box (Data Sheet, Fig.16) just a blackbox for some circuit? do you have any
suggestions, which crystal (type, manuf.) can be used?
Yes. It is a simple quartz crystal .
There are extra hints about this in the AD724 datasheet on page 12 in the
section called "FREQUENCY TUNING".
The crystal is used, among other things, to generate the colour burst reference
frequency - the sub carrier frequency. The AD724 will operate from clock
frequencies which deviate quite significantly from the 17.734480M nominal but
it will output a sub carrier which is Fclock/4.
The NTSC and PAL specs require the a tolerance of +/-10Hz and most monitors
require that the sub carrier frequency be within a few hundred hertz of the
nominal standard. The required tolerance is therefore dependant on the monitor
you intend to use and how close you wish to be to video standards, it is not
limited by the AD724.