sames

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or after pressing the MUTE key, the gain is reduced by > 60 dB.

Receive

The receive input is the differential signal of RI and STB. The gain of the receive path

is 2 dB (test circuit figure 5) with differential outputs, RO1/RO2 (0dB on 1000

Ω versions).

When mute is active during dialling the gain is reduced by > 60dB. During DTMF dialling

a MF comfort tone is applied to the receiver. The comfort tone is the DTMF signal with

a level that is -30dB relative to the line signal.

The receive gain can be adjusted under user control by using the volume control keys (not

on SA2531D). The VOL key gives a 4dB increase or returns the gain to normal in a Toggle

Function. Alternatively the + and - keys may be used. The + key increases the gain to

a maximum of +6dB while the - key reduces the gain to a minimum of -4dB. Each press

of the keys changes the gain by approximately 2dB.

The gain is reset by the next on-hook.

Side Tone

Side Tone is controlled along with Return Loss by a Double Balance Bridge as shown in

Fig. 1.

Figure 1

Double balance bridge (return loss and side tone) with one common ground

A good side tone cancellation is achieved by using the following equation:

ZBAL

=

R5

ZLINE

R1