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Getting matching transformer from telephone

R

Ross Herbert

Jan 1, 1970
0
On Tue, 06 Jan 2009 04:53:17 -0900, [email protected] (Floyd L. Davidson) wrote:

:>On Sun, 04 Jan 2009 20:36:09 -0900, [email protected] (Floyd L. Davidson)
wrote:
:>
:>:
:>:> voice frequency circuits were all 300 - 3400Hz in my day.
:>:
:>:The PSTN is specified from 400 to 2800 Hz, with 24 dB SNR.
:>:
:>:Individual channels on various carrier systems, and some
:>:private line voice circuits are specified with more
:>:bandwidth.
:>:
:>
:>In Australia PSTN is specified for 300 - 3400 Hz bandwidth.
:
:I doubt it.


Well let's give some examples...

When I was involved in junction commissioning (unloaded copper inter-exchange VF
junctions) with Telstra, transmission measurements were carried out over the
300-3400Hz range. This hasn't altered.

When looking at the transmission characteristics of an international telephone
exchange (ITU-T Recommendations)the only frequency range mentioned is
300-3400Hz.
http://www.itu.int/rec/dologin_pub.asp?lang=e&id=T-REC-Q.45-198410-I!!PDF-E&type=items
http://www.itu.int/rec/dologin_pub.asp?lang=e&id=T-REC-Q.45bis-198811-I!!PDF-E&type=items

And the following document recommends all channel terminal equipment be lined up
using 300-3400Hz.
http://www.itu.int/rec/dologin_pub.asp?lang=e&id=T-REC-G.120-199812-I!!PDF-E&type=items

Note that ITU-T G235 (3KHz spacing) is supesrseded and is no longer recommended
for international connections.

As for the specification relating to customer equipment connected to the PSTN
the frequency range used for testing is 100Hz - 4KHz.
http://www.commsalliance.com.au/documents/standards/S004:2008

In other documents from this website the definition of VF telephony or Voiceband
is 300-3400Hz.

There probably a number of other publications if I had the time to research them
but suffice to say that in Australia the VF telephony channel bandwidth is
specified as per ITU recommendations ie. 300-3400Hz.
 
J

John Livingston

Jan 1, 1970
0
Ross said:
On Tue, 06 Jan 2009 04:53:17 -0900, [email protected] (Floyd L. Davidson) wrote:

:>On Sun, 04 Jan 2009 20:36:09 -0900, [email protected] (Floyd L. Davidson)
wrote:
:>
:>:
:>:> voice frequency circuits were all 300 - 3400Hz in my day.
:>:
:>:The PSTN is specified from 400 to 2800 Hz, with 24 dB SNR.
:>:
:>:Individual channels on various carrier systems, and some
:>:private line voice circuits are specified with more
:>:bandwidth.
:>:
:>
:>In Australia PSTN is specified for 300 - 3400 Hz bandwidth.
:
:I doubt it.


Well let's give some examples...

When I was involved in junction commissioning (unloaded copper inter-exchange VF
junctions) with Telstra, transmission measurements were carried out over the
300-3400Hz range. This hasn't altered.

When looking at the transmission characteristics of an international telephone
exchange (ITU-T Recommendations)the only frequency range mentioned is
300-3400Hz.
http://www.itu.int/rec/dologin_pub.asp?lang=e&id=T-REC-Q.45-198410-I!!PDF-E&type=items
http://www.itu.int/rec/dologin_pub.asp?lang=e&id=T-REC-Q.45bis-198811-I!!PDF-E&type=items

And the following document recommends all channel terminal equipment be lined up
using 300-3400Hz.
http://www.itu.int/rec/dologin_pub.asp?lang=e&id=T-REC-G.120-199812-I!!PDF-E&type=items

Note that ITU-T G235 (3KHz spacing) is supesrseded and is no longer recommended
for international connections.

As for the specification relating to customer equipment connected to the PSTN
the frequency range used for testing is 100Hz - 4KHz.
http://www.commsalliance.com.au/documents/standards/S004:2008

In other documents from this website the definition of VF telephony or Voiceband
is 300-3400Hz.

There probably a number of other publications if I had the time to research them
but suffice to say that in Australia the VF telephony channel bandwidth is
specified as per ITU recommendations ie. 300-3400Hz.
Click to expand...

