🚀 go-pugleaf

"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:
>>>
>>>> "Alan Rutlidge"
>>>> "Arny Krueger"
>>>>>
>>>>>>
>>>>>> It takes significant distances for audio electrical signals to
>>>>>> demonstrate things like reflections and standing waves. For most
>>>>>> practical purposes, they never happen because by the time the audio 
>>>>>> goes
>>>>>> that far in the modern world, its probably already been digitized.
>>>>>>
>>>>>>
>>>>> Digitising an audio signal does not eliminate propagation delay if the
>>>>> signal is transmitted over a long distance.  To demonstrate this in
>>>>> combination with the problem of impedance mismatch - make an 
>>>>> international
>>>>> telephone call to a third world country like India.  The echo you hear 
>>>>> is
>>>>> usually 100% attributable to poor impedance mismatch at the other end 
>>>>> of
>>>>> the line.  And guess what?  It's all occurring at audio frequencies 
>>>>> (300Hz
>>>>> to 3400Hz).
>>>>
>>>>
>>>>  ** ROTFLMAO   !!!!
>>>>
>>>> Does this Rutmaniac, fucking idiot actually teach such monumental
>>>> **BULLSHIT**  to IT industry trainees  ????
>>>>
>>>> Only the dullest teenagers would fall for such utter shite !!!
>>>>
>>>>
>>>> FYI
>>>>
>>>> Telecommunications data is digital ( not audio) and travels to places 
>>>> like
>>>> India via optical fibre and geo-stationery satellites  -  not thousands 
>>>> of
>>>> miles of bloody co-ax !!
>>>>
>>>> What a complete MORON  !!!!
>>>>
>>>>
>>>>
>>>> ..........  Phil
>>>
>>> So Phil. It is digital right up to the phone at the far end, is it? That
>>> phone doesn't have a hybrid transformer, does it? That transformer 
>>> doesn't
>>> have a limited sidetone spec, does it? The match of phone to line is
>>> perfect is it? There is no signal reflection from that phone, is there?
>>> That reflected power isn't digitised by the codec is it? That digitised
>>> signal isn't audible at the sending end as an echo is it? And the fact 
>>> that
>>> these signals might use satellites or optical fibre somehow magically 
>>> makes
>>> all these reflected signals disappear, does it?
>>>
>>> Only only planet Phil does the world work this way.
>>>
>>> d
>>
>> Don,
>>
>> Fair go mate.(wink)  You don't honestly thinks a toaster repairman needs 
>> to know much about impedance mismatch do you?  :P
>
> Well actually I am afraid that Hybrid leakage is not actually anything to 
> do with impedance mis-match but rather more to do with Gain of a 
> Transmission System and the way a Hybrid works - The gain of the System 
> can (obviously) be affected by Impedance etc however the echo is not the 
> result of reflected signals due to Impedance mismatch but rather from the 
> fact that a hybrid is not perfect.
> The term signal reflection is often used to refer to hybrid leakage but 
> (and here is the rub) it is not Impedance reflection.
>
> http://www.commsdesign.com/design_corner/showArticle.jhtml?articleID=16502325
>
> http://www.iec.org/online/tutorials/echo_cancel/topic02.html
>

Perhaps the issue of reflected power in an impedance mismatched circuit has 
gone a bit too far and turned the corner.

Any energy not absorbed by a termination must go somewhere - agreed?

When the source and load impedances are matched all the power developed by 
the source will be transferred to the load without loss.  As soon as either 
of the impedances vary relative to the other an impedance mismatch occurs. 
Under such conditions not all of the power developed by the source will be 
transferred to the load.   This results in a loss of power which is directly 
related to the degree (ratio) of impedance mismatch.

Reflection loss is the logarithmic ratio of the incident power to the load, 
that is, the ratio between the power that would be absorbed under correctly 
matched conditions, to the power actually absorbed by the load.

Therefore :

Loss due to reflection (dB) = 10 x log(base10) Incident Power / Reflected 
Power

or in terms of the impedances -

Loss due to reflection (dB) = 10 x log(base10) (Z1 + Z2) sq / 4Z1 x Z2

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