GAI-Tronics L3149B C200 Advanced Deskset - Manual (Page 47)

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Theory of Operation
General Desk Set Overview
The L3149B (C200) Advanced Desk Set is a multi-processor design employing state-of-the-art DSP
technology to achieve superior performance over typical analog and single-processor designs. This
allows for ultimate flexibility in audio processing.
The heart of the design consists of U1, the DSP; and U3, the microcontroller. The DSP is responsible for
audio processing of receive and transmit audio. This includes notch and line filtering, tone detection,
tone generation, audio level compression and setting, and audio routing. The microcontroller is
responsible for controlling the functions of the DSP, programming, the LCD display, keyboard scanning,
support of the relay and dc modules and general I/O and general operational functions.
Receive Audio
Receive audio is presented to the desk through the line transformer providing for the necessary load. The
audio is buffered and switched by U14. U14 controls the source of the receive audio: line 1 for two-wire
or line 2 for four-wire operation. The appropriate receive audio is then passed to the gain stage
consisting of U21 and U12. U12 provides the compression mechanism used by the DSP to keep the
receive audio below the maximum level of approximately –6 dBm.
After the receive audio has been properly compressed, it is passed to A/D section of U15 where it is
converted to digital for use by the DSP and also to P400-12 for data conditioning. The input impedance
is controlled by U10. When U10-1 is asserted, the input impedance is 560 ohms. When U10-1 is
unasserted, the input impedance is bridging.
Speaker Audio
Speaker audio is originated from the DSP in digital format. The DSP combines the receive audio that is
notched for tone control operation, side-tone and beep tones to generate the speaker audio. The speaker
audio is converted to analog using the D/A section of U17, and is passed through the volume control
stage consisting of U13 and U22 before being passed to the handset earpiece and the speaker amplifier,
U18.
The microcontroller or the DSP can mute the speaker amplifier. The microcontroller mutes the speaker
audio using U5-C8, in cases such as parallel mute or the user muting via a key press. The DSP uses a
speaker squelch algorithm to squelch the speaker when no audio is being output to the speaker. This
squelch will remain open for two seconds after no audio is present. The DSP then mutes the speaker
using U1-88.
Transmit Audio
Transmit audio is originated from the DSP in digital format. The DSP combines control tones, paging
tones and voice audio to generate the transmit audio. It is converted to analog by the D/A section of U15.
Upon conversion to analog, the audio is passed through the line-out gain stage consisting of U11 and U10
for level adjustment. U10 also provides the proper line drive and line enable for the transmit audio. The
proper level audio is then routed to the line 1 transformer for output from the desk set.