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Looks like my ISPs News server is stuffed again .....

"Alan Rutlidge" <dont_spam_me_rutlidge@iinet.net.au> wrote in message
news:431c05d4$0$15537$5a62ac22@per-qv1-newsreader-01.iinet.net.au...
>
> "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:
>>


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


>>> 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.

True however there is loss (theoretically 6dB - in reality greater than
6dB)
through the Hybrid Transformer - I had meant to flesh this concept out
further yesterday but inadvertantly hit send before I had finished
following
this idea/posting  further.

> 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.

so are you starting to suspect that the Hyrbrid has something to do
with the
echo then and not cable reflections ?

>> 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.

Correct - the loop will not be singing if it merely consists of two
sets of
Hybrids - ie no gain in the Transmission system - as per my earlier
comment
that stability is not created by padding a system down (as the basic
system
already contains sufficent loss) but rather about keeping total gain to
a
minimum.

> 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.

Exactly the basic system - with no added gain - has a total (using your

figures) of 14dB of loss built in and is by its very nature stable (any

signs of instability in this system would be cause for celebration by
physiscts around the world).

>>> 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.

Lets see - over 6dB (theoretical) loss each way through the hybrid ....
That sounds about right.

>>> 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 Sidetone was there already as a function of the Hybrid in the
phone, the
ASTIC was wired to cancel the sidetone further than a simple Hybrid did
-
This was only possible as the level (not really the impedance except
where/as
it affected the overall gain) of the signal between the hybrid/Astic
and
the receiver was both known and constant.- In a side note the ASTIC was

actually introduced (around 1939 I believe in Australia) to encourage
people to talk louder into Telephones as it provided less Sidetone than
the normal Hybrid Transformer had previously.
In reality since it is not really impedance mismatches that stop us
supressing Hybrid leakage but rather the fact that we do not know the
overall gain of a System (and hence the exact Signal level we need to
cancel out) - nor does that gain remain constant - largely due
to line length variations etc I would have to argue that in a real
world telephone network no matter how well designed is it not possible
to provide an echo free service without either a VSA (Voice Switched
Amplifier the old method
of echo supression which basically gated the signal in one direction at
a
time) or DSP based echo supression.

> 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.

ideal Sidetone is considered to be the level we are used to hearing
when we
speak (sorry I don't recall the figure off hand)

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

again my apologies I had meant to follow these ideas further before
posting -
However even well designed pure digital Transmission systems require
echo
suppression when the round trip delay exceeds a certain amount of time
(IEC
consider this to be 36mS-
http://www.iec.org/online/tutorials/acrobat/echo_cancel.pdf - which is
actually an excellent tutorial on the whole subject and one that I
recommend
highly )  due to the leakage through the hybrid. If rtd is kept below
36mS
echo does still occur however it is heard by the person speaking as
part of
the sidetone (a variant of this effect called 'double tracking' is an
effect
often used in recordings and live concerts to give vocals more power -
but I
digress .....) over 36mS however this starts being noticed as a
separate
echo and becomes a problem.
I suspect that your 'well designed network' is merely one in which no
call
has an RTD of over 36mS. Unfortunately due to the laws of physics this
precludes networks which have paths of over 5,700 Kms (In Australia we
approach that limit before we even consider processes such as those
within
the codec etc which cause additional delay). Of course when/if you add
a
Satellite to the equation you also add 50,000 Kms of path or 166mS and
a
very noticeable echo - I believe (but not working there am unable to
confirm) that India makes (or maybe made) extensive use of satellite
technology as a (relatively) cheap way of providing telecommunications.

- Alternatively ISDN type connections do not naturally have Sidetone or

suffer from echo when both ends terminate on ISDN (try any call through
a
tester such as an IBT 1A) - however this is due to the fact that there
is in
fact no hybrid in such a system - anywhere! TX and RX are maintained as

completely isolated paths through the entire network. Sidetone for ISDN

phones is added deliberately by the handset manufacturer.

Bottom line is (and getting back to the point of the thread - at least
the
point where I joined) echo is not caused by cable reflections but
rather by
Hybrid leakage - which is often incorrectly called a 'reflection - yes
cable
reflections do occur and this is put to good use by the pulse echo
tester -
however they do not occur at levels sufficient to cause noticeable
problems 
at VF (Voice Frequency).

regards
Richard Freeman