What are Codecs ?

I am writing this piece in response to mails from some of my readers who wrote in to ask for explanation about the significance and effect of the different codec options they have in their various voip applications. This post is to give the definition of codecs, explain the different codecs in use in the voip ecosystem and explain how choosing one or the other could affect the performance of your voip application. Codecs are the various mathematical models used to digitally encode and compress analog audio information. Whenever you talk into your microphone while using your favourite voip application to call someone, the analog audio signal generated by your microphone can not be sent over the internet directly but has to be digitised and compressed before it can be sent over the internet. The mathematical models the enable the coding and compression of such analog audio signal is what we refer to as codecs. Originally, the term CODEC referred ti a COder/DECoder: a device that converts between analog and digital. Now, the term seems to relate more to COmpression/DECompression.One of the important features of a codec is its bitrate which refers to the number of bits that is required to be transmitted per second. The higher the bitrate the higher is the bandwith requirement, so the purpose of the various codecs is to strike a balance between efficiency and quality. The table below gives the various codecs that are used in voip applications and their bitrate.

Codec                      Bitrate (kbps)
G.711                      64
G.726                      16, 24, or 32
G.723.1                    5.3 or 6.3
G.729A                     8
GSM                       13
iLBC                       13.3
Speex                      Variable

In the following posts i will describe each codec in detail and the table above will be link to those posts so that by clicking on any codec you can read about all the features of that codec. This is just the first post in the "what are Codecs" series and by the end of the series you will be have a proper understanding of what codecs are, their features and which one is the best for you to select in your voip application for best performance.