External Speaker; Handsfree Unit; Digital Signal Processor (Dsp); Audio-To-Mobile-Station - Ericsson EF738 Service Manual

Accessories

External Speaker

Unless an optional Music Mute unit is utilised the 4Ω External Speaker has to be
connected to the speaker amplifier output of the HandsFree Unit.

Handsfree Unit

The Handsfree Unit includes the following electronic blocks:

• Digital Signal Processor (DSP)

• Audio-To-Mobile-Station

• Audio-From-Mobile-Station

• Microphone Amplifier
• Speaker Amplifier
• Music Mute Control
• Power Supply
Digital Signal Processor (DSP)
The main functions of the Digital Signal Processor are to reduce the ambient audible
noise and to suppress the speaker sound after being picked up by the microphone, so
called echo cancellation. The circuitry around the DSP includes memories like
ROM, EEPROM, and SRAMs.
Audio-To-Mobile-Station
The digital output from the DSP is D/A-converted and amplified by the Audio-To-
Mobile-Station block. The audio signal passes through an analog multiplexer and a
latch when enabled, which occurs when the signal MUTE from the phone goes high,
i.e. when a conversation is initiated by the phone. The audio signal is supplied from
the latch to the phone via the ATMS pin of the System Connector.
Audio-From-Mobile-Station
The analogue audio signal from the phone is received via the AFMS pin of the Sys-
tem Connector and supplied to the Audio-From-Mobile-Station block, where the
level of the signal is reduced by an amplifier and then A/D-converted.
Microphone Amplifier
The low level signal from the microphone is amplified and filtered by the Micro-
phone Amplifier which also includes an A/D-converter. The signal level is reduced
prior to being digitised in order not to exceed the max. allowed peak voltage at the
input of the A/D-converter.
Speaker Amplifier
The digital speaker signal from the DSP is D/A-converted, fed through an analogue
multiplexer, and a latch. The final stage of the Speaker Amplifier includes the
power amplifier which is able to supply approximately 7W into a 4Ω speaker. The
+12V power supplied to the Speaker Amplifier is switched on and off by the phone
via the DCIO pin of System Connector.
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