Well I started thinking more about this C64-VGATE board today, and specifically about the look of the video wave form as compared to what's considered the standard in the industry. And I noticed that what is called the Breezeway, is non existent out of the VIC-II video chip (reference image below for what it should look like).
Standard NTSC Waveform and it's Attributes
In the NTSC standard, there is a 0.6 uS long blanking level just after the end of SYNC and prior to where the color burst is suppose to begin called the Breezeway. As I stated earlier this is virtually non-existent where the VIC-II is concerned, with it's color burst immediately following the end of the SYNC pulse. And another not as significant change is that the VIC-II outputs a rather long color burst, which in itself is not usually going to be a problem, but for the sake of making it a bit more standard it should be shortened (some video converters might take issue with it's longer state).
Luckily no hardware change is required to implement these changes, and everything can be adjusted within the PIC chip's firmware. Specifically in the Interrupt section of the assembly code as seen below...
Luckily no hardware change is required to implement these changes, and everything can be adjusted within the PIC chip's firmware. Specifically in the Interrupt section of the assembly code as seen below...
org 0x004 ; Processor interrupt vector
movlw 0AH ; Wait 4.7 uS (sync)
call dly_405ns
bcf VGATE ; Turn off video for 0.6 uS (add breezeway)
nop
nop
nop
nop
bsf VGATE ; Turn on video for 3 uS (color burst)
movlw 06H
call dly_405ns
bcf VGATE ; Turn off video for 2.4 uS (over-scan deleted)
movlw 05H ; Also shorten tail end of color burst
call dly_405ns
bsf VGATE ; Turn on video (display active video data)
banksel INTCON ; Clear interrupt flag
bcf INTCON,INTF
retfie ; Leave interrupt
In this new firmware version the Color Burst was shortened to 3 uS which is the equivalent of 10 cycles, with the standard being as little as 9 or as much as 11 cycles. The 0.6 uS Breezeway got re-instated. And in order to eliminate any left side over scan and to shorten the Color Burst , the Back Porch was lengthened by 2.6 uS, and set to begin before the VIC-II's color burst ended. This last part is about 1 uS longer than the standard, but isn't a detriment.
By the way where retro computer video is concerned, there is no separate blanking vs. black level, they are one in the same.
Here's how things compare with stock video shown on the left and VGATE enabled video on the right (Color Burst is in the middle)...
 Stock VIC-II Video - No VGATE |  VGATE turned ON |
On the stock video output you can see that the Breezeway is extremely short and practically merged with the Color Burst which looks to be 15 cycles long. Whereas with VGATE engaged, and using the new firmware, we now have a bit more normal looking video with a standard 0.6 uS Breezeway and a 10 cycle Color Burst.
The firmware is now at rev 1.1 and available for download below.
The firmware is now at rev 1.1 and available for download below.
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I presently don't have the means to test this in a PAL machine, but if it's anything like the Atari VGATE version I think it will automatically scale to the slower speed (60 vs. 50Hz). If not then a PAL specific firmware version will need to be developed by adjusting the timing in the interrupt routine.
- Michael
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