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参数资料
| 型号： | TDA7400 |
| 厂商： | STMICROELECTRONICS |
| 元件分类： | 消费家电 |
| 英文描述： | SPECIALTY CONSUMER CIRCUIT, PQFP44 |
| 封装： | 10 X 10 MM, TQFP-44 |
| 文件页数： | 7/28页 |
| 文件大小： | 274K |
| 代理商： | TDA7400 |
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FUNCTIONAL DESCRIPTION OF STEREODE-
CODER
The stereodecoder part of the TDA7400 (see Fig.
12) contains all functions necessary to demodu-
late the MPX signal like pilot tone dependent
MONO/STEREO
switching
as
well
as
"stereoblend" and "highcut" functions.
Stereodecoder Mute
The TDA7400 has a fast and easy to control RDS
mute function which is a combination of the
audioprocessor’s 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. 13 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.
Ingain + Infilter
The Ingain stage allows to adjust the MPX signal
to a magnitude of about 1Vrms internally which is
the recommended value. The 4th order input filter
has a corner frequency of 80KHz and is used to
attenuate spikes and nose and acts as an anti al-
lasing 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
TDA7400 offers an I2C bus programmable roll-off
INGAIN
3.5 ... 11dB
STEP 2.5dB
INFILTER
LP 80KHz
4.th ORDER
DEMODULATOR
- PLOT CANC
- ROLL-OFF COMP.
- LP 25KHz
PLL +
PILOT-DET.
F19
NOISE BLANKER
F38
STEREO
HOLDN
SB CONTROL
DEEMPHASIS
+ HIGHCUT
t=50 or 75
s
REF 5V
VSBL
MPINFL
LEVEL INTERN
HC
CONTROL
VHCCH
VHCCL
-
MULTIPATH
DETECTOR
D
A
MP_OUT
LEVEL INPUT
LP 2.2KHZ
1.th ORDER
MP_IN
LEVEL
FM_L
FM_R
MPX
100K
D98AU855
GAIN 0..10dB
MPLEVELOUT
QUALITY DETECTOR
QUAL
NOISE
+
Figure 12. Block Diagram of the Stereodecoder
TDA7400
15/28
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