🚀 go-pugleaf

"Alan Rutlidge" <dont_spam_me_rutlidge@iinet.net.au> wrote in message 
news:43180e72$0$518$5a62ac22@per-qv1-newsreader-01.iinet.net.au...
>
> "Don Pearce" <donald@pearce.uk.com> wrote in message 
> news:a750c7dhudwb.1h2dtb3ggjv48.dlg@40tude.net...
>> On Fri, 2 Sep 2005 15:27:47 +0800, Alan Rutlidge wrote:
>>
>>> "Richard Freeman" <bogoff@nospam.spam> wrote in message
>>> news:4317f1be$1_1@news.iprimus.com.au...
>>>>
>>>> "Alan Rutlidge" <dont_spam_me_rutlidge@iinet.net.au> wrote in message
>>>> news:4316b902$0$515$5a62ac22@per-qv1-newsreader-01.iinet.net.au...
>>>>>
>>>>> "Don Pearce" <donald@pearce.uk.com> wrote in message
>>>>> news:1jlu9dz4m6nho$.9bip6dnzgr15$.dlg@40tude.net...
>>>>>> On Thu, 1 Sep 2005 17:25:01 +1000, Phil Allison wrote:

- - - - - - -- - - snipped for brevity - - - - - - - - - - -

>>> As an example, a 1 : 1 ratio results in 0dB loss due to reflection.
>>> A 2 : 1 mismatch results in 0.5dB loss due to reflection.
>>> A 4 : 1 mismatch results in approximately 1.9dB loss due to reflection.
>>> An 8 : 1 mismatch results in approx. 4dB loss due to reflection; and so 
>>> on.
>>>
>>> Impedance mismatch does matter.
>>>
>>> Cheers,
>>> Alan
>>
>> We are not actually worried here about the power lost by reflection, but
>> the fact that it causes an echo - particularly noticeable on an
>> international line with long delays. In this regard, even quite small
>> mismatches are important. A mismatch which results in just 0.1dB of 
>> signal
>> loss will cause an echo at -20dB. This will be very audible.
>>
>> d
>
> Absolutely no disagreement there Don about the fact that impedance 
> mismatch contributes to echo.
>
> Echo (due to reflection) is a function of the return loss of the circuit.

The Echo is not due to Line reflections but rather due to Hybrid Transformer 
leakage - see Sidetone which is a deliberate result of this leakage being 
taken advantage of ....

> The formula is -
>
> Return Loss (dB) = 20 x log(base10) (Z1 + Z2) / (Z1 - Z2)
>
> A 2 : 1 mismatch (which results in a power loss of 0.5dB) corresponds to a 
> return loss of 9.2dB which is pretty poor.  A mismatch which results in 
> only 0.1dB loss due to reflection actually achieves a healthy return loss 
> in excess of 40dB, but as we both know that good figure rapidly 
> deteriorates as the mismatch ratio increases.
>
> In a transformer hybrid circuit, the return loss between the two paths of 
> the 4 wire transmission circuits will never go below about 7dB due to 
> trans-hybrid losses even if the impedance ratio between the load on the 2 
> wire circuit is infinitely high with respect to that of the network 
> connected to the balancing network port.  In practice, most telecos build 
> loss into their transmission networks to counteract the effects of 
> instability due to impedance mismatches.

you do not have to build loss into Telephone networks - it is there anyway - 
it is more a case of keeping your gain down to make sure you do not get 
howling due to feedback through the hybrid Transformers at each end -

> In Australia, the loss from 2 wire appearance at the MDF of the exchange 
> to the same at the other end is designed at 6dB for each direction of 
> voice transmission.  A nice idea, as the echo level is reduced by a factor 
> of 2 times the loss of the link in the network, resulting in a minimum 
> stability margin of at least 12dB even under the worst possible 
> conditions.
>
> Some other overseas networks aren't anywhere near as good as ours when it 
> comes to echo performance.  In fact a well designed and impedance matched 
> network requires little or no echo cancellation equipment, resulting in a 
> clearer network to talk over and minimal VF data transmission / fax 
> transmission problems.

Hmmm? what about Side tone ? how do these well designed networks provide 
Sidetone ?