RFX144V24-S23 and RFX96V24-S23 Modem Designer’s Guide
12-4
1070
The acoustic echo canceller is bypassed in the handset mode. However, the coefficients of the canceller are maintained in
order for the canceller to resume functioning quickly when configured back to the FDX Speakerphone Mode.
A side-tone simulator is provided which takes the mouth-piece signal through an attenuator and mixes the attenuated signal
with the far-end signal before sending it to the ear-piece. The attenuation level is controlled by the ECHOAT bits (0F:0-1):
Bit 1
Bit 0
Attenuation
0
0
∞
dB (default)
0
1
18 dB
1
0
12 dB
1
1
6 dB
12.3.5 Microphone Volume Control
The microphone volume control can be used either as a software compensator for the acoustic decoupling design as
mentioned earlier in this section, or as a true four-level volume control. Microphone volume control (compensation gain) is
added to the digital signal before the microphone AGC process. This prevents the possibility of digital overflow in the event
of large microphone compensation on top of high AGC output level. The microphone volume control bits MICLVL (0E:0-1)
can be adjusted at any time while in Speakerphone Mode (CONF = 91h).
Bit 1
Bit 0
Gain
Level
0
0
0 dB
Low
0
1
+6 dB
Medium (default)
1
0
+9.5 dB
Medium-High
1
1
+12 dB
High
An internal software limiter is attached to this volume control. If signal overflow or underflow situation happens after applying
the volume control gain, the signal will be limited to 7FFFh or 8000h in the modem depending on the signal polarity. The
default microphone volume level is "Medium" (+6 dB).
12.3.6 Speaker Volume Control
The digital speaker volume control, instead of a traditional analog potentiometer, should be used all the time in the
Speakerphone Mode. Each time the speaker volume control level is changed, the acoustic echo level is changed
accordingly. Therefore, the coefficients of the acoustic echo canceller must be scaled to reflect the echo change. Otherwise,
the acoustic echo canceller will lose tracking of the echo change and must re-adjust all the coefficients again. The full-
duplexity will be degraded until the canceller reconverges. Changing analog potentiometer setting will most likely cause the
acoustic echo canceller to go through the convergence process again.
The digital speaker volume is controlled through the VOLUP and VOLDWN bits. Each time the VOLUP bit is set to a 1, the
volume increases by 2 dB. Accordingly, each time the VOLDWN bit is set to a 1, the volume decreases by 2 dB. There are
totally fifteen 2 dB increment and decrement steps representing 30 dB volume control range. Volume control range is
programmable-see parameter 79, section 4.2. The VOLUP and VOLDWN control bits are automatically reset to a 0 after the
volume has been adjusted.
VOLUP
VOLDWN
Volume
0
0
No change (default)
0
1
Down by 2 dB
1
0
Up by 2 dB
1
1
No change
Содержание RFX144V24-S23
Страница 64: ...RFX144V24 S23 and RFX96V24 S23 Modem Designer s Guide 3 30 1070 This page is intentionally blank...
Страница 92: ...RFX144V24 S23 and RFX96V24 S23 Modem Designer s Guide 5 10 1070 This page is intentionally blank...
Страница 110: ...RFX144V24 S23 and RFX96V24 S23 Modem Designer s Guide 7 8 1070 This page is intentionally blank...
Страница 156: ...RFX144V24 S23 and RFX96V24 S23 Modem Designer s Guide 9 34 1070 This page intentionally left blank...
Страница 170: ...RFX144V24 S23 and RFX96V24 S23 Modem Designer s Guide 11 8 1070 This page is intentionally blank...
Страница 178: ...RFX144V24 S23 and RFX96V24 S23 Modem Designer s Guide 12 8 1070 This page is intentionally blank...
Страница 184: ...RFX144V24 S23 and RFX96V24 S23 Modem Designer s Guide 13 6 1070 This page intentionally left blank...
Страница 197: ......