TS34118

19-20

2004/09 rev. B

Application Information

Volume Control

If a potentiometer with a standard linear taper is used for the volume control, In situations where this may be

objectionable, a potentiometer with an audio taper (commonly used in radio volume controls) will provide a more linear

relationship as indicated in Figure 12. The slight non-linearity at each end of the graph is due to the physical

construction of the potentiometer, and will vary among different manufactures.

Figure 12. Receive Attenuator Gain versus Potentiometer Position Using Audio Taper

RFI Interference

Potential radio frequency interference problems should be addressed early in the electrical and mechanical design of

the speakerphone. RFI may enter the circuit through Tip and Ring, through the microphone wiring to the microphone

amplifier, or through any of the PC board trances. The most sensitive pins on the TS34118 are the inputs to the level

detectors (RLI1, RLI2, TLI1, TLI2) since, when there is no speech present, the inputs are high impedance and these op

amps are in a near open loop condition. The board traces to these pins should be kept short, and the resistor and

capacitor for each of these pins should be physically close to the pins. Any other high impedance input pin (MCI, HTI,

FI, VLC) should be considered sensitive to RFI signals.

The Final Analysis

Proper operation of a speakerphone is a combination of proper mechanical (acoustic) design as well as proper

electronic design. The acoustics of the enclosure must be considered early in the design of a speakerphone. In general,

electronics cannot compensate for poor acoustics, low speaker quality, or any combination of the two. Proper acoustic

separation of the speaker and microphone, as described in the Design Equations, is essential. The physical location of

the microphone, along with the characteristics of the selected microphone, will play a large role in the quality of the

transmitted sound. The microphone and speaker vendors can usually provide additional information on the use of their

products. In the final analysis, the circuits shown in this datasheet will have to be “fine tuned” to match the acoustics of

the enclosure, the specific hybrid, and the specific microphone and speaker selected. The component values shown in

this datasheet should be considered as starting points only. The gains of the transmit and receive paths are easily

adjusted at the microphone and speaker amplifiers, respectively, The switching response can then be fine turned by

varying (in small steps) the components at the level detector inputs until satisfactory operation is obtained for both long

and short lines