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ERL at the CODEC is the receive level at TLP minus the transmit level at
TLP. To obtain this level as previously discussed subtract the TLP from
the level (in dBm).
The 6 dB Rule for the CELP states that in order for the built in echo can-
celler to function, the ERL at the CODEC shall be 6 dB or greater. (This
rule also applies to the VEC-1 Echo Canceller used on the Universal AD-
PCM voice modules). That is the Received signal level out of the CODEC
must be at least 6 dB higher than the reflected transmit signal into the
CODEC.
The method used for measuring the ERL is to send a tone from the CELP
receive circuit toward the hybrid and measure the amount of signal re-
flected back to the CELP transmit end. A convenient point for making
these measurements is at the interface between the CELP and the CODEC
circuit. Remember to add or subtract the transmit and receive
selections of the CELP to the measured value.
When making these measurements, always use real loads, not dummy
loads, since dummy loads may not have the same complex impedance as
the real thing.
The CELP transmits and receives tones quite well at frequencies
ranging from 300 Hz to about 3000 Hz.
The dB meter used to measure signal levels should always be in Bridged
or High Impedance mode so that it will not alter the impedance of the line
it is measuring.
The following is an example of an ERL calculation:
A tone from the CELP was measured at the Receive TIP/RING interface
at –3 dBm. The reflected signal at the Transmit TIP/RING interface was
measured at –33 dBm.
The CELP has both –16 dBm and +7 dB sections selected with no fine tun-
ing of levels (i.e., the CELP is set for TLP –16/+7). The fine tuning levels
should be used in this calculation if they are actually being used on the
CELP but have been omitted for the purpose of simplification.
1. Determine the output level of the CODEC by subtracting the receive
TLP from the TIP/RING measured receive level.
(–3) – (+7) = –10 dBm
2. Determine the ERL being presented to the CODEC by subtracting the
transmit TLP from the reflected signal level measured at the trans-
mit TIP/RING.
(–33) – (–16) = –17 dBm
3. Subtract the CODEC out level from the CODEC in level. The differ-
ence between these two levels must be ≥ 6 dB.
(–10) – (–17) = 7 dB
The ERL in the example above passes the 6 dB test, but with a safety mar-
gin of only 1 dB. You might want to do something to improve this situa-
APPENDIX D
loss/gain
D-15
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