Grundig STR-622 TWIN Service Manual Download Page 18

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Schaltungsbeschreibung / Circuit Description

STR 622 TWIN

2 - 6

GRUNDIG Service

4.1 Noise Reduction System (NRS)
The NRS consists of a peak level detector and a controlled low pass
filter. Audio for each channel to its peak detector is band limited by an
external band-pass filter, and centred on the transistors CT1073,
CT1078.

The output of each peak detector is a voltage stored in its NRS time
constant. This is a DC level which will vary with the amplitude of the
audio channel signal and is used to control the frequency response of
its NRS filter via a variable transconductance amplifier.

Switch S3 is used to select the NRS system.

4.2 Passive Deemphasis
S4 is used to select the type of passive deemphasis that is applied to
each audio channel. The options are set by external networks and are
J17, and 75

S. 50

S is obtained by adding an internal resistor in

parallel with the 75

S network.

For PANDA operation NRS and 75

S are used together.

5. Video Noise Filter

The baseband signal from the tuner is passed through a 10MHz low-
pass filter consisting of L1116, CC1113, CC1114 and CR1009, to remove
unwanted noise before it is taken to the Video Processor IC1100-(20)
for further processing.

6. Video Processing

6.1 Video Gain Control in IC1100
Baseband Video from the Tuner section is AC coupled into a controlled
amplifier whose gain can be set to give the desired Video amplitude.

6.2 Video Inverter
The polarity of the video signal can be set by the invert switch so that,
for instance, C band reception can be achieved.
After the invert selector switch this signal forms the BASEBAND input
to the Video Matrix and PAL De-emphasis.

6.3 PAL Deemphasis
The baseband signal is applied to the non-inverting input of an
amplifier which has a PAL De-emphasis network in its negative
feedback loop.

After the deemphasis the PAL signal is band limited by the 5MHz Low-
pass Filter. In the following, the signal is AC coupled into the clamp to
remove the 25Hz energy dispersal waveform. The resulting signal is
the NORMAL video input to the Video Matrix.

6.4 Video Matrix IC1100
This is a cross wire matrix of switches which can select any one of the
Video sources and make it available to each of the three outputs. In
addition to the SAT-video signals described above video is also fed to
the matrix from the VCR and DEC sockets, each signal being clamped
to the same level to avoid switching disturbances. All Video Matrix,
Invert and Video Gain are controlled via I

C-bus. The output signal from

the matrix is sent to the TV, VCR and DEC socket.
Megalogic control is possible via contact 10 of the SCART sockets
(passive connection).

6.4.1 TV Socket
By a "High" level switching voltage U

AV OUT 3

CIC1401-(39), the 12V

switching voltage is fed through CT1254, CT1250 to the TV Scart
socket, contact 8, and consequently the TV receiver is switched to the
AV mode.
The OSD signal is taken from the output CIC1331-(16) via CT1260 to
the TV socket (contact 19).

6.4.2 VCR Socket
The audio and video signals to be recorded are provided on contact 1/
6 and 19 respectively of the VCR socket.
On VCR playback the IC1100 applies the audio/video signal to the TV
socket via the switching voltage from the VCR socket - contact 8, U

AVIN 2

CIC1401-(22).

6.4.3 Decoder Socket
On Decoder operation, the switching voltage from the DEC socket -
contact 8, U

AV IN 1

CIC1401-(21) - feeds the descrambled audio/video

signal to the TV and VCR socket.
On RGB operation, the RGB switching voltage from the DEC socket -
contact 16 connects the RGB signal via CT1276 to the TV socket. The
TV receiver changes to RGB mode.
If a switching voltage is simultaneously applied to contact 8 of the VCR
socket, the switching voltage U

RGB TV

is fed out from CIC1401-(38) and

the RGB switching voltage at the TV socket is switched off by CT1270.
VCR priority operation: the VCR-signal is only fed through the video matrix
IC1100 to contact 19 of the TV socket.

6.4.4 Signal Switch-over to Modulator
Dependent on the U

MOD

switching voltage, the SAT video signal or the

VCR video signal is switched by CIC1400 to the modulator, contact 3,
via the transistor CT1406.
On switching over, the TV or VCR audio signal is fed through CT1405
or CT1406 and the amplifier CIC1330 to modulator contact 1.

7. OSD and Synchronisation

The video signal "V

TV OUT

" via CT1365 is used as the feed for the On

Screen Display (OSD) insertion circuit CIC1331 which adds menus
and status information to the video signal to be displayed on the TV.
The sync tips of the video signal "V

TV OUT

" are clamped at a predeter-

mined level by CT1350.
The presence of a standard sync is signalled by a "High" level at
CIC1331-(29). From a level of >4V, IC1401-(19) recognizes via the
"U

EXO SYNC.

" lead that a valid sync signal is present and indicates the

display field.
In the absence of a valid sync signal, the OSD processor CIC1331 is
instructed to generate its own sync signal so that the screen back-
ground becomes blue and the OSD can still be used. Level at CIC1401-
(19) is < 3.2V.

All control of the OSD is effected via the SDA, SCL and CS

OSD

leads.

PA L

De-emphasis

Low Pass

Filter

Clamp

PAL

BASEBAND

NORMAL

DECODER