Audio Design - Motorola G24 Developer's Manual

Audio Interface
.

Audio Design

The audio quality delivered by G24 is highly affected by the application audio design,
particularly when using the analog audio interface. Therefore, special care must be taken when
designing the G24 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 GSM transmit burst current surges (217 Hz TDMA
buzz), 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 G24's audio circuits
through the microphone input signals and then are amplified by the G24'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.
40
Table 2-15: Speaker Port Gains for Speech (Cont.)
AT Command
Value
4
5
6
7
Table 2-16: Microphone and Headset Microphone Port Gains
AT Command
Value
0
...
3
...
16
...
31
Module Hardware Description
0dBm0 Output
0dBm0 Gain
Level (mV
Level (dB)
-24
-21
-18
-15
0dBm0 Input
Maximum Input
Level
Level (mV
(mVRMS)
87
...
63
...
15
...
4
)
RMS
50
70
100
140
)
RMS
350
...
250
...
55
...
11
January 1, 2006