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PSTN line gain

Started by JeremyW, January 31, 2015, 06:18:15 AM

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Quote from: drgeoff on February 12, 2015, 03:40:53 PM
SIP will always have an advantage over PSTN/POTS regarding echo and consistency of levels.

When you use POTS the analogue audio signals to and from your handset are on a common pair of wires.  In your OBi they are separated and amplified or attenuated according to the level settings that you have been tweaking.  Then the two signals have to be again on a single pair of wires to travel to/from your telco's switch.  There they are separated again for carriage over the digital trunk network.

Those separation and joinings are never perfect; there is always some leakage of one direction into the other.  Some of the imperfection is caused by the non-ideal characteristics of the 2-wire/4-wire conversion circuits and some is due to the actual line impedance being different from the value assumed in the design of the 2-wire/4-wire converter. Those leakages manifest themselves as echo.

In the majority of cases the end result is not troublesome but there may well be some, possibly yours, where there are enough significant shortcomings that their combined effect is troublesome.

Accept all of that, except I'm not comparing the PSTN performance to SIP at the moment - SIP has always been better whichever box I've used. I should have clarified: the first part of my testing was connecting my DECT phone directly to the master socket where the performance was as perfect as you could hope for on a PSTN line. The next part of the test was reconnecting the phone and the line to the 110 so I'm comparing the PSTN performance via the 110 with the direct connection.

The overall performance of the 110 is significantly better than I'm used to with other ATA's plus there's more settings and technical help from people like yourself. My expectations therefore are set quite high so I really want to make sure I have the optimum set up for my usage with no untried tweaks.


There's really only three places to tweak:

1. Phone Port - Raising your volume from the phone to the OBi will affect all calls - SIP and PSTN. Maybe an increase of 1dB on your transmitted voice level will not upset your SIP callees, but may give you that extra margin to tweak with on the Line Port.

2. Line Port tx/rx levels. Already much discussed.

3. Line Port impedance setting. The UK setting is a theoretical perfect match between the OBi and the PSTN line. However, the real impedance of the PSTN line will be a mix of the impedance of the local wiring, which will be different from the main cable impedance to the telephone exchange, and also the telephone exchange equipment itself (plus the other possibilities mentioned by drgeoff). Echo is a sure sign of impedance mismatch, which will be worse at higher tx levels. If the mismatch is 5%, then 5% of a big signal reflected back to the hearer is more than 5% of a small signal. There's no easy answer to this one, but a few trials of different impedances may give better results.


Thanks. I'll sit it out for a while and see how it goes before twiddling with the impedance. As I say, it seems to have settled down for the moment. Again, thanks for all the useful help.