🚀 go-pugleaf

"Richard Freeman" <bogoff@nospam.spam> wrote in message 
news:431bf695_1@news.iprimus.com.au...
>
> "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 -

Not entirely true.  Most of the US Inter Exchange Network (IEN) [digital] is 
designed to have a nominal 0dB loss exchange to exchange.  The only overall 
transmission losses occur on the lines between the exchange and the 
customers.  Even before the advent of digital transmission and switching 
technology, the old Strowger exchange had a fraction of a dB loss through 
the exchange on a local call.  IEN echo however was not a problem as it was 
a two wire circuit through the exchange and the cable network to the 
telephone.

> 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 a closed 4 wire IEN circuit losses of approximately 7dB occur across the 
transformer hybrids at each end of the 4 wire transmission path even under 
the worst possible mismatch conditions.  This effectively provides a total 
of 14dB loss to the singing loop.  Therefore, provided the total gains in 
the singing loop don't exceed the total losses, the circuit will remain 
unconditionally stable.  Any difference in favour of the losses over the 
gains in the singing loop is known as the Stability Margin.
Example :  If the losses total 14dB (trans-hybid losses as the worst 
possible condition) and the gain only 1dB in each direction of transmission 
to overcome adjacent port losses in the transformer hybrids (total 2dB gain 
in the singing loop), therefore:- 14dB (loss) - 2dB (gain) = 12dB stability 
margin.  In simple terms the closed 4 wire loop can never become unstable.

>
>> 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 ?

The sidetone is developed within the telephone.  Older phones (pre the T200 
/ T400 series) used what was known as an Anti-SideTone Induction Coil 
(ASTIC) which is a purposely leaky hybrid, was designed to feedback a small 
amount of the speech energy from the transmitter (microphone) to the 
receiver.  This sidetone is purposefully locally introduced to make the 
caller think the phone was "working okay."  The level of sidetone is 
critical.  Too much and the talker will speak softly (thinking the other 
party can hear him /her loud enough).  Another problem is background noise 
(if loud enough) picked up by the transmitter can tend to drown out incoming 
speech and make the speech unintelligible.  Conversly, too low a level or 
absence of sidetone tends to cause the user to speak too loudly in the false 
belief the distant party can't hear them.  If they shout loud enough they 
could overload the A/D converter in the exchange causing distortion.

BTW.  The modern telephone still achieves sidetone, but instead of a bulky 
ASTIC, it is achieved with semiconductor technology.

Cheers,
Alan