[adrian_b]

Actually in the 625-line TV standard there are only 576 visible lines.

The other lines are for the vertical retrace, when the video signal is blanked.

With square pixels, the B&W image would have been 576 x 768, which requires a 7.5 MHz analog video bandwidth (@ 50 Hz vertical & 15625 Hz horizontal frequencies).

Most 625-line B&W TV sets could display 576 x 768 images very well and some of the early personal computers with video outputs for TV used this format.

Nevertheless the broadcast TV signal was limited by a low-pass filter to lower horizontal resolutions, corresponding to 5 MHz analog video bandwidth in Western Europe and to 6 MHz analog video bandwidth in Eastern Europe. The reason was to provide space in the TV frequency channel for the audio signal, which used a carrier offset from the video carrier by 5.5 MHz in Western Europe and by 6.5 MHz in Eastern Europe.

So the broadcast B&W signal was worse than what the B&W TV sets could display, corresponding to 576 vertical pixels by about 510 to 620 horizontal pixels (depending on the country).