🚀 go-pugleaf

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:
>>>>
>>>>> "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

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