u-law encoding – 8 bits in a DS0

The previous blog explained why we have 8 bits in a DS0, here we’ll explain how we get these 8 bits.

When your voice is digitally sampled the amplitude or how loud you are speaking is captured, this happens 8000 times a second. However this digital sample is not what is sent. For a T1 line an algorithm called u-law is used to compand the data. This is a logarithmic type of algorithm that has the effect of giving greater resolution to lower value samples i.e. when you are talking quietly, and less resolution to higher amplitude samples. This intuitively makes sense, if someone is shouting then you can afford to loose a little differentiation between sounds but when someone is whispering you want to hear or accentuate the different levels so you can make out what they say. Typically when your voice is sampled, an Analog to Digital converter that has a 13 bit output is used, the u-law algorithm as well as performing this logarithmic compression has the effect of reducing the voice sample to the 8 bits that is sent.

Up until now we have confined our discussions to voice T1’s. Next time we’ll take a look at how data is transmitted via a T1 line.

WhichVoIP team

About the author  ⁄ Calum

Calum has been creating articles and blogs for WhichVoIP for many years. He has vast knowledge of the telecommunicators sector.

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