Receive Signalling Circuits
6.3-16 Theory of Operation
7.3 Alert Tone Circuits
When the software determines that it needs to give the operator an audible feedback (for a good key
press, or for a bad key press), or radio status , it sends an alert tone to the speaker.
It does so by sending SPI BUS data to U0103 which sets up the audio path to the speaker for alert
tones. The alert tone itself is generated by the AFIC.
The allowable internal alert tones are 410, 820, and 1640Hz. In this case a code contained within
the SPI BUS load to the AFIC sets up the path and determines the tone frequency, and at what
volume level to generate the tone. (It does not have to be related to the voice volume setting).
Inside the AFIC, this signal is routed to the alert tone generator; the output of the generator is
summed into the audio chain just after the RX audio de-emphasis block. Inside U0103 the tone is
amplified and filtered, then passed through the 8-bit digital volume attenuator, which is typically
loaded with a special value for alert tone audio. The tone exits at U0103-23 and is routed to the
audio PA like receive audio.
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