The most important part is proper gain structure. If that's wrong no there is no way to control echo. No amount of tweaking of compensation networks will bring one into balance... No Convolution processing can control it. On old style equipment i.e. stuff built by Tellabs, the gain structure had to be "right" within about .5 DBm0.
Alignment meant dialing up a milliwatt test signal, measuring that signal at the 2 wire point and adjusting pads on the module so that the 4 wire transmit point was at a fixed and correct level. If memory serves, on an analog microwave system, 0 DBm into a module was supposed to be -16 DBm on the 4 wire transmit point. The "picture" below may help to clarify:
=======================================================================
-------> 2 wire TX |
| |
0DBm | /------o -16DBm |
|/ |
C.O. milliwatt o-------------x 4 wire |
\ |
\------o +7DBm |
|
<------- RX |=======================================================================
So... given that we know the C.O. milliwatt is 0 DBm we also know that the signal seen at the point marked 2 wire is the sum of 0DBm minus the line loss, usually around 3 to 4 DB. When that signal passes through the hybrid and correctly adjusted associated attenuator it will appear as marked and discussed. Conversly, +7 DBm is inserted at the 4 wire RX point and the associated attenuator adjusted so that sufficient signal is seen at the 2 wire point at 0 DBm.
The microwave system that connects to the 4 wire point has 23 DB of gain so that the layout above can be mirrored for a complete analog 2 wire/4 wire/2 wire circuit with an overall loss of between 6 to 8 DB.
The old bell specifications called for minimum 12 DB longitudinal loss across the 4 wire points for a hybrid on a local circuit and 16 DB for long haul. There were milage specifications, but I don't remember them anymore. Just getting the gain structure right was usually enough to meet that requirement. If not, then we got into a backend adjustment to impedance match the 2 wire circuit to the hybrid... Interestingly enough, on an in use circuit, the losses and impedances didn't tend to change much over a period of years.
I think this has gone on long enough... suffice to say, gain/levels are crucial to echo control... It you send is too hot, you WILL have echo and I don't care how good your card is. These principals applied to channel banks that I adjusted in olden days as well... Mostly Northern Telecom DE4, but others as well. We used special equipment to measure signal levels at the T1 point. I have to presume E1 equipment is/was similar, but I have no experience there.
'nuff said
Steve Underwood wrote:
Andrew Kohlsmith wrote:
[...]If you are trying to do cellular, satellite, VoIP or any telephony with high latency and do not use echo cancellation you are on to a looser. Sure, the problem is worse with some interfaces in combination with certain lines (you can't separate the two), but echo performance will always be lousy without proper echo cancellation. With echo cancellation almost any FXO interface should work well. Every cell phone call to the PSTN is echo cancelled. Every cheapo or expensive VoIP interface box echo cancels. Hybrids of any design are really lousy, and do little more than stop howling. You can hand tweak some of them connected to a particular line and get great performance. However, they always drift, the lines get altered, or in some other way they get screwed up again. Echo cancellation is a requirement, not an option.
Not at all. Any of the channel banks I've tested have better echo and audio quality than the X100P. I believe it comes down to the Part68 interface being better able to accomodate different lines but YMMV. I have never had decent results with an X100P. All of the tricks and hacks you see on the wiki with it are proof that it's a substandard card, IMO.
If the X100P's interface matches a line well it will work well. If it matches it badly it will work badly. Same with the channel banks, or any other analogue line interface. Almost all use a compromise line match. Adaptive line matching is rare. The echo you get is the luck of the draw, regardless of what FXO hardware you are using.
Regards, Steve
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