Gain Adjustment
From Pbxnsip Wiki
Purpose
When analog components are involved in a phone call, the volume of a call may vary. For example, if you run long FXO lines to your office, the signal in your office will be significantly damped. This is a serious problem, because the audio volume is perceived as one of the most critical quality indicator.
Digital equipment is usually calibrated during fabrication. When you are using digital transmission lines (like ISDN, T1 or SS#7) do not change the input gain. This typically just confuses the whole signal chain and might cause unnecessary echo.
The PBX itself does not change the gain. This would lead to a significant reduction in signal quality; therefore the PBX does not even attempt to do this. It is the "job" of the PSTN gateway.
In order to help calibrating the gain, the PBX offers in 2.0.3 a new feature that measures the actual audio levels on calls. This feature does cost CPU performance; therefore we recommend using it only during the installation or possibly during audio quality troubleshooting.
Traditionally, gain is indicated in dB (decibel). We calculate that value from the highest possible signal value, which would be 0 dB.
In order to use the gain measurement feature, you need to turn the "Analyze audio levels" setting on in the system administration/logging settings (see Log Setup). Please be aware that this feature does cost a lot of CPU performance, therefore we strongly recommend to turn this flag off after the measurements.
PSTN Gateway Gain Adjustment
In order to calibrate the gain on the gateway, you need to call something of which you know the audio levels. A good example is the ring back tone, which in USA typically has a volume of -31 dB. This value is quite stable; therefore you can use it for an exact adjustment of the input gain for the FXO gateway.
In order to see the volume, log in to the web interface of the PBX. You need to log in as domain or system administrator and watch the current calls. When you hear the ring back tone, the PBX takes a few seconds to display the volume. During ring back, only one side of the call should have a significant volume and the other side should be silent.
If you need to change the gain, make sure that the PSTN gateway takes the change (possibility you need to reboot the gateway) and make another test run. Repeat this until you have a satisfactory result.
Microphone Gain Adjustment
Some phones also offer the feature of adjusting the microphone gain. Generally speaking, a high input gain has the problem that the line echo of the handset cord is amplified and has to be compensated in the handset. Plus a loud voice might sounds very distorted.
In order to adjust the handset gain, you can use a phone that you think is “good” for calibration purposes. Just establish a phone call between the devices to be calibrated and the good phone and compare the audio levels. By speaking only into one device, you can find out which side of the PBX means which phone (the one with the higher level is the phone that you are talking to – keep in mind the numbers are negative). Then hold the two handsets to both ears and speak for a few seconds. Then you should be able to compare the audio level of the two phones and tell if the phone microphone level needs to be changed.
You can use this method also to determine if a phone is generally too loud. If you are getting close to 0, that means that this phone is too loud (at least for this speaker).
