I wanted to know if it's possible to use a digital oscilloscope (rigol ds1054z in fact, ) to show up composite video on the scope as w2aew showed in his topic.
It would need a Z-input in addition to the x and y inputs, could we use the ext-trigger for the scope or is that for something else?
You can use any oscilloscope (within reason) to display a picture. All you need is the ability to drive the X axis at line scan rate and the Y axis at frame scan rate and to be able to feed the video to the Z input. You can use the internal timebase as the line scan generator and trigger it from sync pulses in the video but you have to generate the sawtooth (ramp) waveform for vertical scanning (Y axis) yourself and also reset it from the sync in the video.
I'm not sure of the exact spec of that Rigol but if it has a Z input it should work.
Some digital oscilloscopes expose a Z-input in XY-mode. It can be used to operate the oscilloscope with respective X and Y deflection signals as raster graphic display. Z modulation is usually on-off only, no grey tones possible. As for the DS1054, I trust in your capability to read a user manual, does it mention Z input or blanking in X-Y mode?