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"Richard" <thermof@atps.net> wrote in message 
news:1126077325.490966.165880@z14g2000cwz.googlegroups.com...
> 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.

Trans hybrid loss in a two transformer design hybrid (4 Wire Hybrid In port 
to 4 Wire Hiybrid Out port) is closer to 7dB loss where the impedance of the 
termination connected to the 2 Wire Line port is an infinite mismatch to 
that of the impedance of the network connected to the Balance Network port 
(ie.e an infinite : 1 ratio).  I have measured slightly less (6.7dB) in 
proactice on some hybrids, but 7dB is closer to the norm.

The 7dB figure is derived from the fact that a minimum of 3dB loss will 
occur between adjacent ports.  Add a little extra (say 0.5dB) for 
transformer losses and a practical adjacent port loss closely approaches 
3.5dB.  3.5dB + 3.5dB = 7dB.  A diagram would be easier to illustrate, but 
as this is a text only NG, posting a pic is not possible.

>
>> 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 ?

No.  Albeit that hybrid mismatch is a major contributor to echo in the 4 
wire transmission path, it is not the sole cause.  Mismatches due to cable 
gauge changes onn a simple 2 wire circuit will produce an echo (reflection) 
due to impedance mismatch at the point they join.  Echo and signal 
reflection are one of the same thing.  Just we associate echo with long 
return path delays of 35mS or more.  It's purely an auditory perception 
thing as delays less than 28mS are very hard for the human brain to 
recognise.

>
>>> 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.

It would be pointless creating a 4 wire transmission system containing 
hybrids if no active transmission components or ADC / DACs were involved.

>
>> 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).

Agreed.  If losses exceed gains the circuit must be unconditionally stable.

>
>>>> 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.

Yes.  Especially if transformer hybrids are used.  The adjacent port loss in 
each hybrid is very close to 3.5dB.  As there is a hybrid at each end of the 
link, the minimum losses for each direction of transmission would be close 
to 7dB.

>
>>>> 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

Once again a circuit diagram would assist in the explanation.  A very simple 
circuit of t phone would have the transmitter and the receiver in series. 
The sidetone level on the receiver would be very high..  The idea of the 
ASTIC is to reduce the level of the speech current generated by the 
transmitter reaching the receiver, whilst maximising transmitter signal to 
line and also maximising the received line incoming speech signal reaching 
the receiver.  ASTICs are usually single transformer leaky hybrids.  Good 
examples are to be found in the old Telecom 800 series phones.

> -
> 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 the old phones (say 800 series and earlier) there was no gain, except in 
the hearing aid version of the 800 series which featured a volume control 
where the recall button is usually located.

- 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.

As I said previously.  Early phones had no ASTIC and sufferred from very 
high sidetone.  This high sidetone level caused people to talk softer 
because they believed the other party could hear them okay based purely on 
the effect of the local sidetone level.  This became an even bigger problem 
on long distance trunk calls which suffered significantly more transmission 
loss than local calls.

> 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.

I beg to differ.  Getting impedance matching and hybrid balancing correct 
negates the requirement for echo cancellation in either the digital or 
analogue domains.  This is immediately apparent on facsimile and data calls 
through the PSTN (not ISDN) where echo cancellation can't be used.  To get 
reasonable error free data throughput through the PSTN, the echo performance 
of the transmission path must be reasonably good to begin with.

>
>> 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)

True.  When we hold a telephone handset to our head, we cut off some of the 
natural sidetone we would normally experience in non telephone coversation. 
This means that we need to replace some of that lost natural sidetone with 
an equivalent in the telephone.  The correct level of sidetone effectively 
regulates how loud we speak into the telephone.  Ever noticed how spoken but 
profoundly deaf people speak - quite often too loud or too soft.

The correct sidetone level in a phone should be approximately 13B below the 
speaker's transmission level (local end).

>
>> 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).

The IEC tutorial takes a very simplisitic approach to the issues surrpunding 
echo cancellation in typical telephone networks.  It completely ignores the 
issue of VF data (fax and modem calls) through the PSTN and how echo 
cancellors are supposed to handle such calls.

Furthermore the document dips to mediocrity with the first sentence on Page 
5 which reads " Unfortunately, the hybrid is by nature a leaky device." 
What utter crap.  I've measured return losses in old 1954 two transformer 
hybrids which exceed 60dB!   A million to 1 times power isolation is nothing 
to be sneezed at.  Hybrids, when correctly balanced are supposed to provide 
isolation between the 4 wire transmission paths - not leakage.

You've obviously never conducted a Near End return loss measurement on a 
customer's telephone line with an EDL423 Network Transmission Quality 
Tester.  Run a transmission test on a loaded cable (nomimal Z = 1200 ohms) 
where it interfaces to a LIB7 LI in an AXE exchange (Zin = 600 ohms fixed). 
Where the cable interfaces into the exchange there is a 2 : 1 impedance 
mismatch.  Even though the mismatch only contributes to 0.5dB additional 
forward transmission loss, the return loss at this point is a mere 9.2dB - a 
mismatch in anyone's book, with more than 10% of the transmitted signal 
reaching this point being reflected back to the source.   The only thing 
reducing this reflected signal back to the customer is the transmission loss 
of the cable itself which by Australian standards must be less than 6.5dB @ 
820Hz.


Cheers,
Alan


>
> regards
> Richard Freeman
>