Material prepared by W. Grover (1998-2002) 1 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering

Material prepared by W. Grover (1998-2002)

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EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering

Network Loss PlanningNetwork 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).


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Volume ObjectivesVolume 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 <6dB were too

loud and >21dB were too faint.


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Overall Loudness Rating (OLR)Overall Loudness Rating (OLR)

• New standard circa 1990.

• Loss accumulated from speaker’s mouth and listener’s ear.

• OLR = SLR + CLR + RLRSLR – Send Loudness RatingCLR – Circuit Loudness RatingRLR – Receive Loudness Rating

Mouth toInterface

Loss

Interface toInterface

Loss

Interfaceto EarLoss

OLROLR Good/ExcellentGood/Excellent Poor/BadPoor/Bad

5-15dB 90% 1%

20dB 80% 4%

25dB 65% 10%

30dB 45% 20%


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StabilityStability

• 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 2-wiresHybrid Hybrid

Amplifier

2-wires 2-wires

receivetransmit

Amplifier

2-wires 2-wires

receive transmit

Reflection(ZB ZL)


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Stability (2)Stability (2)

• Reflection at the hybrid re-inserts the signal with “balance balance return lossreturn loss” (BRLBRL or BBSS) into the return side of the 4-W loop.

1020log B LS

B L

Z ZB

Z Z

Minimum return loss seen atthe 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:3 3STHL dB B dB

6STHL B dB Ideal loss

7STHL B dB Loss in practice (~3.5 db splitting loss)


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3dB

3dB

BS+6dB

G

Net Gain of one side of 4-W loop(total amplifier gain minus line losses)

T2-W to 2-W total attenuation

6T dB G


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• Total round-trip closed loop loss (“singing marginsinging margin”):

2( 6 )Sm B dB G 2( )Sm T B

• Generally found to be adequate if:

2( ) 6ST B dB

• Otherwise, singingsinging may result.– out of control runaway oscillation in the loop.– can continue even after the original impulse ceases.


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• 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.


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• Define new term for variance of round trip loss:

2 2 22 (2 )STot B Tn 2

SB Variance of trans-hybrid loss

2T Variance of gain or

loss in each 4-W sectionn Number of 4-W sections

• What if we want only 0.1% chance of singing?

2(T+BS)

Recall:We have singing ifm = 2(T+BS) < 6dB

Recall:3 standard deviationsfrom the mean isequivalent to 0.1%

6dB

0.1% 3Tot

2( ) 3 6S TotT B dB


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Typical values: 2.5SB

dB 1T dB 6SB dB

2 2 22 (2 )STot B Tn 2 22 (2 )

STot B Tn

2 22 2.5 (2 1)n 12.5 4n

2( ) 3 6S TotT B dB 2( 6) 3 12.5 4 6T n

1.5 12.5 4 3T n

This is basis for a generally acceptedapproximation or rule of thumb for 99.9%chance of stability (i.e. no singing): 4 0.5T n


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• 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?

4 0.5T n

4 0.5 6T

7T dB


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Echo-DelayEcho-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.

Talker Listener

Be+6dB

TTalker Echo:

Loss = Be + 2T

Listener Echo:

Loss = 2Be + 2T


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Echo-Delay (2)Echo-Delay (2)

• Recall Bs:– Balance Return Loss– Minimum return loss seen at any voice-band frequency

• What is Be?– Hybrid Echo Return LossHybrid Echo Return Loss– Average return loss in voice-band.

Frequency

B(f)

Return Lossat Frequency f

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( ) ( )( ) 20log

( ) ( )B L

B L

Z f Z fB f

Z f Z f

Be (echo)

BS (stability)Why BWhy Bee and not B and not BSS??


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• 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 DelayOne-Way Delay Loss to Satisfy 50%Loss to Satisfy 50%

10 ms 11.1 dB

20 ms 17.7 dB

30 ms 22.7 dB

40 ms 27.2 dB

50 ms 30.9 dB


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• The echo objective is for 99% of all connections to have acceptable or better echo effects.

• Factors to consider:• Be = Expected hybrid echo return loss.

Be = Standard deviation of Be

• 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


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• For the connection to be acceptable, we want:

2eE B T 2 0eM B T E

Mean margin against

objectionable echoM has a standard

deviation:M 2 2 2 2(2 )

eM B T En

What if we want a 99% chance of acceptable echo?

Recall:2.33 std. dev.from the mean isequivalent to 1% 1%

0

2.33M

2eM B T E E 2eB T

2 2.33e MB T E


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In-Class ExampleIn-Class Example

Documents

Material prepared by W. Grover (1998-2002) 1 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering