How To Troubleshoot Echo problems on a SPA-3000

It is not uncommon for users to report unacceptable echo on incomming PSTN calls on the SPA-3000 and SPA-3102 devices. If you search the Forums and support pages you will see many different explainations and potential solutions.

There are many things that can cause echo on your SPA-3000. Echo can be created by your telephone handset, Impedence settings (as a result of condition of your phone Line and distance from the exchange), one of the gain settings or just the latency introduced by the transcoding.

In reality, most of the time the SPA-3000 isn't causing the echo, it's just making it more noticeable. This is due to the fact that the SPA-3000 passes calls from the PSTN to the LINE1 by converting it to VoIP internally and then back to analogue (transcoding). This process does not produce any echo, however is can add about 30ms of latency to the call. This added latency can make echo that was previously unnoticed suddenly become annoying. A pure digital system has no echo (the TX and RX path are 100% separated). It's the interaction of the Digital and Analogue that cause problems.

Types of Echo
There is a number of different types of echo you can experience so the first task is to determine where the echo is occurring. Understanding which end is having problems, and which call types are affected will greatly reduce the time it takes to resolve the problem.

Your End
Remote End
Echo
PSTNPSTNRemote end
VoIPPSTNRemote end
VoIPVoIPRemote end
PSTNPSTNYour end
VoIPPSTNYour end
VoIPVoIPYour end

There are two gain settings at the bottom of the Regional tab - FXS Input Gain and FXS Output Gain. The are another two located at the bottom of the PSTN line tab down the bottom SPA to PSTN gain and PSTN to SPA gain.

If you are experiencing echo, you should first try to eliminate your handset as the cause, then depending on the type of echo, try lowering the gain setting values.

Gain Settings
Valid values are from 12 (loudest) to -15 (quietest).
Volume increases(decreases) by a factor of 2(1/2) for each change of 6. So if you change from 12 to 6, the you will half the volume. If you change from -12 to -6, it will be twice as loud.

If you are still having echo issues relating to the PSTN line, read one...

  1. Make sure you are running the latest firmware (3.1.10) and you have everything back to factory defaults or at least undo all the previous tweaking.

  2. Switch *off* all echo cancellation in all your device. There are 6 places in the sipura to switch off echo cancellation. The idea is that we want to hear how bad the echo is with different configurations.
    • PSTN Line -> "Echo Canc Enable",
    • "Echo Canc Adaptive Enable",
    • "Echo Supp Enable" and
    • Line 1 -> "Echo Canc Enable",
    • "Echo Canc Adaptive Enable",
    • "Echo Supp Enable".
  3. Unplug everything from your phone line except the SPA-3000. This includes all the extension cables even with nothing connected to them. These can cause impedance problems that lead to echo.

  4. Set the Impedance on your lines.
    • PSTN -> "Port Impedance" = 220+820||120nF as a starting point.
    • Regional -> "FXS Port Impedance" = 220+820||115nF   OR   220+820||120nF as a starting point.
  5. In the PSTN tab set - (Doesn't seem to affect echo, but I believe that these are the correct numbers for Australia.)
    • "Tip/Ring Voltage Adjust: = 3.1V" and
    • "Operational Loop Current Min = 16mA".
  6. Turn down the jitter buffers! (This reduces the delay across your SPA-3000.)
    • "PSTN -> Network Jitter Level: = low",
    • "PSTN -> Jitter Buffer Adjustment: = disable".
    • "LINE1 -> Network Jitter Level: = low",
    • "LINE1 -> Jitter Buffer Adjustment: = up and down".

    • If you are using a poor quality ISP or VoIP service, as well as, the PSTN then you could change the:
    • "LINE1 -> Network Jitter Level: = medium".
  7. Set the preferred codec for the PSTN to be g711a and lock it in. (Note: This is the opposite that I have found, will need to test!)
    • "PSTN -> Preferred Codec = g711a",
    • "PSTN -> Use Pref Codec Only = yes".

