Cirrus Logic AN168 - Manual (Page 20)

AN168
20 AN168REV2
discussed earlier in this note.
2) Configure the CS6422 from reset with the ex-
ception of the following:
a) Mic set to '1' or '0', depending on whether
the internal mic preamp is used or not
b) HDD = ‘1’
c) GB = 0.75 dB/ms
d) TSD = ‘1’
e) ACC = ‘Cleared’
f) Taps = 55.5 ms
g) RSD = ‘1’
h) NseRmp = 12 dB/s
i) PCSen = ‘low’
j) AErle = 18 dB
k) AFNse = -42 dB
l) NECD = ‘1’
m) ASdt = -18 dB
The register configuration which implements the
above is:
reg 0: 5480 (d480 if internal mic preamp is
used)
reg 1: 0a82
reg 2: 0804
reg 3: 2006
reg 4: 5008
reg 5: 018a
3) Inject 2.8 Vpp white noise into NI (capacitively
coupled), and measure the RMS signal level at
NO. The measured signal should be band-lim-
ited with either a C-message filter or at least a
5-pole low-pass filter with a corner frequency
of 4 kHz in order to remove out-of-band noise.
4) Record the band-limited RMS voltage level at
NO.
5) Turn OFF the white noise source.
6) Set ACC to ‘Normal’.
7) Turn ON the white noise source.
8) Record the band-limited RMS voltage level at
NO after 5 seconds or more of white noise.
The ERLE is the difference in dB voltage levels of
(8) and (4):
The signal at NO can be monitored through
headphones to actually hear the echo canceler train
to the acoustic path.
If the above tests are to be repeated, it is important
to set ACC = ‘cleared’ and to turn off the white
noise source between each test, otherwise results
may be inconsistent.
Typical values obtained using the above technique
range between -9 dB and -30 dB. In general, the
closer together the speaker and mic are and,
consequently, the lower the speaker driver gain, the
better the performance.
1.5.3.2 Loop Gain Method
The loop gain method allows the worst-case ERLE
to be determined without the use of a white noise
source or an RMS voltage meter. In this method,
the adaptive filter is trained on the acoustic path
using speech or white noise, its coefficients are
then frozen, and a closed loop is formed between
TVol, NSdt, RVol, and the acoustic path, which
includes the echo canceler. As in the acoustic
coupling loop gain test, the receive volume is
incremented until howling occurs, and this number
is recorded.
The echo canceler coefficients are then cleared,
effectively removing the EC from the path, and the
receive volume is adjusted again until the point at
which howling occurs. This new value is recorded.
ERLE 20 meas 8
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20 meas 4 ( )
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