[Asterisk-Users] Why echo occurs

[Rich Adamson]

> > The sidetone is 'always' generated within analog and digital phones.
> > It never comes from any source outside the phone. In analog phones,
> > it derived from the hybrid within the phone. On digital phones, its
> > basically firmware.
> 
> I never said that sidetone was generated outside the phone.

Your original posting said the sidetone was coming from the distant
phone and did not even come close to implying that sidetone is
something always engineered into the local phone, regardless of
whether its analog or digital. Sidetone is always local phone
generated by design.

> The hybrid is the conversion from the dual channel (4 wire,
> transmit/receive) to the single channel (2 wire, the POTS line).
> 
> The "audio injection point" that I was talking about in my
> previous email, is the location of the hybrids.  The hybrid is
> supposed to automaticaly cancel echo, but it takes precise
> impedance matching to pull it off.

A 100% perfect hybrid would never generate any feedback or echo.
But, to date no one has been successful at designing such a beast.
So a better way to say that is "imperfections in the hybrid can
cause echo" as opposed to "the hybrid is supposed to automatically
cancel echo". There is no such thing as an echo canceller in a
hybrid.

> In an analog phone, the sidetone is a side-effect of the hybrid.

Not true at all. Sidetone _is_ designed into the hybrid in analog
phones on purpose and has been for for at least 30 years.

> In a digital phone, the sidetone is on purpose.

Just exactly like the analog phones.
 
> > The conversion from four-wire (analog or digital) to two-wire requires
> > the use of a hybrid (physical component in analog phones, mostly
> > firmware in digital phones).
> 
> The hybrid is an analog device.  

Not true. Better take a look at the Silicon Labs chip sets that are
used in the digium TDM card (as one example). The hybrid is 100%
digital.

> When I am talking digital, I am
> talking about technology like ISDN.  In a single bearer channel,
> I get 56Kbps out and 56Kbps in.  I do not see an echo of the
> output on the input.  (This would cause massive issues when used
> as a data call.)  The echo comes when and if I hit a conversion
> to analog then hit a hybrid.  If the conversation is happening
> purely digital end to end, then you will not get echo.  Just like
> IP to IP.
> 
> Say I have a PRI into the PSTN.  I call a friend who has POTS service.
> Now days, the path will be digital from my PRI all the way to my
> friend's central office.  At that point it gets split off the trunk,
> converted to analog, passed through a hybrid, and placed on the wire
> pair to my friend's house.  Then, through the hybrid in his phone.
> So, the echo I hear is from the hybrid in the central office and
> the echo my friend hears is from the hybrid in his phone, which is
> so close to him, that it becomes sidetone.

In most current-day CO hardware (line cards included), the echo that 
"we" hear from the distant end is almost always associated with the 
phone hybrid, not the CO hardware. (Cellular and other types of non-
telco stuff can be very different, and shouldn't be used as the basis
for evaluting echo.)