View Full Version : VGA to NTSC conversion?


N2IXK
02-14-2015, 03:35 PM
Does anyone have a recommendation for converting VGA signals from a PC into NTSC composite video? I am looking to incorporate a laptop into my shop AV system, and purchased one of these guys from eBay:

http://www.ebay.com/itm/400858906982

It basically works, playing back .avi, .mp4 and other formats through VLC media player. But the output signal is NOT really NTSC compatible, with the negative going sync pulses being far too large, and the luma/chroma content compressed into a much smaller portion of the overall signal. As would be expected, contrast is poor and colors are washed out. I have attached 3 shots taken from my waveform monitor showing what is going on. First shot is the output from my bar generator, showing that the WFM itself is properly calibrated. Video signal looks great, covering the range from -40 IRE (sync tips) to +100 IRE (peak white), with pedestal at 7.5 IRE.

The second shot shows the output from the VGA converter. Pedestal level is OK, but the sync tips are WAY offscale, with maximum positive signal level at around 30 IRE. Third shot shows the complete signal onscreen, shifted upward with the vertical position control.

I opened the unit up to see if there were internal adjustments to tweak, but nothing found besides the NTSC/PAL selection DIP switches. The heart of the thing is a big chip labeled "XY1945" (No manufacturer logo), with some flash memory and a serial EEPROM and not much else inside. A Google search for the chip datasheet turns up nothing. I was hoping to get some basic application data to see if this thing is just poorly designed, or this is an inherent flaw in the chip itself.

Has anyone out there found a VGA to NTSC converter that actually puts out a standards compliant composite signal?

Chip Chester
02-14-2015, 06:01 PM
Although there's not a manual associated with the ebay listing, it does mention that all the color/video elements are adjustable, and there is a menu button with nav buttons on the side.

What happens when you adjust the signal parameters while viewing with your scope?

The menu may only be visible on the pass-thru VGA monitor, or also on the composite output. Additionally, performance may be hampered if the VGA pass-thru is not terminated.

There are also switches on the side... No labels, but they're likely not there for nothin'.

Good luck!
Chip

Username1
02-14-2015, 09:10 PM
You can probably find it by searching different strings using the descriptions
of the chip itself.... Like I found this on the description of a data sheet
of a chip that does what your box does... " vga to pal ntsc video encoder"
type that into google and try variations.... Below are a few interesting items....

http://www.google.com/url?q=http://www.nxp.com/documents/data_sheet/SAA7104E_SAA7105E.pdf&sa=U&ei=oPTfVPj5GZGZyQSWk4HoAw&ved=0CCYQFjAD&usg=AFQjCNF3KcJdVMuUpDvv_c2H5XX4u-I69w

http://www.semiconductorstore.com/cart/pc/viewPrd.asp?idproduct=12766&cm_mmc=GoogleProducts-_-Averlogic%20Technologies-_-Description-_-AL128B-PBF&gclid=CM610ZTt4sMCFWRk7AodWwoAXw

This one is a little expensive, but it's a finished box
http://www.networktechinc.com/vga-converter.html

http://www.google.com/url?q=http://www.analog.com/static/imported-files/data_sheets/AD725.pdf&sa=U&ei=gf3fVM-dMYS1yQT5pICACQ&ved=0CFcQFjAL&usg=AFQjCNEzx-AZ1Qf4nR8l7RKcmgEd1XdUNw

I wonder if this is close to what you got....
http://www.jammaboards.com/store/vga-to-cga-pal-ntsc-video-converter-pcb-gbs8100.html

You may just have to engineer it to get it to do exactly what you want.

.

old_tv_nut
02-14-2015, 09:28 PM
Are you loading the output with 75 ohms when scoping/using it? It may have a non-constant output impedance or the syncs inserted as a current, or something goofy like that, expecting the output to be loaded - a bad practice that may work correctly when the output is loaded, but not with the 1 Megohm scope input.

