oO cozy Oo wrote: ↑Wed Sep 07, 2022 7:07 pm
So now i have to run in VGA now Im sad now....
Version 4 of the core will have correct hdmi video timing but that won't be coming for a while.
Spectrum clones don't attempt hdmi for a reason -- modern tv standards are incompatible with spectrum video signals. The Spectrum Next is a spectrum internally and it generates video with the same structure as the originals. It's not possible to display this on hdmi so instead, for hdmi, the structure of the video frame is changed to match hdmi's requirements. This changes the relationship between cpu and raster position which some games like Old Tower rely on to draw multicolour effects. So unfortunately at the moment, only VGA and RGB can display such things accurately because the spectrum video frame remains intact.
VGA and RGB work because they are analogue standards. Almost anything goes -- you can be well out of spec and they will still put something on the screen. Modern TVs supporting VGA or RGB can still present a challenge here, as although they will do their best to create a display like their older analogue counterparts, their digital nature means they sample the signal and process it for display on their panels. This can be done badly or well with corresponding results.
The typical solution for generating hdmi video from incompatible signals is to construct a framebuffer so that the old source writes pixels into a ram at one rate and the hdmi reads pixels from the same ram at a different rate. However this solution is not possible in the Spectrum Next because of its high resolution and the insufficient fpga size. Instead, there is another solution that will be pursued in version 4 of the core where the hdmi frame and the spectrum will be frame locked so that their frames are in sync and the amount of buffering between the two will be kept at a minimum level. This will not make much difference :- I can't recall the exact numbers but the frame rate of the 128K is slightly over 50 Hz and the frame rate of the 48K is slightly under. In fact, the 128K was "fixed" to be more compliant as the 48K was incompatible with some tvs (colour may not be there, worse artifacts, etc) and this led to some incompatibility between the 128K and 48K where games written to do multicolour effects on the 48K no longer worked on the 128K.
Anyway, the hdmi will mean the frame rate will be a constant 50 Hz with the 128K becoming very slightly slower and the 48K very slightly faster to suit. Most people don't even know that the 128K was faster. The difference will be more noticeable for the Pentagon as its frame rate is around 48.x Hz due to hardware bugs that are now an essential part of what makes a Pentagon a Pentagon. It was incompatible with many displays in its day. In comparison, the analogue nature of VGA and RGB means it is possible to support the frame rates as they were in the originals.
