GR47/48 Design Guidelines
BA/SEM/MS 03:0015 Rev A
LZT 123 7596
16
•
Portable Hands Free - This state activates a different
amplification factor in the GR47 and activates a microphone bias
level of 2V in ATMS when a call is in progress.
This is the default state at power-on.
Audio Circuit Electrical characteristics
All sources must be AC-coupled except the Portable HandsFree
microphone, which shall be DC-coupled in order to supply DC current
to the Portable HandsFree microphone. AC coupling prevents incorrect
biasing or damage of the ATMS input. The capacitor must have a value
greater than shown below to avoid attenuation of low frequencies.
Application driving impedance (0.3 – 3.5 kHz)
≤
300
Ω
AC coupling capacitance
≥
1
µ
F
Module input impedance (0.3 – 3.5 kHz)
> 50 k
Ω
Low frequency cut-off. (-3 dB)
300 Hz
±
50 Hz
High frequency cut-off. (-3 dB)
> 3500
±
50 Hz
Hands-Free
mode
0V
Output DC bias level
Portable
Hands Free
Mode
2.0V
±
0.1V
Additional Gain in Portable Hands Free Mode
28.5 dB
Table 1. ATMS Levels. Audio Levels
3.5.2 Advanced Portable Hands Free Functionality
This functionality consists in the detection of a push button press
connecting
ATMS
to
AGND
for a certain period of time. This will
create a change in the microphone DC bias level.
Microphone bias current at 2V
1
< 0.3 mA
Activated
2
< 100
Ω
DC impedance for push button
Deactivated
3
> 10 k
Ω
[TBC]
Portable Handsfree electrical data
AFMS is the analogue audio output from the module. When it is active,
the output is derived from the PCM digital audio by the decoder part of
the CODEC. The PCM data comes from PCMI on the system
connector.
1
Bias current >0.7mA will cause false indication of button activation (Threshold is 100mV).
2
2V x ( 100
Ω
/ ( 2700
Ω
+ 100
Ω
)) = 71 mV. Threshold is 100mV.
3
Lower impedance will reduce bias on microphone.
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