NEC DTE-29U1TH, Service Manual

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Service Manual CP-785F
In the TDA936x series, the following geometry parameters can be adjusted for all picture tubes
:
Horizontal shift
Vertical amplitude
Vertical slope
S-correction
Vertical shift
The types which are intended to be used in combination with 110¡ picture tubes have an East-
West control circuit. The additional controls for these types are:
EW width
EW parabola width
EW upper and lower corner parabola correction
EW trapezium correction
Vertical zoom, horizontal parallelogram and bow correction.
Chroma and luminance processing
The chroma band-pass and trap circuits (including the SECAM cloche filter) are realised by
means of gyrators and are tuned to the right frequency by comparing the tuning frequency with
the reference frequency of the colour decoder. The luminance delay line and the delay cells for
the peaking circuit are also realised with gyrators. The circuit contains a black stretcher
function which corrects the black level for incoming signals which have a difference between
the black level and the blanking level.
Colour decoder
The ICs can decode PAL, NTSC and SECAM signals. The PAL/NTSC decoder does not need
external reference crystals but has an internal clock generator which is stabilised to the
required frequency by using the 12 MHz clock signal from the reference oscillator of the -
Controller/Teletext decoder.
The Automatic Colour Limiting (ACL) circuit (switchable via the ACL bit in subaddress 2OH)
prevents that oversaturation occurs when signals with a high chroma-to-burst ratio are
received. The ACL circuit is designed such that it only reduces the chroma signal and not the
burst signal. This has the advantage that the colour sensitivity is not affected by this function.
SOFTWARE CONTROL
The CPU communicates with the peripheral fonctions using Special function Registers (SFRS)
which are addressed as RAM locations. The registers for the Teletext decoder appear as
normal SFRs in the -Controller memory map and are written to these functions by using a serial
bus. This bus is controlled by dedicated hardware which uses a simple handshake system for
software synchronisation.
For compatibility reasons and possible re-use of software blocks, the TV processor is controlled
by I2C bus. The TV processor control registers cannot be read. Only the status registers can
be read ( Read address 8A ).
The SECAM decoder contains an auto-calibrating PLL demodulator which has two references,
via the divided 12 MHz reference frequency (obtained from the -Controller) which is used to
tune the PLL to