GSM/GPRS Modules
GSM Module Audio Design Guide
GSM Module Audo Design Guide Confidential/Release
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Appendix A
Table 2
：
Reference Document
NO.
Name
Remark
[1]
M1x_AT_Commands_Manual
AT Commands Manual
[2]
Mxx_Hardware_Design
Hardware Design
Table 3
：
Description of ECHO and TDD noise
Noun
Explanation
ECHO
Talking quality can be obviously affected when echo problem is present.
Description of ECHO issue: The far end could hear its own voice from the
module side (the near end) in talking.
ECHO issue can be caused by the near end of some reasons, such as
receiving circuit coupling to the microphone circuit, unsealed mechanical
structure, high SPK loud voice, or high sensitivity microphone. ECHO issue
has a negative effect on talking.
TDD noise
TDD noise could be present at the far end of the near end while talking. RF
power aptitude generates a burst keeping 576us in every 4.615ms. It could
be coupling to audio circuit. The envelope curve of the RF burst could be
present due to filtering effect and make a noise in the audio circuit path as a
constant pulse at 217Hz and its harmonic frequencies. Another of the reason
is from the power supply. The burst consumption of current can cause
obvious ripple at the supply voltage at 217Hz. If the ripple at the supply
voltage conducts to audio circuit through power supply or ground, TDD noise
could be present at the far end or the near end.
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