參數(shù)資料
型號(hào)： CD6420
廠商： Cirrus Logic, Inc.
英文描述： FULL DUPLEX SPEAKERPHONE CHIP
中文描述： 的全雙工免提通話芯片
文件頁(yè)數(shù)： 29/52頁(yè)
文件大?。?/td> 815K
代理商： CD6420
CS6420
DS205PP2
29
The basic constraint on getting good echo canceller
performance is that the maximum output should
not clip when coupled to the input. For example, if
in a speakerphone, AO provides 1 V
rms
to a speak-
er, the reflections reaching the microphone should
present no more than 1 V
rms
to the Acoustic ADC.
In fact, it is advisable to allow 6 dB or even 12 dB
of margin, such that in the above example, the sig-
nal present at the Acoustic ADC is 250 mV
rms
.
After this coupling level is established, the desired
signal gain must be established. To continue from
the previous example, the transmit gain must be ad-
justed to make sure the near-end talker is easy to
hear at the far-end. If the signal from the near-end
talker clips at the ADC, it is not significant to the
echo path because the AEC should not be updating
anyway.
In general, to minimize noise, system gain should
be concentrated before the ADC. However, this is
not practical in all cases, mostly because of the cou-
pling constraint. The CS6420 offers the AGC’d
gains provided by TVol and RVol to help provide
the desired gain.
The CS6420 offers two different programmable
gain sources: TGain/RGain and TVol/RVol. TGain
and RGain switch in different size sampling capac-
itors at the ADC to provide a choice of 0 dB, 6 dB,
9.5 dB, and 12 dB of analog gain. TVol and RVol
introduce digital gain and attenuation in 3 dB steps.
The difference is significant in that the digital gain
will gain up the noise of the ADC as well as the de-
sired signal, whereas the analog gain will not.
Testing Issues
The following tests are suggestions for measuring
echo canceller and half-duplex performance.
ERLE
Echo Return-Loss Enhancement (ERLE) is a mea-
sure of the attenuation that an echo canceller pro-
vides. The number is an expression of the ratio of
the level of signal without the echo canceller com-
pared to the level of signal with the echo canceller.
When measuring ERLE, it is important that any po-
tential acoustic loops be broken; so to measure the
ERLE of the Acoustic Canceller, the NO output
should be disconnected from the rest of the net-
work. This will prevent feedback which could oc-
cur when all of the CS6420’s failsafes are disabled.
The following example outlines the steps necessary
to measure the ERLE of the acoustic echo cancel-
ler.
It is important to choose a good test signal for the
tests to be valid. As mentioned in the
Adaptive Fil-
ter
section, the CS6420 does not work optimally
with white noise. The best signal to use would be a
repeatable speech signal, like a recording of some-
one counting or saying “ah.”
Use the Microcontroller Interface to disable trans-
mit and receive suppression as well as half-duplex.
Allocate all the taps to the Acoustic Canceller in or-
der to allow full-duplex without the Network Can-
celler being trained. The gains should be set
appropriate for good system performance.
The first measurement is a baseline figure of per-
formance with no echo canceller. Use the Micro-
controller Interface to clear the acoustic canceller
coefficients. Inject the test signal at NI and measure
the rms voltage at NO. This measurement gives the
baseline coupling level (denominator).
Use the Microcontroller Interface to set the acous-
tic canceller coefficients to normal which will al-
low the adaptive filter to adapt. Inject the test signal
at NI and allow a few seconds for the filter to adapt.
Measure the rms voltage at NO. This measurement
gives the cancelled echo level (numerator).
Convert both voltages to decibels and subtract the
echo cancelled level from the baseline level to cal-
culate the ERLE. At the factory, with known good
components, we typically see 30 dB of ERLE with
speech.
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