    • Obviously adjust this if you're accessing your PSTN line via VoIP from a remote network. Set the LINE1 to allow g711a as well as whatever else your prefer.
    • "LINE1 -> Use Pref Codec Only = no".

    • The g711a codec is fast to encode and decode. Using this codec again reduces your latency and may make the echo less obvious or easier to catch with the echo canceller.
  8. Power cycle the SPA-3000 (pull the power plug). Believe it or not, this sometimes fixes the problem. Especially after you have changed the physical phone wiring!


  9. Make some test calls. The telco test number 1800801920 is a good one to start with. It has a recorded voice telling you your local phone number. While it's talking, talk back and work out how much echo you are getting. Talk loud and also talk soft.


  10. Look at what you have got. If you can hear an echo then the problem could be that your probably sending to much power down the line. This is probably reflecting back somewhere as an echo. If you're close to the exchange and have good wires then this is probably the case. You need to crank back on the power. Go to PSTN -> "SPA To PSTN Gain" and turn it down. Be aware that at some point if you turn it down to much, the SPA does a double negative and turns it way up. I believe the range of this variable is about -127 -> 127 (from my testing). Turn it down until the person can still hear you but reduced echo.
    Note: if you enable "Echo Supp Enable" then you will negate these parameters. It seems that the Sipura echo suppression is actually just an automatic gain control. It's really annoying - leave it off.


  11. Make a test call to someone with a known good phone out via the SPA's PSTN line or get someone to call in to the PSTN line. Best if it’s just a boring old Telstra phone hard wired to a socket on the wall. Don't call a mobile!

    If the remote party is hearing echo, it could be that your phone is so loud that it's feeding back into the microphone. Turn down the PSTN To SPA Gain until you can comfortable hear the person and no more. If the remote user can still hear echo, try using a different phone plugged into the SPA. Go for the basics first, a cruddy old Telstra phone is what I use for testing. If this solves the problem you may have a bad phone or an impedance miss match between your phone and the SPA.
    • Try changing the Regional -> FXS Port Impedance to "600".
    • Try changing the FXO port impedance to "600" or "global".

    • If this doesn't help, change it back. The impedance will only affect what the remote party hears, it won't help echo you are hearing.
  12. After you have the echo down to a reasonable level, go back into the "PSTN" tab and switch on the "Echo Can Enable = yes". Check to see if the echo has improved. If the echo is tolerable at this level, leave the adaptive echo canceller off. You should have the echo level down to a level that can be stomped on by the echo canceller. If you are using a sip device to talk through your PSTN line, you should probably do all the echo cancellation at that device and leave it switched off in the SPA.

    The adaptive echo canceller is a lot more aggressive but also can cancel out some of the incoming conversation. In particular if you're calling in a loud environment then the voice going down the line from your end can trick the echo canceller to start canning some of the real conversation. It makes the incoming party sound a bit scratchy. Leave it off unless you really need it.

    The "Echo Supp Enable" switches on automatic gain controls. This means the Sipura will be constantly turning up and down the volume of the call for you and the remote party to try and keep the sound levels at a "good" level. Sipura's definition of good may not suit your situation. The constant variation of the volume annoys me so I recommend leaving it "off".


  13. Test call. Talk loud, talk soft. Is your conversation clear at both ends? Should be a lot better. If echo re-appears, at a later date, remember to try power cycling the SPA before you tweak with anything. It may also be at the other end of the call.

In the end it basically boils down to this -
If you are hearing the echo, the real problem is at the other end of the connection. In a normal phone conversation, the latency is so low that you don't notice it. Your brain automatically tunes out its own voice when you're talking (as long as the delay between talking and hearing isn't to long). When you insert the SPA, the delay is increased and sometimes the echo can become noticeable.

The only way you can counter this is to reduce the latency (jitter buffer) and/or reduce the gains so the echo volume is reduced below the level that it is noticed. At this point, the SPA-3000 echo canceller should be able to kill off the rest.

This information has been adapted from Ben Sharif's Trixbox Without Tears giude. If you are interested in setting up Trixbox, I recommend you check check out his guide.

 

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