levels.txt
13399 00024 US 
Date: Fri, 13 May 83 16:44 PDT
From: Stewart.PA
Subject: Back door levels
To: VoiceProject^.pa
cc: Stewart

Here is the strategy for testing the back door levels.

General comments:

Inside the analog board, all sources should produce "the same" level and all sinks should accept this level.  This is easy for codecs and line.  However, because of the loss in the local loop, telephone sets are designed to transmit a relatively louder signal and receive a relatively weaker one.

The amplifiers which handle the telephone set and the telephone line will have to compensate for this effect.

The TeleSetSource amplifier will introduce loss to bring down the teleset transmit level to the standard level.  The TeleSetSink amplifier will introduce loss to reduce the standard level to that expected by the telephone set.

The TeleWallSource amplifier will introduce gain to boost the weak signal from the central office to the standard level.  The TeleWallSink amplifier will introduce gain to bring the standard level up so that it will compensate for the loss going to the central office.

This design strategy has a deleterious effect on hybrid performance.  Suppose the round trip loss to the CO is 10 dB.  Then the transmit and receive arms of the four wire side of our hybrid must have 5 dB gain each.  If the native hybrid had 20 dB return loss, then the final system will have only a 10 dB return loss.

What is the standard level?

According to CCITT, a test tone at 0 dBm analog should be 3.17 dB below the digital clipping level.  Touch tones are (I think) transmitted at 0 dBM and voice levels have an average power of about -10 to -13 dBm.

Lets work with voltage rather than power, so that we can ignore impedance levels for the moment.  0 dBm is 1 milliwatt in 600 ohms, corresponding to  (P = E^2/R)  0.775 v RMS or 0.775 * 1.41 * 2 = 2.19 v peak-to-peak.

I've lost the message describing the original analog board level design, so lets reconstruct it from the schematics.  Digital clipping level at the output of the 2912 (pin 4) is +/- 3.2 v or 9.3 dBm.  The resistive divider (R7, R8) connecting the codec to the crossbar has a ratio of .333, putting the peak voltage at the crossbar at 2.13 v p-p.  Thus the codec clipping level is -0.24 dB and the voltage at the crossbar corresponding to 0 dBm is 3.17 dB lower or -3.41 dBm.  The voltages involved at the crossbar are .523 v RMS or 1.48 v p-p.

The transmit section of the codec (CodecSink) has an intrinsic 3 dB gain, plus gain setting resistors, the gain is (1 + R5/R4) + 3 dB.  R5 = R4, so the overall gain is 2.82 (1/0.354).

The line-out amplifiers have a gain of .687, (1/1/455) so that 0 dB at the crossbar produces about a 1 v p-p signal at line out.  Line in has a nearly inverse gain of 1.5, to restore the standard level.

TeleSetSource has a gain of 0.386 (or -8.26 dB).  After accounting for the -3.41 dB reference level, the extraordinary gain of TeleSetSource is -4.85 dB.

TeleSetSink has a gain of 1.33 (2.5 dB), but the Telephone set receiver is fed through a 910 ohm resistor, which forms a voltage divider together with the receiver element.  The division ratio of this voltage divider must be measured.

Aside on two wire local loops.

Consider a phone system in which only the local loop is two wire and that the connection is four wire inside the subscriber equipment and inside the central office, with a hybrid at each end.  Suppose further that the local loop has 5 dB loss in each direction.  The phone company can choose to have equal transmit and receive levels on its end of the local loop, thus getting the full return loss benefit of the CO end hybrid.  The subsciber must add gain to compensate for loop loss and consequently must put up with poorer hybrid performance.

How to get the resistors right.

Measure round trip loss for a local call.  Use differential scope to measure the voltage on tip and ring at the sending and receiving ends.  Measure the loss at several frequencies.  Half the loss in dB (square root of voltage ratio) can be assigned to the loss in each direction.  Call the one-way loss X dB.

Check: Call the tone generator number and measure the differential voltage at this end.  The value should be -X dBm.

Measure the (additional) loss introduced by the hybrid by measuring the single ended signal at the transmit port and the two wire differential signal at the phone line.  The rule is that

A Triplett 630 VOM was placed in series with tip and ring on 4478 to measure the DC loop currents drawn by various units.

