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Audio Interface
36 C24 Module Hardware Description December 15, 2008
.
Audio Design
The audio quality delivered by C24 is highly affected by the application audio design, particularly
when using the analog audio interface. Therefore, special care must be taken when designing the
C24 audio interface. Improper design and implementation of the audio interface will result in
poor audio quality.
Poor audio quality is a result of electrical interferences, or noises, from circuits surrounding the
audio interface. There are several possible sources for the audio noise:
•
Transients and losses on the power supply
•
EMI from antenna radiations
•
Digital logic switching noise
Most of the audio noise originates from the DC supply current, which appear on the main power
supply lines and antenna, but also indirectly penetrate the internal application's supplies and
signals. The noises are transferred into the C24's audio circuits through the microphone input
signals and then are amplified by the C24's internal audio amplifiers.
To minimize the audio noise and improve the audio performance the microphone and speaker
signals must be designed with sufficient protection from surrounding noises.
The following guidelines should be followed to achieve best audio performance:
•
Reference the microphone & speaker circuits to the C24 AGND interface signal.
•
Keep the audio circuits away from the antenna.
•
Use RF filtering capacitors on the audio signals, as described in Table 2-3 .
•
The audio signals should not be routed adjacent to digital signals.
•
Isolate the audio signals by a surrounding ground plane or shields.
•
Filter internal supplies and signals that may indirectly affect the audio circuits, from noises
and voltage drops.
Table 2-15: Microphone and Headset Microphone Port Gains
AT Command
Value
Gain Level
(dB)
Maximum Input
Level (mV
RMS
)
0 9 350
... ... ...
3 12 250
... ... ...
16 25 55
... ... ...
31 36 11