Helprequired (Speech to signal transmsion)

[guess.who]

Hi guyz..
i know the basic building blocks are
A/D
Speech Coding
Channel Coding
Interleaving
Encryption and then transmission... but i am confused abt the data rates....specially the first step...
at one place i have read that A/D conversion of speech is done at 8Kbps(13 bits) and then speech coding reduces/compresses the size to 13 kbps.... How is 13 kbps smaller than 8 kbps.....??
and at another place i have read GSM uses PCM (64kbps) to digitize voice and then uses speech coding ...with this statement the compression justifies itself....
So is it 8Kbps or 64Kbps...If 64 then what did the writer try to meant in the first statement....
Thanks in advance...

[hassen]

hi
about A/D conversion
do you know The frequency band of a telephone circuit is 4KHz
and do you know
The theorem of Nyquist-Shannon, provides that the frequency of sampling of a signal must be equal or greater than double the maximum frequency contained in this signal, in order to convert this signal a form analog to digital form.
so the frequency of sampling is 2*4KHz=8KHz
The method to convert an analog signal to a numeric signal,and don't forget it must be sure that the latter contains all the information of the analogue signal of origin .
1 sampling
2 quantification 13bit for in order to code the smallest sample
3 compression logarithmic 13 to 8bit in order to reduce the frequency band
and finally digital circuit in the field telephonic its band is 8Khz * 8bit=64Khz
br hassen

[guess.who]

first of all Thaaaanks a lot hasen... well yes i did know abt the nyquist theorm and did understand what u were saying there but how u converted 8k to 64k went right up my head again,,,, please elaborate the latter part..... Thanks in advance.

[mirza]

first of all Thaaaanks a lot hasen... well yes i did know abt the nyquist theorm and did understand what u were saying there but how u converted 8k to 64k went right up my head again,,,, please elaborate the latter part..... Thanks in advance.

Hello Guess.who
Thanks for Nice Question and reply
It would great if you can give the reference for further understanding

Thanks

Mirza

[guess.who]

Dear Mirza
brother , i just had a couple of manuals (hard copy) through which i consulted.....