Bell set from 4477   36 mA
Bell set from 4478 32 mA
ITT set (reverted) 32 mA
ITT set (reverted) with ringer shunted across loop   36 mA
TeleWall ckt   33 mA
TeleWall ckt with ringer shunted across loop   29 mA

I measured the RMS dial tone voltage across tip and ring.
p-p on scope .45
RMS (fluke) .123  (blue phone off hook)
RMS (fluke) .133  (black phone off hook)

This is -16 dBm.  For a single tone it would be -19 dBm.  According to notes on the Network, Dial tone is -13 dBm per frequency, giving a CO to here loss of 6 dB.

I made frequency dependent measurements at various points, with a phone call open between 4477 (a black phone) and 4478 (A Lark with it's TeleSet unplugged.)
p-p 4478 is peak-to-peak voltage measured between tip and ring at the 4478 (Lark) end
rms 4478 is RMS voltage measured between tip and ring at the 4478 (Lark) end
rms 4477 is RMS voltage measured between tip and ring at the 4477 end
rms twsrc is RMS voltage measured at TeleWallSrc inside the analog board
rms line1 is RMS voltage measured at Line1Src inside the analog board
dB loss is the power ratio in dB between the tip-ring differential voltages at the two ends of the phone connection (rms 4478 and rms 4477).
dB twTXgain is the power ratio in dB between rms line1 and rms 4478
dB hyb is the power ratio in dB between rms line1 and rms twsrc (hybrid return loss)

The dB loss entries represent the frequency specific end-to-end loss of the local telephone call.  Half this values should be the one way loss to or from the CO.  The figures are consistant with the measurements of dial tone level above.

  f     p-p   rms   rms   rms     rms   dB   dB           dB
        4478  4478  4477   twsrc  line1  loss  twTXgain  hyb
3000   1.48   .568   .092   .198   .374   15.8   3.6          -5.5
2000   1.41   .544   .126   .147   .374   12.7   3.3          -8.1
1500   1.32   .512   .134   .105   .374   11.6   2.7          -11.0
1000   1.36   .529   .142   .147   .374   11.4   3.0          -8.1
 800   1.44   .555   .142   .176   .373   11.8   3.5          -6.5
 600   1.52   .583   .139   .200   .372   12.5   3.9          -5.4
 400   1.56   .599   .132   .209   .371   13.1   4.2          -5.0
 300   1.55   .590   .116   .199   .370   14.1   4.1          -5.4
 
 I made measurements of rms voltage across tip and ring of a call placed between 4477 and 4478.
 The instrument at the 4477 end was a standard black phone.  The instrument at the 4478 end was a standard black phone.  In addition, there were on-hook and not-reverted Larks hung on both 4477 and 4478 (The ring detect circuits might present a small load.)
 
 The rows of the tables are DTMF buttons held down as signal sources.  Thus 4478  7,8 means that the 7 and 8 buttons were held down on the phone on line 4478.  The frequencies for button 7 are  852 and 1209 Hz.  The 7 and 8 buttons together generate only 852 Hz.
 
             rms      rms   dB
            4477     4478   gain
4477 7     .976      .272   -11.1
4477 7,8   .753      .201   -11.5
 
4478  7     .287      1.068   -11.4
4478  7,8   .202      0.774   -11.7

 The same configuration, except using the reverted Blue phone at the 4478 end with the black phone unplugged.
            rms       rms   dB
            4477     4478   gain
blue  7             .812
blue  7,8           .498
4477 7     .976     .274   -11.0
4477 7,8   .753     .199   -11.6

The blue phone has about the same audio load as the black phone
The blue phone generates lower level DTMF (by 1.8 dB)

The same configuration, except using the Lark hybrid as a receiver with the black phone unplugged and blue phone un-reverted.

rms xbar is the RMS voltage appearing at TeleWallSrc
dB twRXgain is the power ration between rms xbar and rms 4478

             rms      rms   rms   dB    dB    
            4477     4478   xbar  loss   twRXgain
4477 7     .976     .192    .252   14.1   2.4
4477 7,8   .753     .140    .184   14.6   2.4

I placed a call to 9-494-0020, which is a 1000 Hz test tone.  Probably the tone is at 0 dBm, but I am not sure.  RMS voltage measurements were made accross tip and ring with a load provided by the blue, black, and Lark hybrid for respective columns.  

meas.           blue  blk   EP    xbar
test tone rms    .48   .47   .315   .413    rms voltages at various points
test tone dBm  -4.2  -4.3  -7.8   -5.5     dBm equivalents

The ratio of EP and xbar = 2.4 dB, as above.
If the tone is 0 dBm at the CO, then the local loop loss is  about 4.2 dB.  I discount this, givent he good evidnece for loss in the 5.5 to 6 dB area above.


