60/50 or the 20% plus issue...

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60/50 or the 20% plus issue...

Post by Eratosthenes » Thu May 02, 2019 7:14 pm

Moin, moin! (==Hi!)

Even though I do not use the Next (Just The Board) so much any more I sometimes fiddle around with it. Since I've got some new parts (see attached pic) I've reactivated my Next some days ago...
My Next now having an as new ORIGINAL keyboard mat... ;-)
NextNewKeyboard02.jpg (81.49 KiB) Viewed 424 times
Having used the first cores I was happy that the Spectrum diddn't speed up when the display settings have been changed from 50Hz (PAL?) to 60Hz. I haven't tested this for a longer time.

Now, I'm using V2 (Core 2.00.26) this seems to have changed.
Several games I've tested are running quicker than I'm used to when using HDMI@60Hz... - Looks like them being aprox. 20% faster as usual.

Does anybody know if something has changed in the core?

THNX 4 info.

der Eratosthenes

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Re: 60/50 or the 20% plus issue...

Post by Alcoholics Anonymous » Mon May 27, 2019 5:13 pm

The machine itself doesn't run faster but a 60Hz machine does mean the ula interrupt occurs at 60Hz instead of 50Hz which is 20% more frequent. If the game relies on the interrupt to pace itself, it will run faster.

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Re: 60/50 or the 20% plus issue...

Post by Timbucus » Mon May 27, 2019 5:16 pm

Also that would have been the actual effect on an real NTSC Spectrum...
I'm Infinite Imaginations when not in work... PAWS for thought.

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Re: 60/50 or the 20% plus issue...

Post by SevenFFF » Mon May 27, 2019 5:44 pm

The duration of one T state remains broadly the same, and the number of Ts per video line also does. But there are 20% fewer video lines per VBI interrupt - precisely because there are 20% more interrupts per unit of time.

This is the same effect as seen in Timex Sinclair machines or NTSC Spectrums.

Games that are synced to the VBI interrupt will appear to run 20% faster.

Unsynced games will appear to run at the same speed.

Games that run code in every available T state in a 50Hz frame will not run as expected, and may crash depending on how the bottom border code interacts with the ISR code.

Games designed specifically for the Next generally will be designed for the worst case Ts per frame (HDMI 60Hz timings). If these games have specdrum/covox/soundrive DAC sound they will have to do something clever to switch sample rates if they allow the user to press F3 to switch 50/60 mode during the game.

In short, 50/60 mode shouldn’t primarily be seen as a tool to support different monitor types. It should be used to accurately give the experience of running particular Spectrum models, and to support programs written for the characteristics of those models.
Robin Verhagen-Guest
SevenFFF / Threetwosevensixseven / colonel32
NXtel Spectron 2084blog

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