Good, accurate stuff, Ross.

I really don't know where Floyd L. Davidson is coming from. He is grimly
sticking to an assertion the the UK and Australian PSTN is specified as
"400 to 2800 KHz", but has been unable to quote his source in terms of
unambiguous specifications.

I can only assume - from his location - that he has some experience as a
technician in military comms systems, and has some view of the US Bell
system. Hence the belief in "400-2800". As has been fully demonstrated
by several others, this is a profound misunderstanding of the
international PSTN.

This thread has wandered far and wide from the OP's question, but it's
getting a bit too far off the subject for me. It's also degenerated into
a classic battle of the Trolls.

If Floyd wishes to make further wild and inaccurate statements, perhaps
he would like to open a new thread ?

John
 
R

Ross Herbert

Jan 1, 1970
0
On Wed, 07 Jan 2009 06:52:48 -0900, [email protected] (Floyd L. Davidson) wrote:

:>
:>Good, accurate stuff, Ross.
:
:All of it referenced the specifications for individual
:channels on various facilities. None of it had to do
:with the overall minimum allowed specification for an
:end to end connection via the PSTN.

Hold it, that's a dumb statement.

The ITU recommendation for international VF channel bandwidth is 300-3400Hz, so
why would it be necessary to "allow a minimum" bandwidth of 400-2800Hz? If the
300-3400Hz applies to ALL channels used in an end-to-end international link,
then it follows that the overall bandwidth is 300-3400Hz.

Obviously the ITU spec is greater than the so-called "minimum" of 400-2800Hz, so
it is not necessary to "allow a minimum" bandwidth of less than this.

The fact that some administrations may not have adopted the ITU recommendation
and continue to use 400-2800Hz simply means that they are not prepared to
upgrade their equipment and are therefore behind the times. Any communication
carried over such links will mean the overall bandwidth is degraded even if some
sections do conform to the ITU recommendation of 300-3400Hz.

:
:>I really don't know where Floyd L. Davidson is coming from.
:
:More than three decades in the long distance
:telecommuncations industry (but not with the Bell
:System).

And I have 37 years of Telco experience from 56 - 93 in both metro and long-line
installations. In all those years we used 300-3400Hz.
 
R

Ross Herbert

Jan 1, 1970
0
On Tue, 06 Jan 2009 19:47:17 +0000, Eeyore

:
:
:Ross Herbert wrote:

:> With regard to a POTS line the VF bandwidth is some 300 - 3400Hz - hardly
hi-fi
:> - so optimal flat frequency response is not an issue.
:>
:> The fact that the secondary impedance of the 600 ohm transformer does not
match
:> the input impedance of the sound card is totally unimportant in this
:> application.
:
:So your attitude is "it's so bad it doesn't matter messing it up even more".

Not at all. Any practical measurement of the degradation of a 300-3400Hz signal
using the 600 ohm transformer would be insignificant. If your assumption that
audio degradation when using a 600:600 transformer was valid then why is there
no available audio transformer, with a 3.5Kv isolation rating, to match a 600
ohm telephone line to a high impedance (eg. 10K ohm) input? The answer has to
be, "it's not necessary".

:
:A proper 600:600 transformer will be more expensive than a 10k:10k one too.
:

That may be true but how many readily available 10K:10K audio transformers have
a 3.5Kv isolation rating?

In order to conform to the required isolation rating of 3.5Kv between the mains
powered PC and the telephone line, the only transformer which is readily
available, and complies, is an approved 600:600 transformer.
 
T

tony sayer

Jan 1, 1970
0
Ross Herbert said:
On Wed, 07 Jan 2009 06:52:48 -0900, [email protected] (Floyd L. Davidson) wrote:

:>
:>Good, accurate stuff, Ross.
:
:All of it referenced the specifications for individual
:channels on various facilities. None of it had to do
:with the overall minimum allowed specification for an
:end to end connection via the PSTN.

Hold it, that's a dumb statement.

The ITU recommendation for international VF channel bandwidth is 300-3400Hz, so
why would it be necessary to "allow a minimum" bandwidth of 400-2800Hz? If the
300-3400Hz applies to ALL channels used in an end-to-end international link,
then it follows that the overall bandwidth is 300-3400Hz.
Click to expand...

