CS6422-IS Cirrus Logic Inc, CS6422-IS Datasheet - Page 40

• Manufacturer Part Number: CS6422-IS
• Description: Speech Network Standard 12dB 20-Pin SOIC
• Manufacturer: Cirrus Logic Inc
• Type: Audio Processorr

Specifications of CS6422-IS

• Package: 20SOIC
• Maximum Transmitter Gain: 12 dB
• Typical Supply Current: 50 mA
• Typical Operating Supply Voltage: 5 V
• Applications: Speakerphones
• Mounting Type: Surface Mount
• Package / Case: 20-SOIC
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Other names: 598-1201-5

speaker, the reflections reaching the microphone
should present no more than 0.9 V
tic ADC. In fact, it is advisable to allow 6 dB or
even 12 dB of margin, such that in the above exam-
ple, the signal present at the Acoustic ADC is
250 mV
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 CS6422 offers the AGC’d
gains provided by TVol and RVol to help provide
the desired transmit and receive gains.
The CS6422 offers two different programmable
gain sources: TGain/RGain and TVol/RVol. TGain
and RGain provide analog gain at the input to the
ADC of 0 dB, 6 dB, 9.5 dB, or 12 dB. 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
desired signal, whereas the analog gain will not.
Furthermore, gains introduced by TVol and RVol
will not result in clipping, since both gains are
AGC’ed, unlike the gains at TGain and RGain
which are not.
4.3.2
The following tests are suggestions for measuring
echo canceller and half-duplex performance.
4.3.2.1
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
40
40
rms
Testing Issues
.
ERLE
rms
to the Acous-
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 signal 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 CS6422’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 Section 4.1.1.2,
“Adaptive Filter”, the CS6422 does not work opti-
mally with white noise. The best signal to use
would be a repeatable speech signal, like a record-
ing of someone counting or saying “ah.”
Use the Microcontroller Interface to disable trans-
mit and receive suppression, half-duplex, and the
Network Echo Canceller. 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.
Again, measure the rms voltage at NO. This mea-
surement gives the cancelled echo level (numera-
tor).
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.
CS6422
CS6422
DS295F1