Found a hidden VGA feature!

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While I was hacking around with my ATtiny85 VGA / Sound generator, I found something interesting about the VGA signal, something I had never heard of or seen before.

You can send both Horizontal and Vertical sync on the Horizontal sync line in mode 720x400!

By mixing the syncs much like NTSC composite sync, all 5 monitors I tested work perfectly. I have never seen this documented, but it seems to work. In 720x400 or 640x400 mode, the vertical sync is positive and horizontal sync is negative, so I just bring the sync line high for one line during the vsync period, and then let hsync operate normally.

Perhaps the 2 pins are simply mixed internally anyhow, so this is why it works?

Anyhow, this is great, as an ATtiny85 only has 4 usable pins once you supply an external clock, so now I am getting 4 colors and sound into my monitor (2 color bit pins, 1 combined sync pin, and 1 sound output pin).

If this combined sync hack is actually documented someplace, I would sure like to see it.

I will be updating this project soon now that I have gained another IO pin to work with...

https://www.avrfreaks.net/index.p...

Cheers,
Brad

I Like to Build Stuff : http://www.AtomicZombie.com

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Experimenting further, I found that this single pin symc will work in any video mode. I tried 640x480, 800x600, and 1024x768, and they all work fine, even with both sync pulses being negative.

So it would appear that inside the monitor, both sync pins are actually ORed together onto the HSync line.

I don't have an old CRT to try this on, but so far all 5 LCD monitors are happy with mixed sync into the single HSync line.

It's a shame that the internal clock / PLL on the ATTiny85 is so damn unstable, or I would free up yet another pin!

I would still like to know if using a single sync line on analog VGA is some kind of "legitimate" option, or just an accidental hack that I just discovered.

Brad

I Like to Build Stuff : http://www.AtomicZombie.com

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A lot of rgb monitors had a 'sync on green' option. Ever since composite video there have been sync separator chips, so the algo must be public domain. You are a strange little man Atomic Zombie. Sort of like Woz. May you meet your Jobs soon.

Imagecraft compiler user

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Glad to see you practicing your dark arts again!

If you don't know my whole story, keep your mouth shut.

If you know my whole story, you're an accomplice. Keep your mouth shut. 

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AtomicZombie,

Congratulations! You have discovered a VGA/RGB sync mode that's been in existence for only about 35 years. It's called "Digital Composite Sync", "DCS", or "4-wire RGB".

The "Sync-On-Green" Bob Gardner refers to is also known as "3-wire Sync" or simply "SOG". In this method the sync pulse is "attached" to the bottom of the green signal of the R-G-B triad of data lines. Logically, this sync is the same as "DCS" described above. That is, it is a composite of horizontal and vertical sync pulses passed over one signal path, rather than two as with the more common 5-wire RGB type. (Also know as "Digital Separate Sync" or "DSS".)

DCS is formed by ORing the H-Sync and V-sync together. However, I don't exactly recall if the Inclusive or the Exclusive OR function is used to perform this combining.

The majority of existing VGA monitors can handle 5-wire and 4-wire sync types. A smaller number can handle Sync-On-Green signals.

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Chuck-Rowst wrote:
AtomicZombie,

Congratulations! You have discovered a VGA/RGB sync mode that's been in existence for only about 35 years. It's called "Digital Composite Sync", "DCS", or "4-wire RGB".

The "Sync-On-Green" Bob Gardner refers to is also known as "3-wire Sync" or simply "SOG". In this method the sync pulse is "attached" to the bottom of the green signal of the R-G-B triad of data lines. Logically, this sync is the same as "DCS" described above. That is, it is a composite of horizontal and vertical sync pulses passed over one signal path, rather than two as with the more common 5-wire RGB type. (Also know as "Digital Separate Sync" or "DSS".)

DCS is formed by ORing the H-Sync and V-sync together. However, I don't exactly recall if the Inclusive or the Exclusive OR function is used to perform this combining.

The majority of existing VGA monitors can handle 5-wire and 4-wire sync types. A smaller number can handle Sync-On-Green signals.

Thanks for the info! I knew about the sync on green, but never read about DCS in any of the vast amount of data I collected over the years.

Even now, a seach on both terms only bring up this thread!...

https://www.google.ca/#q=%224-wire+RGB%22+dcs

Good to know it will work on all monitors though. I found "sync on green" not to work on any of my 8 monitors.

Brad

I Like to Build Stuff : http://www.AtomicZombie.com

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Torby wrote:
Glad to see you practicing your dark arts again!

Thanks!
I finally ordered boards for my XMega system and intend to get the info online soon. With boards on the way, I am committed now (take the word committed any way you like)!

Cheers,
Brad

I Like to Build Stuff : http://www.AtomicZombie.com

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Take a look at this video generator on EBay:

http://www.ebay.com/itm/Quantum-Data-802B-Video-Test-Generator-for-Part-...

Look at the three buttons along the bottom edge under the label "SYNC GATE". They are labeled "ACS", "DCS" and "DSS".

Yes, there is a lot of information in the electronic industry which is "undocumented". That is, it is common knowledge within a certain electronic industry sector, but you will never find it in a textbook, or in any on-line tutorials and the like.

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Many analog front end chips meant to receive component (YUV) or VGA do support composite digital sync. Back in the days some monitors had 5 BNC connectors and the manuals said you need the three RGB BNCs only if Sync-On-Green is used, four BNCs (RGB+HS) if digital composite sync is used, or all five BNCs if digital separate sync is used (RGB+HS+VS).

So many modern, if not all modern displays could support it, but my experimentation with some unix workstations (HP and SUN) had all different view what signals are required so it was difficult to mix PCs to SUN monitors and SUN workstations to PC monitors.

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Sun used unique video resolutions as well as non-standard H & V-sync frequencies to prevent "thrifty" users from utilizing off-the-shelf monitors in their workstations. They also used a rather non-standard D-shell connector as their interface to help accomplish the same goal - make their users pay top dollar for Sun monitors.

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Speaking of odd "retro" monitors and systems, I had to fix an AS/400 system this week!

Brad

I Like to Build Stuff : http://www.AtomicZombie.com

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Quote:
Sun used unique video resolutions

To be fair, some of this is because Sun was first.
Sun3s were shipping with high-res (1280x1024?) B&W monitors in 1985, while the PC industry was still doing EGA. (and they did a lot of that B&W stuff for a long time, since "color was just for games.")

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westfw,

Based on my own experience I'd bet that "standard" 1280 x 1024 high-res format was NOT running at 30/60 frames per second.

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Probably not. "High persistence phosphors" - brings back memories. "Kids today don't realize how lucky they are to be able to go out and buy a 19 inch high-res color computer display that looks great for about two days pay. In my day you were looking at two weeks pay for a 12 inch B&W "monitor" that wasn't designed for computer use and probably flickered. Or you might be able to splurge on a special-purpose monitor (built in small numbers for specialty markets) at a price comparable to a pretty nice used car." (There was also surplus. I think my first monitor was a 9-inch B&W thing that had belonged to an airline, and cost $40 (at a time when min wage was $4 or so?))

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I remember wearing sunglasses to reduce the flicker on my 2080 Amiga monitor when I was doing interlaced video stuff!

Brad

I Like to Build Stuff : http://www.AtomicZombie.com