Ummmm... those figures are not -quite- that meaningful unless we have
some sort of reference level ..

i.e. 300 to 3400 +/- ? dB ...
 
R

Ross Herbert

Jan 1, 1970
0
:
:What do you suppose happens to the bandwidth of a signal as it
:is passed through successive audio devices that are band
:limited to 300-3400 Hz?
:
:Would you expect to get 300-3400Hz +- ndB response in an end
:to end loopback test when you use white noise as the test
:signal? (n =ITU spec)
:
:Going from a handset to a PBX to MUX to a CO into the network
:and back if each device has a 300-3400Hz response the looped
:back signal should be more band limited than the first device
:in the signal chain with 300-3400Hz response originating test
:signal by quite some bit.
:

I suggest you do some research on the various ITU documents, eg.
http://www.itu.int/rec/T-REC-P/e
http://www.itu.int/rec/T-REC-O/e

or pick any relevant section from the main page
http://www.itu.int/ITU-T/publications/recs.html
 
S

Salmon Egg

Jan 1, 1970
0
:What do you suppose happens to the bandwidth of a signal as it
:is passed through successive audio devices that are band
:limited to 300-3400 Hz?
:
:Would you expect to get 300-3400Hz +- ndB response in an end
:to end loopback test when you use white noise as the test
:signal? (n =ITU spec)
:
:Going from a handset to a PBX to MUX to a CO into the network
:and back if each device has a 300-3400Hz response the looped
:back signal should be more band limited than the first device
:in the signal chain with 300-3400Hz response originating test
:signal by quite some bit.
:

I suggest you do some research on the various ITU documents, eg.
http://www.itu.int/rec/T-REC-P/e
http://www.itu.int/rec/T-REC-O/e

or pick any relevant section from the main page
http://www.itu.int/ITU-T/publications/recs.html[/QUOTE]

If you know anything about circuit theory, you understand that a
filter's frequency behavior is determined by poles in the complex s
plane where s - -a + jw and a is positive to keep stability. Whatever
the poles are for a single filter section, lining them up in tandem will
have multiple poles. The effect for your basic two-pole bandpass filter
is to sharpen the skirts and reduce your 3 dB bandwidth,

Bill
 
E

Eeyore

Jan 1, 1970
0
Floyd L. Davidson said:
ITU-T standards and Bellcore standards.
Click to expand...

Bell don't count as they're not International.

Since everyone else says ITU-T say 300-3400Hz I suspect we're seeing
another case of you being stuck in a time warp.

Graham
 
T

tony sayer

Jan 1, 1970
0
Wecan do it said:
What do you suppose happens to the bandwidth of a signal as it
is passed through successive audio devices that are band
limited to 300-3400 Hz?

Would you expect to get 300-3400Hz +- ndB response in an end
to end loopback test when you use white noise as the test
signal? (n =ITU spec)

Going from a handset to a PBX to MUX to a CO into the network
and back if each device has a 300-3400Hz response the looped
back signal should be more band limited than the first device
in the signal chain with 300-3400Hz response originating test
signal by quite some bit.


Peace
dawg
Click to expand...

Well either over long or local circuits -some- level reference needs be
quoted .....
 
M

Mark Evans

Jan 1, 1970
0
In uk.telecom Stuart said:
It is always grounded, directly or indirectly, otherwise the screen fails
to be effective.


So how does phantom power work then? You stick a voltage down the signal
leads in parallel and the the return is via what?
Click to expand...

One signal lead is connected to positive the other signal lead is
connected to negative. Much the same way as telephones are powered.
For screening to work it must not be carrying an electrical current.
 
T

tony sayer

Jan 1, 1970
0
Floyd L. Davidson said:
In the context presented, that is generally true... but
I'd like to point out that the shielding on multipair
telephone cables is 1) grounded, 2) commonly conducts
current, and 3) the noise reduction provided is
*enhanced* by the current!

Both ends of the cable are well grounded and any induced
current in the shield will be greater than in any
individual cable pair. The current in the shield
induces an opposite current into the cable pair, thus
reducing the total noise signal in each pair.
Click to expand...

Do you manage to get grounds in that soil over there, seemed to think it
was more permafrost;)..

Those pix on the Floyd website look rather, well "cool" :)...
 
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