When a headset is in serted in J940, the internal speaker an d microphone is dis abled. The user can now use the headset for handsfree conversation. BOOM_MIC_DET

informs U1000 whethe r a headset is inse rted or not. The state of BOOM_MIC_DET is controlled by Q1551.

When the headset is inserted the biasing voltage for the AUX_MIC will no longer be pr esent at the base of the NPN transistor Q1551. This will switch Q1551 on, thus,

allowing BOOM_MIC_DE T to be pulled high by source 2.75V. When the headset is not inserted, the AUX_MIC biasing voltage is present at the base of Q1551. This

condition will switch Q1551 off, thus, pulling the BOOM_MIC_DET to ground through R1540.

There are two distinct audio paths for the microphone. One is for the auxilia ry microphone(headset mic) and the other is for the internal microphone. This is not the case

with the speaker audio paths. The headset speaker and the int ernal speaker share the same audio path which comes from the GCAP(U1500). In order to disable the inter-

nal speaker when the headset is attached, the phase of the audio signal on the SPKR- line is phase shifted by 180 º. When the internal speaker is enabled the SPKR+ and

SPKR- have the same audio signal e xcept that SPKR- is phase shifted 1 80º. When a headset is attached, the SPKR- is not phase shifted 180º . The SPKR+ and SPKR-

have the same audio signal and are in phase. This condition will no t allow the internal s peaker to function, thus, disabling the inte rnal speaker.