1. Material prepared by W. Grover (1998-2002) 1 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Network Loss Planning Received Volume Control –Subscribers must have a received signal level within an appropriate range. –i.e. Not too loud and not to quiet. Stability or Oscillation Control: “Singing” –Manage reflections that can result if there’s a poor mismatch of the 2-wire line impedance and the hybrid balance impedance. –Singing can result. Talker Echo –Talker should not hear his/her own voice reflected back (with a significant enough delay).

2. Material prepared by W. Grover (1998-2002) 2 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Volume Objectives Reference Equivalent (RE) or Overall R. E. (ORE) –A measure of perceived loudness of the signal. –ITU in Geneva used group of telephone users to judge loudness. –Measured by adjusting an attenuator in a simulated network. –Rated “highest tolerable volume”, “preferred volume” and “lowest tolerable volume”. –Results showed that attenuator settings of 21dB were too faint.

3. Material prepared by W. Grover (1998-2002) 3 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Overall Loudness Rating (OLR) New standard circa 1990. Loss accumulated from speaker’s mouth and listener’s ear. OLR = SLR + CLR + RLR SLR – Send Loudness Rating CLR – Circuit Loudness Rating RLR – Receive Loudness Rating Mouth to Interface Loss Interface to Interface Loss Interface to Ear Loss OLRGood/ExcellentPoor/Bad 5-15dB90% 1% 20dB80% 4% 25dB65% 10% 30dB45% 20%

4. Material prepared by W. Grover (1998-2002) 4 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Stability Long distance connections all have 2-W to 4-W to 2-W conversion (as do most local connections). If there’s a poor mismatch of the 2-W line impedance with the hybrid balance impedance, signal energy passes across the hybrid reflecting from one 4-W direction into the other. 2-wires Hybrid Amplifier 2-wires receivetransmit Amplifier 2-wires receivetransmit Reflection (Z B  Z L )

5. Material prepared by W. Grover (1998-2002) 5 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Stability (2) balance return lossBRLB SReflection at the hybrid re-inserts the signal with “balance return loss” (BRL or B S ) into the return side of the 4-W loop. Minimum return loss seen at the hybrid in any frequency in the voice-band Additional 3+dB loss at hybrid when converting 4-W signal to 2- W signal, and another 3+dB going from 2-W to 4-W (6db total). Total trans-hybrid loss of returned signal: Ideal loss Loss in practice (~3.5 db splitting loss)

6. Material prepared by W. Grover (1998-2002) 6 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Stability (3) 3dB B S +6dB G Net Gain of one side of 4-W loop (total amplifier gain minus line losses) T 2-W to 2-W total attenuation

7. Material prepared by W. Grover (1998-2002) 7 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Stability (4) singing marginTotal round-trip closed loop loss (“singing margin”): Generally found to be adequate if: singingOtherwise, singing may result. –out of control runaway oscillation in the loop. –can continue even after the original impulse ceases.

8. Material prepared by W. Grover (1998-2002) 8 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Stability (5) Loss in a 4-W circuit may depart from its nominal value for a number of reasons: –Variation in line losses and amplifier gain with time, temperature, etc. –Gain or loss will differ at different frequencies (usually tested at 800 Hz and/or 1600 Hz). –Measurement errors. –Circuit errors.

9. Material prepared by W. Grover (1998-2002) 9 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Stability (6) Define new term for variance of round trip loss: Variance of trans-hybrid loss Variance of gain or loss in each 4-W section Number of 4-W sections What if we want only 0.1% chance of singing? 2(T+B S ) Recall: We have singing if m = 2(T+B S ) < 6dB Recall: 3 standard deviations from the mean is equivalent to 0.1% 6dB 0.1% 3  Tot

10. Material prepared by W. Grover (1998-2002) 10 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Stability (7) Typical values: This is basis for a generally accepted approximation or rule of thumb for 99.9% chance of stability (i.e. no singing):

11. Material prepared by W. Grover (1998-2002) 11 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Stability (8) Example: We have a long distance circuit with 6 4-W sections. What is the minimum attenuation (T) required for a 99.9% chance of not singing?

12. Material prepared by W. Grover (1998-2002) 12 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Echo-Delay If the reflection at the hybrid is strong enough, telephone users will hear it. Talker echoTalker echo is when talker hears his/her own voice. Listener echoListener echo is when listener hears talker’s voice twice. TalkerListener B e +6dB T Talker Echo: Loss = B e + 2T Listener Echo: Loss = 2B e + 2T

13. Material prepared by W. Grover (1998-2002) 13 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Echo-Delay (2) Recall B s : –Balance Return Loss –Minimum return loss seen at any voice-band frequency What is B e ? –Hybrid Echo Return Loss –Average return loss in voice-band. Frequency B(f) Return Loss at Frequency f B e (echo) B S (stability) Why B e and not B S ?

14. Material prepared by W. Grover (1998-2002) 14 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Echo-Delay (3) SubjectiveSubjective annoyance of echo depends on relative echo level and on the delay. –The stronger the echo and the longer the delay, the more troublesome the echo. One-Way DelayLoss to Satisfy 50% 10 ms11.1 dB 20 ms17.7 dB 30 ms22.7 dB 40 ms27.2 dB 50 ms30.9 dB

15. Material prepared by W. Grover (1998-2002) 15 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Echo-Delay (4) The echo objective is for 99% of all connections to have acceptable or better echo effects. Factors to consider: B e = Expected hybrid echo return loss.  Be = Standard deviation of B e T = Nominal 2W-2W loss in connection.  T = Standard deviation of T Ē(t) = E = Expected echo attenuation for delay t at which 50% of users find echo tolerable.  E = Standard deviation of E

16. Material prepared by W. Grover (1998-2002) 16 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Echo-Delay (5) For the connection to be acceptable, we want: Mean margin against objectionable echo M has a standard deviation: What if we want a 99% chance of acceptable echo? Recall: 2.33 std. dev. from the mean is equivalent to 1% 1% 0 2.33  M E

17. Material prepared by W. Grover (1998-2002) 17 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering In-Class Example