I placed a call from the Lark at 4478 to 4477 and measured the rms voltage accross tip and ring at 4478 generated by dtmf buttons on the blue phone, both reverted and via the electonics.

Measured at tip and ring
blue 7,8 via elec phone   0.225
blue 7,8 via revert      0.493

These values are 6.8 dB different.
After applying the fixes detailed below, the value was .464 via electronics)

I then measured the voltage accross the blue phone receiver element in reverted and electronic configurations. (The tones were generated by the DTMF pad 7 button at 4477).

electronic phone  .02
reverted           .079

These values are 9.6 dB different.
After applying the fixes detailed below, the value was .067 via electronics)

I placed a "front door" call from the blue phone to itself and measured the voltages across the receiver element generated by the DTMF pad.  (Column labelled rms elec.)
I also placed a reverted back door call to 4477 and measured the voltages generated by the 4477 DTMF pad (column labelled rms reverted).  These two columns use different DTMF pads, and the blue pad is weaker than most black pads (see above).

          rms   rms
          elec  reverted
7+8     .011   .047   (12.6 dB different)   (probably wrong??)
7        .023   .069   (9.5 dB different)    checks with above test

In order to check whether the electonic version was being insufficiently powered by low talk battery, I measured the voltage between the blue phones tip and ring in both reverted and non-reverted configurations for DTMF buttons on the blue phone.

          rms   rms
          elec  reverted
7+8     .523   .502
7        .882   .821

See above that DTMF levels are about tthe same if reverted or not, so dc drive currect is sufficient to supply telephone set

Notes:

1.   The Lark presents a smaller impedance to the telephone line than a standard desk set.  This means it gets less signal from the line.  I have not measured the impedances at audio frequencies but the Lark should be 600 ohms (that is the transformer rating) and the standard phones may be 900 ohms.

Suppose the phones are 900 ohms.  We can use the measured voltages at the two ends to estimate the loop resistance as though it were a single resistance.

The voltage at the sending end is 0.976 and the voltage at the receiving end is 0.272.  The difference, .704, is developed accross the loop resistance.  .704/Rl = .272/900, so Rl = 2300 ohms.  We can apply half the value for the resistance to the CO.

These numbers are consistant.  A .976 v source will develop .275 volts across 900 ohms at the other end of a 2300 ohm loop, while the same source will develop only .202 volts across a 600 ohm load.  Thus we have a received signal loss of 2.7 dB due to the impedance difference.

The change should not affect the transmit side much.  The lower drive impedance of the Lark should actually improve the transmit levels by 0.7 dB.

Conclusion, TeleWall source gain should be increased by 3 dB.

2.   The front door levels are lower than those of a back door call by about 9.5 dB.

Connecting TeleSet to TeleSet via the crossbar gives 2.0 dB more signal than connecting them via the codecs.

What accounts for the other 7.5 dB difference?  Is it the TeleSet source or sink or some combination?

The difference between electronic phone receive level and reverted receive level is 9.5 dB.  If 3 dB is due to the Telewall receive gain, then the other 6 is due to low TeleSetSink gain.  This computation would have the TeleSetSource gain low by 1.5 dB.

The difference between electronic phone transmit level and reverted transmit level is 6.8 dB.  If 2 dB is due to the Codec chain, then the remaining 4.8 must be split between TeleSetSource and TeleWallSink.  If TeleSetSource gain is low by 1.5 dB, then the remaining 3 dB would be due to TeleWallSink.

Proposal:

Boost TeleWallSource by 3 dB
	Change R70 from 27K to 47K
Boost TeleWallSink by 3 dB
	Change R62 from 11K to 18K
Boost TeleSetSource by 1.5 dB
	Change R76 from 390 ohms to 510 ohms
Boost TeleSetSink by 6 dB
	Change R82 from 24K to 47K

Boost CodecSink gains by 1.5 dB
	Change R5 from 22K to 30K
	Change R14 from 22K to 30K

I modified two analog boards and tested one of them, giving the (after fixes) numbers above.