I feel obliged to draw attention to the teletext editor in my Test Card Generator software, not least because it is itself written in BBC BASIC! It will display the control codes as symbols (abbreviations), and includes a simple 'sixel' graphics editor. I also have a standalone teletext editor based on that program but it's not currently available for download.guesser wrote:If you are playing with editing teletext stuff you may want to look at a dedicated teletext editor as they have options to show visible control codes
As far as test and demo pages are concerned, there is a carousel of quite a lot of them (mostly downloaded from Prestel) supplied with BBC BASIC for Windows (including the free trial version) as MODE7DEM.CAR in the EXAMPLES\GENERAL folder. It's also supplied with the cross-platform (Win32 / Linux-86 / MacOS / Android-86) versions of BBC BASIC.
I can confirm what other people have said, that you should not confuse MODE 7 with the official Videotex/Teletext specification. Specifically, any row containing double-height characters is sent only as one 40-character packet; what appears in the 'bottom half' is automatically determined by the decoder.
This has an interesting consequence: you can create a page containing double-height text which will instantly 'flip' to an entirely different content just by changing the very first character on the very first row! This happens because if every row contains a double-height control character only the even-numbered rows (0, 2, 4, 6 etc.) are visible: the odd numbered rows don't contribute to the display at all. If you then overwrite the double-height code in the top row, only the top row and the subsequent odd-numbered rows (1,3,5...) are visible. This is a tough test for a software-based Videotex/Teletext display.