FX0 sound levels (PSTN db level)

Unanswered Question
Dec 16th, 2009

Is there a way to change the line level on the fxo ports?  The sound on my phone is turned all the way up for incoming calls, but for internal calls like picking up voice messages I have to turn it down.  It just seems a little bit low for such a contrast in sound between internal and external.

I used to have a phone system where it would automatically analyze the PSTN lines and adjust the db accordingly.  They would never be at the same levels, so I'm thinking I might have to test and change each one.

My voice going out seems to be okay, it's just the external incoming calls that need to be amplified a bit.


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ciscojoe837 Thu, 12/17/2009 - 12:43

Hi Marcos,

Yes, this is a very useful document.  Thank you.  I think I can test the Input Gain and Output Attenuation values, but I would not be sure of the Impedance value needed or why I would be changing it.  I know I could leave it at default and that would probably be right, but how would I know or test its setting of 600c or 900c?  Plus there are two other values complex 1 and complex 2.

As well, my 7921 sounds great on speaker, but when I take it off speaker it sounds a little canned.  The person on the other end says they can hear me fine, but it's like they are alway on a cell phone even when they are calling from a PSTN line.  Do you think this document would cover this problem as well.  I kind of blame it on the phone itself and it being wireless, but I don't know for sure.  I think it use to be a clearer as well but it's hard to judge.



Marcos Hernandez Thu, 12/17/2009 - 13:09

Start with default for the impedance and just modify the input gain in 3 db increments while you test.



ciscojoe837 Fri, 12/18/2009 - 09:23

Okay.  Just to be sure, would I change the Input gain to increase the volume on an incoming call or is it the Output Attenduation?  I'm confused by where these two would get applied.  They mention a transmit side and a receiver side of the interface, but I can picture "the interface" both transmitting and receiving so its not clear to me.



Marcos Hernandez Fri, 12/18/2009 - 11:12

Input and output is from the perspective of the FXO port. A signal coming into the FXO port from the PSTN, will be amplified by increasing the input gain and attenuated by decreasing the input gain. The signal going out of the FXO port towards the PSTN will be amplified by reducing the output attenuation (making it more negative) and will be attenuated by increasing it (making it more positive).



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