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RFX144V24-S23 and RFX96V24-S23 Modem Designer's Guide
12.2.2 Microphone and Speaker Gains
When the placements of the microphone and speaker are fixed, the coupling characteristics through open air is basically
determined. The analog pre-amplification gain for the selected microphone element must be determined so that the input
signal falls within the normal input range of the codec when talking loud at one foot distance from the microphone. Two
clamping diodes should be used on the microphone output signal before the DC-blocking (see Figure 14-4).
Minimum comfortable speaker loudness for a specific application determines the minimum amplification gain in the speaker
driver. This gain can be increased as long as condition (2) stated above is met. If the gain for the minimum desired loudness
violates the condition, the microphone pre-amplification gain must be reduced and then compensated up to 12 dB through
the software microphone volume control (see section 12.3.6). Each time a gain is added in the software, the speakerphone
becomes more switching depending on the other operating conditions. This is the penalty caused by a poor acoustic
separation.
12.2.3 Hybrid Interface
In order for the line echo canceller to function properly, the hybrid interface must be linear and condition (1) stated above
must be met. It is recommended that the same gain factor and frequency response be used as used in Figure 14-3 for
normal 2-wire telephone interface.
12.3 SPEAKERPHONE CONTROL
While in configuration 91h, the speakerphone control bits at interface register locations 0Eh and 0Fh can be used for most
common speakerphone function controls in real time. Overwriting these registers with different control values will either alter
the operating parameters or reconfigure the speakerphone into different operation modes. These include mode select,
volume control, muting, tone transmit, AGC enable/disable and handset operation. Other advanced features are also
provided allowing more controls over the speakerphone voice processing (see Section 4.2).
12.3.1 Speakerphone Mode Registers
Interface Register 0Eh (Default = 15h):
Bit 0-1
MICLVL (Microphone Volume Level).
These two bits control the microphone volume level.
Bit 2:
MUTEX (Mute XIA ADC).
When set, the microphone is muted.
Bit 3:
MUTEI (Mute IIA ADC).
When set, the speaker is muted.
Bit 4:
SP/HS (Speakerphone/Handset Switch).
1 = speakerphone
0 = handset (replacing speaker/microphone setup.)
Bit 5:
VOLDWN (Volume Down).
When set, the speaker volume is decreased by 2 dB.
Bit 6:
VOLUP (Volume Up).
When set, the speaker volume is increased by 2 dB.
Bit 7:
TTONEE (Tone Transmit Enable).
When set, tone transmit mode is enabled.
Interface Register 0Fh (Default = 18h):
Bit 0-1
ECHOAT (Handset Echo Attenuation).
These two bits control the simulated handset echo level.
Bit 2:
N/A (Reserved).
Bit 3:
AGCXE (AGC in XIA ADC Enable).
When set, the microphone AGC is enabled.
Bit 4:
AGCIE (AGC in IIA ADC Enable).
When set, the speaker AGC is enabled.
12-2
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