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I am in midst of some online telephony course, and there are questions regarding which I have no one to ask but this community.

  1. G.711 volume encoding. I have some difficulty to understand how exactly G.711 encodes the absolute volume, if at all. The standard does seem to address it in mapping the code to actual input voltage. But does the actual voltage range choice as input for encoder - changes anything in mathematics of G.711 or is it only so G.711 encoders be compatible one with another. Because seems like G.711 deals with encoding any arbitrary 256 steps range.

  2. Given the following sentence "Power of 3.17 dBm is 2.0749 mW in 600 Ohms (Ω) of telephone impedance". Why do we have to reference impedance, isn't 3.17 dBm translated directly to 2.0749 mW?

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  • \$\begingroup\$ Part 2 hint: Impedance doesn't make a lick of difference \$\endgroup\$ – Matt Young Jul 27 '14 at 15:06
  • \$\begingroup\$ Given dBm, mW is redundant. It's been calculated for you, so that you don't have to do the conversion yourself. You will, in most situations, only get the rated dBm when terminated with the expected impedance, but it's not redundant to specifiy it once, somewhere. Power, however expressed, will be different into a different impedance but mW will still match dBm. \$\endgroup\$ – david Jul 28 '14 at 7:49
  • \$\begingroup\$ I think this redundancy was given so the maximum voltage of sine wave can be calculated later, but I may be mistaken. Thanks for the answer anyway. \$\endgroup\$ – Boris Jul 28 '14 at 11:30
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  1. It doesn't encode absolute volume. 0dBm (1mW) is chosen as the nominal 'full volume' power level because it is a standard for analogue telephone lines, and the +3.17dBm maximum provides an overload margin. If the telephone equipment uses a different standard (eg. -10dBm) then the signal will be encoded relative to that level, but can still be reproduced at 0dBm (or any other 'volume') if required.

    The encoder itself is basically just an A/D converter, and the input voltage it requires depends on the particular circuit used (convertor type, reference voltage etc.). The incoming audio signal would typically be terminated with a 600 Ohm resistor, then the voltage across it amplified to match the input range of the ADC.

  2. It's redundant, but for good reason. We are not machines, and redundancy helps us to understand concepts and memorize facts.

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  • \$\begingroup\$ I think this redundancy was given so the maximum voltage of sine wave can be calculated later, but I may be mistaken. Thanks for the answer anyway. \$\endgroup\$ – Boris Jul 28 '14 at 5:31

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