How to calculate bitrate of PAL?
In the early days of analog television broadcast with CRT display receivers, it was found that instead of sending a whole picture frame information (576 PAL video lines) at once at a frame rate of 25 Hz, it was visually better to send half the lines as even field (0,2,4,...,574) and odd field (1,3,5,..,575) at 50 Hz. This is called interlaced video 576i for PAL and 480i for NTSC. If you send all the lines at once (and at half the rate of 25 Hz) then it's called progressive and denoted as 576p and 480p. These days they are used for 720 or 1080 etc line HD or FHD or 4k formats in the same line.

The reason for better visual characteristic is due to the fact that faster field rate yields smoother motion and yet reduced vertical resolution is compansated by several factors.