SDA 5648

SDA 5648X

Semiconductor Group

25

Circuit Description

Referring to the functional block diagram of the PDC / VPS decoder, the composite video signal with

negative going sync pulses is coupled to the pin CVBS through a capacitor which is used for

clamping the bottom of the sync pulses to an internally fixed level. The signal is passed on to the

slicer, an analog circuitry separating the sync and the data parts of the CVBS signal, thus yielding

the digital composite sync signal VCS and a digital data signal for further processing by comparing

those signals to internally generated slicing levels.

The output of the sync separator is forwarded, on one hand, to the output pin VCS, and on the other

hand, to the clock generator and the Timing block. The VCS signal represents a key signal that is

used for deriving a system clock signal by means of a PLL.

The data slicer separates the data signal from the CVBS signal by comparing the video voltage to

an internally generated slicing level which is found by averaging the data signal during TV line no.

16 in the VPS mode or by averaging the data signal during the clock run-in period of the teletext

lines during the data entry window (DEW) in PDC mode.

The clock generator delivers the system clock needed for the basic timing as well as for the

regeneration of the data clock. It is based on two phase locked loops (PLL’s) all parts of which are

integrated on chip with the exception of the loop filter components. Each of the PLL’s is composed

of a voltage controlled oscillator (VCO), a phase/frequency detector (PFD), and a charge pump

which converts the digital output signals of the PFD to an analog current. That current is

transformed to a control voltage for the VCO by the off-chip loop filter. The generated VCO fre-

quencies are 10 MHz and 13.875 MHz for VPS mode and PDC mode, respectively.

All signals necessary for the control of sync and data slicing as well as for the data acquisition are

generated by the Timing block.

In PDC mode, only teletext rows 8/30 containing Broadcast Data Service Package (BDSP) infor-

mation are acquired. The relevant bytes of 8/30 format 1 (8/30/1) and 8/30 format 2 (8/30/2) are

extracted. The 8/30/1-bytes are stored in the acquisition register in a transparent way without any

bit manipulation, whereas the Hamming coded bytes of packet 8/30/2 are Hamming-checked and

bytes with one bit error are corrected. The storage of error free or corrected 8/30/2-data bytes in the

storage of 8/30/1- data, however, is not indicated by the DAVN output. The presence of 8/30/1 data

In VPS mode, the extracted data bits of TV line no. 16 are checked for biphase errors. With no

That transfer is signalled by a H/L transition of the DAVN output, as well.

In both operating modes data are updated when a new data line has been received, provided that

must be aware that the storage of new data from the acquisition interface is inhibited as long as the

registers are set to FF (hex) until they are updated by the reception of new data packet contained

in the CVBS signal.