Motorola GM-950 Manual page 42

Receive Signalling Circuits
The allowable internal alert tones are 304, 608, 911, and 1823Hz. In this case a code contained
within the SPI BUS load to the ASFIC 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).
For external alert tones, the µP can generate any tone within the 100-3000Hz audio band. This is
accomplished by the µP generating a square wave which enters the ASFIC at U0201-C3.
Inside the ASFIC, 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 U0201 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 U0201-J4 and is routed to the
audio PA like receive audio.
5C.3-18 Theory of Operation