My musings on this...
I have been thinking about this for some time too, and trying to figure out a way of getting QL video out of an HDMI socket.
There are several hurdles and problems, but some thoughts below:
1) QL screen memory use is a little strange in the layout - so that tends to rule out all but a large CPLD or FPGA to do the video so that compatibility is maintained
2) Access to the screen RAM causes the CPU to have to wait, so keeping screen RAM only for screen would be the ideal, RAM shadowing so that reads are being done from faster RAM and not from the screen would allow some speed gain - so this makes the screen RAM effectively write only as far as the QL processor is concerned.
3) Allowing the writes to video RAM to be interleaved with video generator reading the video RAM - one way I thought this could be done is to write to a set of fast latches (using a voltage translating part gives the possibility to use 5v on the QL side and 3.3V on the video generation side). A flip flop indicating that data is present to read into RAM would avoid wasting cycles writing into video RAM and allow a possible co processor a look in. Also - this flip flop could be used to generate dtack back to the QL - if for some reason the last written byte to the latches was not cleared out by the video generator - the next write would just be delayed by holding off dtack.
4) The video generator would spend alternate cycles reading the video RAM, and would check for data in the latches to write into the video RAM during the other half. Using a fast static RAM would eliminate any refresh worries, and offer something like 12-13ns cycle time for a 10ns part.
5) Further speed improvement and bandwidth increase could be done by using 16 bit video RAM, and allowing the latch side to be accessed upper byte, lower byte or both bytes depending on interface width. This means the video generator would get a full 16 bits per access, which allows for mode 4 and mode 8 to be easily accommodated, and if a 16 bit or 32 bit processor was in use, the writes to video RAM would also be 16 bits.
6) output from the video generator could drive a simple resistor DAC and offer a VGA style connector, or it could drive digital inputs to a DVI/HDMI conversion chip.
7) Mapping QL screen resolutions to video timings that will work with VGA / HDMI monitors, and allowing possible extra resolutions (1024 horizontal) and (512 / 768) vertical and more colours.
8) Using a mechanism like the Aurora where additional video RAM space is provided at the top end of memory and shadowed by normal RAM for read / write will allow for additional resolutions and colour depths - a 256K or 512K RAM could be used but clearly the standard 1Mb QL memory map would not work too well with 512K, and 256K of video RAM is expensive on memory for a stock QL - 4MB card of Dave's is a good way out of that issue