TDA7403 datasheet(13/25 Pages) STMICROELECTRONICS

13 / 25 page

DESCRIPTION OF STEREODECODER

The stereodecoder part of the TDA7403 (see Fig.

16) contains all functions necessary to demodu-

late the MPX signal like pilot tone dependent

MONO/STEREO

switching

well

"stereoblend" and "highcut" functions.

Adaptations like programmable input gain, roll-off

compensation, selectable deemphasis time con-

stant and a programmable fieldstrength input al-

low to use different IF devices.

Stereodecoder Mute

The TDA7403 has a fast and easy to control RDS

mute function which is a combination of the

audioprocessor softmute and the high-ohmic

mute of the stereodecoder. If the stereodecoder

is selected and a softmute command is sent (or

activated through the SM pin) the stereodecoder

will be set automatically to the high-ohmic mute

condition after the audio signal has been soft-

muted.

Hence a checking of alternate frequencies could

be performed. To release the system from the

mute condition simply the unmute command must

be sent: the stereodecoder is unmuted immedi-

ately and the audioprocessor is softly unmuted.

Fig. 14 shows the output signal VO as well as the

internal stereodecoder mute signal. This influ-

ence of Softmute on the stereodecoder mute can

be switched off by setting bit 3 of the Softmute

byte to "0". A stereodecoder mute command (bit

0, stereodecoder byte set to "1") will set the

stereodecoder in any case independently to the

high-ohmic mute state.

If any other source than the stereodecoder is se-

lected the decoder remains muted and the MPX

pin is connected to Vref to avoid any discharge of

the coupling capacitor through leakage currents.

Input Stages

The Ingain stage allows to adjust the MPX signal

to a magnitude of about 1Vrms internally which is

the recommended value. The 4.th order input fil-

ter has a corner frequency of 80kHz and is used

to attenuate spikes and noise and acts as an anti-

aliasing filter for the following switch capacitor fil-

ters.

Demodulator

In the demodulator block the left and the right

channel are separated from the MPX signal. In

this stage also the 19 kHz pilot tone is cancelled.

For reaching a high channel separation the

TDA7403 offers an I2C bus programmable roll-off

adjustment which is able to compensate the low-

pass behaviour of the tuner section. If the tuner

attenuation at 38kHz is in a range from 20.2% to

31% the TDA7403 needs no external network be-

fore the MPX pin. Within this range an adjustment

to obtain at least 40dB channel separation is pos-

sible.

The bits for this adjustment are located together

with the fieldstrength adjustment in one byte. This

gives the possibility to perform an optimization

step during the production of the carradio where

the channel separation and the fieldstrength con-

trol are trimmed.

Deemphasis and Highcut

The lowpass filter for the deemphasis allows to

choose between a time constant of 50

µs and

µs (bit D7, Stereodecoder byte).

The highcut control range will be in both cases

tHC = 2

⋅ tDeemp. Inside the highcut control range

(between VHCH and VHCL) the LEVEL signal

is converted into a 5 bit word which controls the

lowpass time constant between tDeemp...3

⋅ tDeemp.

There by the resolution will remain always 5 bits

independently of the absolute voltage range be-

tween the VHCH and VHCL values.

The highcut function can be switched off by I2C

bus (bit D7, Fieldstrength byte set to "0").

PLL and Pilot Tone Detector

The PLL has the task to lock on the 19kHz pilo-

tone during a stereo transmission to allow a cor-

rect demodulation. The included detector enables

the demodulation if the pilot tone reaches the se-

lected pilottone threshold VPTHST. Two different

thresholds are available. The detector output (sig-

nal STEREO, see block diagram) can be checked

SOFTMUTE

COMMAND

STD MUTE

D97AU638

Figure 14. Signals during stereodecoder’s

softmute

TDA7403

13/25

TDA7403 数据表(PDF) 13 Page - STMicroelectronics

TDA7403 数据表(HTML) 13 Page - STMicroelectronics

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

数据表下载