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2 Application Overview
This echo cancelling application uses the internal echo-cancelling
algorithm of the GR47 Advanced Hands-free Mode to create a simple car-
kit application. A block diagram of the GR47 audio system is shown in
Figure 2 which outlines the settable parameters available to the designer.
In addition Table 4 details the parameter options and default values.
The application note solution requires an external audio amplification
stage to achieve a reasonable level of volume. One proposal is outlined in
section 3.2.
A benefit of this application proposal is its simplicity as it requires little
external circuitry. However, its successful echo-cancelling has a limited
audio level range. This is dependant on the application characteristics,
surroundings and position of the speaker and microphone parts. Sony
Ericsson strongly recommend early testing of the suitability of this solution
to the customer application before committing to hardware design.
Note : If, after following the guidelines within this application note, the
echo-cancelling performance has not achieved an acceptable level,
consider implementing an external echo-cancelling circuit as presented in
Application Note LZT 123 5102.
3 Hardware Set Up
The diagram presented in Figure 3 shows the recommended physical
arrangement of a car-kit application using the GR47 advanced hands-free.
The microphone can be connected directly to ATMS and requires no
external amplification. There is sufficient selectable gain within the GR47
to achieve the desired audio level for most applications.
The microphone should be placed above the rear-view mirror in the centre
of the car at the same height as the sun-visors (distance from driver:
approximately 50cm). Alternatively, a 'goose-neck' microphone holder
can be used to position the microphone closer to the driver's mouth. This
type of mechanical extension may be required in extreme, noisy
environments and will allow the microphone gain to be greatly reduced
and the speaker output increased whilst still achieving an acceptable level
of echo-cancellation.
The speaker should be placed between 60cm and 90cm from the user's
ear, in direct line of sight. This can be achieved by mounting the speaker
in the driver's door, centre armrest, or below the dashboard (but not
obscured).
The distance between the speaker and the microphone should be greater
than 100cm (preferably not line of sight).
LZT 123 7895 R1A
Speakerphone/car kit hands free design
5
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