Keep in mind- it could happen at either end of he equation. VoIP creates, IMHO, the worlds best audio delivery system. And it's uncompromising- it will faithfully reproduce whatever sound has been sent to it. Analogue equiment doesn't- it compresses the sound during transport, reduces volume, attempts to kill off hiss, etc. The net effect is that far-end echo tends to be amplified- or at least, not suppressed as well, creating local echo components- you hear your own voice repeated back. Due to latency issues surrounding VoIP, you'll really hear it- before, you might have had it suppressed or turned into sidetone, as previously mentioned.
My 'holy checklist' for finding echo cause runs about like this:
1. Check gains everywhere- under zaptel, on the microphones, etc. Turn them to absolute minimum. I've seen many systems where, under the mantra of 'more is better', people start off by hiking their headset gains, or system administrators hike the Zaptel gains to fix a problem with headset/handset volume. Most audio engineers will tell you to do your amplification only at the last possible point before your speaker- and it's good advice. Repeated inline amplification will increase distortion, and will make echo sound worse than it actually is.
2. Reproduce it. Each call is different on the PSTN and with the far end users- and patterns matter. The echo might only be to a particular PC, headset model, or remote party. Don't rule anything out, or in, until you know the pattern. If some calls sound fine, and others not- you need to know why.
3. Verify your station equipment- make sure that you're using appropriately qualified equipment, in suggested ways. (a fisher price mic plugged into a laptop's line in jack will sound like garbage, really- I know personally). It doesn't have to be expensive to sound good- but expensive stuff can sound bad just as easily if not configured properly.
4. Look at the hardware. Is your zaptel card bad? bad line card? Bad PRI interface? Some things are easy to diagnose, some not- so look at this last- you'll be happier if you solve it earlier than at this stage, because the diagnostics, by and large, are either very expensive, or very much T&E.
5. Finally, look at Asterisk and the OS, and what it's telling you. Does netstats show a long latency, or high jitter? Are you running 0.7? Trying to run conference calls with ztdummy hooked to a bad system timer? The network, assuming all else is OK, is merely a source of latency- it's a problem amplifier. Repeated packets, with significant jitter or delay, can occasionally sound like echo- but it's a software problem, as there is time stamps and sequence numbers involved- more frequently, it will trigger skips or drops, as the packets are ignored- but that slows down processing of good packets. Focus here last, unless you know your network is in bad shape- it's easy to get lost in the forest, and miss the trees.
Good luck- if I can help you, let me know.
-Paul Davidson
PlanCommunications, LLC
Message: 1
Date: Tue, 20 Jun 2006 12:14:13 +1000
From: Michael Knill <[EMAIL PROTECTED]>
Subject: Re: [Astlinux-users] Bad echo on incoming calls on Zap trunk
To: Discussion of AstLinux - Asterisk on Compact Flash
<[EMAIL PROTECTED]>
Message-ID:
< [EMAIL PROTECTED] >
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Further clarification sorry.
There are some other things you can do besides echo cancellation to
alleviate the echo such as reduce the tx level etc. Have a read of the
doco.
Regards
Mike
Michael Knill/Australia/[EMAIL PROTECTED]
Sent by: [EMAIL PROTECTED]
20/06/2006 10:07 AM
Please respond to
Discussion of AstLinux - Asterisk on Compact Flash
<[EMAIL PROTECTED] >
To
Discussion of AstLinux - Asterisk on Compact Flash
<[EMAIL PROTECTED]>
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Subject
Re: [Astlinux-users] Bad echo on incoming calls on Zap trunk
David
I dont know a great deal about setting up Zap cards, not ever owning one,
however I do know about echo.
Below is an excellent document from Cisco explaining echo on VoIP. From
what I have seen on boards, I think this is one of the most misunderstood
subjects.
There is nothing at your end that would cause echo to be received by you
(except low quality handsets as metioned). If there were problems at your
end then echo would be received by the other party, not you. PS the very
long cable you have going to the phone socket is probably quite short in
comparison to the length of cable going to the exchange. Its all just
twisted pair !!
All that VoIP does to echo is it compounds the problem due to the
increased latency. The echo was already there on the analogue phone line,
however you would only notice it as louder sidetone in your receiver due
to the very low latency.
Echo cancellation is the only option.
http://www.cisco.com/en/US/tech/tk652/tk701/technologies_white_paper09186a00800d6b68.shtml
Mike
David Caldwell <[EMAIL PROTECTED]>
Sent by: [EMAIL PROTECTED]
19/06/2006 09:53 PM
Please respond to
Discussion of AstLinux - Asterisk on Compact Flash
< [EMAIL PROTECTED]>
To
[EMAIL PROTECTED]
cc
Subject
Re: [Astlinux-users] Bad echo on incoming calls on Zap trunk
Hi all,
I think my echo problem might be due to a very long cable run from the TDM
cards to the phone socket. I will relocate the Astlinux server and test
for echo again.
In the meantime, I've been trying to find a tool called 'ztmonitor'
mentioned in a number of articles about echo and gain used for tweaking
the Zap cards. Is this included with Astlinux or is there an alternative I
can use?
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