9
Circuit description new circuits
34
L7.3A
9.6
SINGLE CHIP PROCESSOR IC7225 (=TDA8374)
Introduction :
In this chip most of the video, audio and sync circuits are
integrated and in the diagrams the IC is split up into 5 parts:
5A,5B,5C,5D and 5E.
–
IC7225-5A, video detector ,see diagram A4.
–
IC7225-5B, source select en PAL demodulator ,see
diagram A6.
–
IC7225-5C, video control, see diagram A4.
–
IC7225-5D, horizontal and vertical sync, see diagram A3.
–
IC7225-5E, mono sound demodulator, see diagram A4.
9.6.1
IC7225-5A, IF video detector, see diagram A4
Tuning system
For the tuner 1000 there are two possibilities: VST type or PLL
type and in both cases the tuner is controlled by the uC. The
VST tuner is controlled via V_TUNE, AFC and the BS1, BS2
band switching signals; the PLL tuner is fully I2C controlled. IC
7225-5A contains the IF amplifier and the IF detector, the IF
signal is present at the output pin 11 of the tuner. BS1 and BS2
(pin 17-18) switching signals used for band switching of a VST
tuner.
IF bands pass filter , the IF bands pass characteristic is
determined by the band-pass of the SAW filter 1015, listed
below you will find the possibilities :
•
For PAL BG sets a SAW filter with 5.5 MHz bandwidth is
used (33.4 to 38.9 MHz).
•
For PAL 1 sets a SAW filter with a bandwidth of 6.0 MHz is
used (32.9 to 38.9 MHz).
•
For PAL BG/SECAM BG/LL' sets a SAW filter with 6.5 MHz
band switch is used to enable BG/I/LL' reception (33.9 to
40.4 MHz).
•
For PAL BG/SECAM BG/DK sets a SAW filter with a
bandwidth of 6.5 MHz is used (32.4 to 38.9 MHz).
IF-demodulator
IF-demodulation is done with reference circuit L5260 at pin 3
and 4 of IC7225-5A; AGC control of tuner via pin 54 IC7225-
5A. Top sync level is used for AGC inside IC7225-5A. AGC
adjustment is done by 1(C control via entry into the SAM
service menu. C2202 at pin 53 determines time constant of the
AGC. Base band CVBS signal at pin 6 of IC7225-5A (normal -
3V2) is fed to the sound trap filters (1206, 1207, 1208, 1209
and 5206) and returns to pin 13 of IC7225-5B for source
selection and video processing (diagram A6). The NTSC_ SW
signal from the control microprocessor switches on transistors
7216 and 7217 to activate filter 1209 when an NTSC signal is
received. Demodulation for sound IF takes place for the various
signals as follows: For single mono signal, the CVBS_Sound
signal goes to filter 1104 for 4.5 MHz and filter 1102 for
5.5MHz. The sound _IF is connected to pin 1 of IC7225-5E for
demodulation.
9.6.2
PAL/NTSC Processing (IC7225-5B, see diagram A6)
IC7225-5B contains : source selector and PAL/NTSC colour
decoder.
The main functions are :
–
Source Selection
The input selector has CVBS_INT<
CVBS_EXT1,Y_CVBSEXT2, C_EXT2 as inputs which can
be selected via the 12C bus.
–
Colour Decoder; the main functions are :
–
PLL/VCO
The PLL operates during the burst key period. It
synchronizes the VCXO oscillator with the frequency
and phase of the incoming chroma burst. For
alternating burst (PAL standards), a loop filter enables
the PLL to synchronize with the average burst (180
degrees phase).
–
PAL/NTSC Identification
–
Secam Interface
The 0 and 90 degrees reference signals from the VCO
oscillator are supplied via the HUE phase circuit to the (B-
Y) and (R-Y) burst demodulators respectively. The
demodulated burst from the (B-Y) burst demodulator is
supplied both to ACC detector and NTSC indent circuit; the
NSTC indent circuit is an integrator. Once the integrator
output exceeds the NTSC killer level, an NTSC burst has
been identified (IDN=1). Therefore the IDN information
provided the ASM (Automatic System Manager) with
NTSC colour indent information. The demodulated burst
from the burst from the (R-Y) burst demodulator is supplied
to the PAL identcircuit via a AL switch; the PAL ident circuit
is also an integrator. Once the integrator output exceed the
PAL killer level, a PAL burst has been identified (IDP=1).
The IDP information provides the ASM with PAL-colour
ident information.
The SECAM interface allows bi-directional communication
between IC7241 (TDA8395) and the ASM for SECAM
identification. If the VCO oscillator is oscillating at 4.43
MHz, the reference frequency of 4.43mhz is superimposed
on a DC level and is supplied from pin 33 of IC7225 to pin
1 of IC7241. If PAL/NTSC signals are identified, the
reference frequency is continuously available at pin 33 and
superimposed on a 1.5V DC level. When SECAM is
identified by IC7241, the reference signals which is
generated in packets form and superimposed on a 5V DC
level is supplied to pin 33 of IC7225.The ident signal
(IDSM=1) is supplied to the ASM.
–
ASM (Automatic System Manager)
–
(R-Y)/(B-Y) demodulation
The ASM is able to decode PAL/NTSC colour standards
and in combination with IC7241(TDA8395), multi standard
applications can be realized. The different possibilities are
controlled by the 12C bus input commands which are
communicated to the ASM via the 12C bus. The 12C bus
input commands also indicates which crystals must be
connected to pins 34 and 35. This is essential for correct
calibration of the horizontal oscillator.
For PAL/NTSC standards, the (B-Y)/(R-Y) baseband
signals are extracted from the chroma signal by the (B-Y)/
(R-Y) demodulators, filtered and supplied via the output
switch to pins 29 and 30. If SECAM is identified by the
SECAM decoder IC7241 and no PAL/NTSC is identified by
the ASM, then the ASM will open the switch (B-Y)/(R-Y).
This implies (B-Y)/(R-Y) signals from the SECAM decoder
will be directly connected to pin 29 and 30 without extra
loading.
BS1
BS2
VHF1
0
1
VHF2
1
0
UHF
1
1
