[Asterisk-Users] Re: Chaining more than one zap echo canceller?

[Rich Adamson]

> > A transformer would not work correctly on a phone line due to talk path
> > being a DC voltage.  1:1 transformers only work with AC voltage.  
> > 
> Yes it would work - otherwise hybrids, and the transformers in my Adtran 
> interfaces, wouldn't work. The voice and ringing signals are AC voltages 
> riding on top of a DC battery voltage and will pass the transformer 
> quite nicely. However, that being said, don't count on a simple 
> transformer doing the job. It would screw up other things in the 
> interface which depend on the presence of the DC voltage (like line 
> signaling) - and you wouldn't want a 1:1 transformer anyway.
> 
> Impedance matching of this kind is non-trivial. Impedance is frequency 
> dependent, and matching the impedance over frequency is really difficult 
> - especially since the phone line is a distributed impedance, i.e., the 
> R, L and C that make up the impedance are not all in one place like they 
> are in a coil or a capacitor. Phone lines are classical transmission 
> lines, the theory for which was worked out by Lord Kelvin in the 1890's 
> when planning the first transatlantic telegraph cable (and was later 
> applied, amazingly, to the transmission of nerve impulses). Line 
> impedance refers to the "characteristic" impedance of the transmission 
> line. This is the transverse impedance, that is, the impedance looking 
> across the line. It depends upon the longitudinal resistance per unit 
> length and the capacitance per unit length between the wire pair. 
> Inductance is usually not much of an issue (unless LOTS of the line is 
> coiled up somewhere or unless frequencies over 100MHz are involved). If 
> the line is terminated in its characteristic impedance, it will appear 
> perfectly resisistive and will optimally transmit energy into the load. 
> If it is not terminated properly, there will be reflections down the 
> line (i.e., echo at the far end). It is constant at a given frequency 
> because of the distributed nature of the RLC on the line - the more 
> distant elements make a smaller and smaller contribution. On the other 
> hand, the longitudinal resistance, that is, the resistance measured from 
> one end of the line to the other, is not constant but depends upon the 
> length of the line and the resistance per unit length. Line attenuation 
> is highly dependent on longitudinal resistance.
> 
> However, none of this is really the issue with echo at the caller's end 
> of the line. The impedance mismatch at the near end hybrid, while 
> causing reflections and non-optimal coupling, really has its effect by 
> introducing a phase shift in the transmitted signal. When the hybrid 
> attempts to subtract the correct portion of the transmitted signal (the 
> sidetone) from the received signal, echo cancellation is compromised 
> owing to this phase shift. (As an exercise, try subtracting one sine 
> wave from another when one of them is phase shifted. You cannot get a 
> zero result no matter how you adjust the amplitudes.) In other words, to 
> get your impedance matcher to work, you will need to match impedance 
> over frequency in such a way as to eliminate any unexpected phase shift, 
> otherwise cancellation will not be improved. In fact, if you are not 
> very careful, you may just make things worse.
> 
> Stephen R. Besch

And, for those that actually understood even a piece of Steve's words
above, think about how interrupt service latency (and/or pci controller
delays) impact the TDM card (and associated echo can routines). If the
interrupt service latency has any amount of jitter, attempts to cancel
the echo (by digitally substracting the reflection) quickly falls outside
the operating limits of the cancellation routine.

As has been mentioned at other times on this list, telco's use to use
a four-winding transformer (two primary and two secondary) that passed
the DC component through, but the 1:1.5 winding ratio transformed the
impendance from 600 ohms to 900 ohms very well. (I went searching for
these old repeat coils some time ago and no manufacturers them anymore.
Several sites will sell a junk box full of them, but modern electronics
have essentially replaced those bulky transformers.) Other impedance
matching transformer winding ratios use to be available as well.

Rich