TSH512CF datasheet(16/19 Pages) STMICROELECTRONICS

16 / 19 page

TSH512

16/19

VOX description: Voice Operated Transmit

The Voice Operated Transmit section (VOX) re-

duces consumption when there is no audio signal

to transmit. When the VOX detects that no audio

signal is present, it mutes the Output Buffers of

TX1 and TX2 and provides the logic signal

VOX-MUTE to switch-off external LED drivers if

needed.

The audio signal of TX1 is amplified with a gain

depending on Rsens and Csens. Rsens and Cs-

ens are connected to pin 7. The high-pass filtering

has the following cut-off frequency:

fHPF = 1/(2.πRsens.Csens)

Figure 5 : Vox delay and sensitivity schematic

On pin 6, Rpeak and Cpeak integrate the rectified

audio signal with a short time constant. This fil-

tered signal follows the audio amplitude.

Figure 6 : Vox integrator and monostable

schematic

The self-adaptative VOX threshold consist in the

constatation that the ambient background noise

variation is slow compared to the voice or the mu-

sic. On the pin 28, RCOMP and CCOMP integrates

the amplitude to follow the background amplitude.

Therefore, the comparator switches when an au-

dio signal appears over the background noise.

Refering to the ’application schematic’, CCOMP will

be typically a 100nF capacitor and RCOMP will be

determined depending on the audio signal.

As soon as an audio is detected, the output of the

monostable switches to ’high’ state and enables

both output buffers. The output of the monostable

is the pin 27 and is called ’VOX-MUTE’.

The monostable holds the TSH512 in transmit

mode during a delay fixed by the value of CTRIG

connected to pin 29

Please note that the VOX function is activated with

the audio coming into the first transmitter TX1.

When the application needs a permanent trans-

mission, it is possible to inhibit the VOX function.

Just remove CTRIG capacitor and connect pin 29

to ground.

As soon as the TSH512 is powered-on, the inter-

nal reset circuitry sets the VOX-MUTE

to high

state to enable transmission. The transmission re-

mains during the monostable timing and continue

if an audio signal triggs the monostable

Figure 7 : VOX state at power-on

VOX

DELAY

1.4V

µA

------------





Ctrig

⋅

time

VOX-MUTE

POWER SUPPLY

off

VOX Delay

(Ctrig)

high state if retriggered by audio

time

VOX-MUTE

POWER SUPPLY

off

VOX Delay

(Ctrig)

high state if retriggered by audio

time

VOX-MUTE

POWER SUPPLY

off

VOX Delay

(Ctrig)

high state if retriggered by audio

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