Datasheet

Mic2+
Mic2-
Mic1+
Mic1-
470 nF
470 nF
470 nF
470 nF
OUT-
LPF
LMV1089
OUT
+
Osc2
Osc1
AC Voltmeter
LMV1089
www.ti.com SNAS441H –SEPTEMBER 2008–REVISED MAY 2010
TEST METHODS
Figure 7. FFNS
E
, NFSL
E
, SNRI
E
Test Circuit
FAR FIELD NOISE SUPPRESSION (FFNS
E
)
For optimum noise suppression the far field noise should be in a broadside array configuration from the two
microphones (see Figure 34). Which means the far field sound source is equidistance from the two microphones.
This configuration allows the amplitude of the far field signal to be equal at the two microphone inputs, however a
slight phase difference may still exist. To simulate a real world application a slight phase delay was added to the
FFNS
E
test. The block diagram from Figure 7 is used with the following procedure to measure the FFNS
E
.
1. A sine wave with equal frequency and amplitude (25mV
P-P
) is applied to Mic1 and Mic2. Using a signal
generator, the phase of Mic 2 is delayed by 1.1° when compared with Mic1.
2. Measure the output level in dBV (X)
3. Mute the signal from Mic2
4. Measure the output level in dBV (Y)
5. FFNS
E
= Y - X dB
NEAR FIELD SPEECH LOSS (NFSL
E
)
For optimum near field speech preservation, the sound source should be in an endfire array configuration from
the two microphones (see Figure 34). In this configuration the speech signal at the microphone closest to the
sound source will have greater amplitude than the microphone further away. Additionally the signal at
microphone further away will experience a phase lag when compared with the closer microphone. To simulate
this, phase delay as well as amplitude shift was added to the NFSL
E
test. The schematic from Figure 7 is used
with the following procedure to measure the NFSL
E
.
1. A 25mV
P-P
and 17.25mV
P-P
(0.69*25mV
P-P
) sine wave is applied to Mic1 and Mic2 respectively. Once again,
a signal generator is used to delay the phase of Mic2 by 15.9° when compared with Mic1.
2. Measure the output level in dBV (X)
3. Mute the signal from Mic2
4. Measure the output level in dBV (Y)
5. NFSL
E
= Y - X dB
SINGLE TO NOISE RATIO IMPROVEMENT ELECTRICAL (SNRI
E
)
The SNRI
E
is the ratio of FFNS
E
to NFSL
E
and is defined as:
SNRI
E
= FFNS
E
- NFSL
E
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