Sound (GIPS), needs to know at whatever point you use it in a business application. The manner in which you do that is by downloading and printing a duplicate of the iLBC permit, marking it, and returning it to them. In the event that you need to find out about iLBC and its permit, you can do as such at http://www.ilbcfreeware.org.
iLBC works at 13.3 kbps (30-ms outlines) and 15.2 kbps (20-ms outlines).
Speex is a Variable Bitrate (VBR) codec, which implies that it can progressively adjust its bitrate to react to changing system conditions. It is offered in both narrowband and wideband forms, contingent upon whether you need phone quality or better.
Speex is a thoroughly free codec, authorized under the Xiph.org variation of the BSD permit.
An Internet draft for Speex is accessible, and more data about Speex can be found at its landing page (http://www.speex.org).
Speex can work at somewhere in the range of 2.15 to 22.4 kbps, because of its variable bitrate
Of course, MP3 is a codec. In particular, it's the Moving Picture Experts Group Audio Layer 3 Encoding Standard.117 With a name that way, it's no big surprise we call it MP3! In Asterisk, the MP3 codec is ordinarily used for Music on Hold (MoH). MP3 isn't a communication codec, as it is advanced for music, not voice; in any case, it's well known with VoIP communication frameworks as a technique for conveying Music on Hold.
117 If you need to gain proficiency with about MPEG sound, do a web scan for Davis Pan's paper entitled "A Tutorial on MPEG/Audio Compression."
Know that music can't as a rule be communicated without a permit. Numerous individuals expect that there is no lawful issue with associating a radio broadcast or CD as a Music on Hold source, yet this is once in a while obvious.