User Manual

GM47/48 Technical description

BA/SEM/MS 02:0004 Rev B

ATMS and AFMS are the audio input and output for the module. The

analogue audio signals can be used in two different modes, Normal

and Portable Handsfree.

Handsfree

This mode is referred to as Audio To Mobile Station (ATMS) and

Audio From Mobile Station (AFMS). It is used by audio accessories

such as Handsets and Handsfree equipment.

Portable Handsfree

This mode activates a different amplification factor in the Mobile

Equipment (ME). It also activates a microphone bias level in ATMS.

This is the default mode.

3.6.1 Audio To Mobile Station (ATMS)

ATMS is the analogue audio input to the module. It connects to the

audio input of the CODEC in the module. The CODEC then converts

the analogue audio to digital audio, in PCM format, which is

connected to the internal PCM bus in the module. The internal PCM

bus connects the encoded audio to PCMO on the system connector.

ATMS is also used as the microphone input from the Portable

Handsfree. If this is the case, a DC bias is provided from the ATMS.

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.

The ATMS input is a passive network followed by the transmit part of

the CODEC.

Parameter Limit

Application driving impedance (0.3 - 3.5 kHz)

< 300 Ω

AC coupling capacitance

> 1 µF

Module input impedance (0.3 - 3.5 kHz) >50ΚΩ

Low frequency cut-off (- 3 dB) 300 Hz ± 50 Hz

High frequency cut-off (- 3 dB) > 3500 Hz

Maximum allowed input level 1.5V

= 530mV

Output DC bias level Handsfree mode 0 V

AC coupling capacitance must be supplied by the application, unless a DC coupled microphone

is used.