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This is an excerpt from wikipedia about 'full rate' speech coding standard.

Full Rate or FR or GSM-FR or GSM 06.10 was the first digital speech coding standard used in the GSM digital mobile phone system. The bit rate of the codec is 13 kbit/s, or 1.625 bits/audio sample.

And this one is an excerpt from Wikipedia about bit.

In computing parlance, bit is the abbreviation for a single binary digit, represented by a 0 or a 1.

How could I represent 1.625 by 0 or a 1?

Actually, that's my lecturer's question that I could not answer. Some links to papers are more than welcome.

Thanks in advance.

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You can't. But you can represent 8 samples in 13 bits, which is what is being done here.

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My real problem is about how GSM-FR calculate the bit rate. Seems like I found my answer here. – pepito Jun 24 '12 at 10:36

The keyword that you are missing here is "rate" a bit rate. 13 kilobits per seconds means that 13,312 digits are processed a second, of which the possible binary values are 0 or 1. The sampling rate is how frequently an analog signal is converted to a digital representation (analog audio is continuous, a binary representation is discrete). You cannot really record a partial bit, the bit/audio sample rate is just a rate of measure for comparative purposes (another encoding technique might sample the analog signal more frequently or with a higher fidelity and thus that bits per sample rate would be higher.

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