Chip Chester
02-15-2015, 01:12 AM
He has a real (nice) Tek waveform/vector scope combo, and as I recall it has looping inputs for A and B. Some of the newer ones were "self terminating", I believe by a little switch within the BNC connector. No looping connection=terminated. Looping connector installed=unterminated. Trouble is, sometimes they didn't actually switch (Sony!) so you had to add a terminator anyway.

But in general, that scope can be configured to show the truth about a composite analog signal. And bars in his first shot look good. I'd wager either misadjusted unit (perhaps curable via menu), bad cable, or his stated suspicions about the device quality are justified.

Chip

N2IXK
02-15-2015, 12:18 PM
Thanks for all the responses! Yes, the output of the device is working into a properly terminated 75 ohm load (input of a 8 x 4 crosspoint switch). No setting of the various menu items provides any real improvement in the waveform, as far as the ratio between sync and video signal is concerned. The 2 DIP switches are used to select PAL or NTSC output, and are properly set for NTSC. The signal is shown on a dedicated NTSC waveform monitor, (Tektronix 1730), not a general purpose scope. Will rig up a terminating plug for the unused VGA passthrough to see if that helps at all.

Neither of the 2 chips you suggested are the one I have here, but that "jammaboard" unit looks VERY similar as far as circuitry if not layout/packaging. The chip on the pictured unit is a VX1937, manufactured by VXIS, and searching for a datasheet shows a chip with the right package and number of pins, which requires the same 27 MHz timebase crystal as in my unit. Looks like we have a match!:banana:

Unfortunately, the only datasheet I can find for it is a 2 page summary, lacking any actual data like pinouts, etc. that would be needed for reverse engineering the thing. This is a start, though.

Findm-Keepm
02-15-2015, 10:17 PM
I have a VGA to NTSC converter box, about 4"x5"x1" used originally to deliver announcements over an in-school TV system. 640x480 only, and does only 16-bit color, so I have to use WinXP or older for the VGA video. Stable on a 12V wall wart, and made in 1992, so older than anything I see on eBay. I got it with some upstream modulators and a Panasonic HD CRT TV set back in 2009 from a Public Surplus auction that Portsmouth Public Schools ran. The modulators were not what I hoped they would be, and the Panasonic got traded, so it's all I have left for my 31 dollars...

NJRoadfan
02-16-2015, 02:15 AM
The adapters generally don't do a good job. If you can find one, a video card or laptop with built in S-Video/Composite output is a bit better behaved (my old Powerbook G4 with S-Video out does a decent job). Also the compressed chroma may be a screwy color space conversion somewhere in the chain. An improper Rec.601 YUV to RGB conversion in software only gets worse when a ChinaBay adapter converts RGB back to YIQ! A vectorscope with SMTPE bars wouldn't be pretty, thats for sure.

Very few video cards were produced that had standards compliant video levels, those outputs were generally found on high end NLE capture cards that were designed to output the final product to analog tape formats. In order to test the levels of my VHS to DVD projects, I actually have to burn them to DVD and playback in a dedicated player. For test pattern generation I stick with DVDs or my ye olde Video Toaster.

drussell
02-16-2015, 03:06 AM
A good quality, professional-style scan converter probably works much better than the cheap Chinese versions.

I have used various models of Extron scan converters before in several applications with good results. Something like this:

http://www.ebay.com/itm/Extron-VSC-500-Video-Scan-Converter-Tested-/141576224364

Although I had to replace some of the electrolytic capacitors in a couple of model 200s and 500s that were purchased used also. (This particular auction that has already been done.)

Not a big deal though, especially if you get them nice and cheap and they just need some capacitors to power up without blinking off and on... :)

N2IXK
02-16-2015, 12:02 PM
Just bought an Extron VSC200 on eBay. The "Wun Hung Low"" unit worked well enough for initial system testing, so it will just get tossed into the junkbox once the Extron arrives. The Extron is a proper bit of professional rackmount gear, so hopefully the output signal is closer to NTSC standards too.


I had a feeling that the Chinese unit would be somewhat wanting, but never thought that the output would be quite THAT bad.

andy
02-16-2015, 12:27 PM
I have a VGA to NTSC converter box, about 4"x5"x1" used originally to deliver announcements over an in-school TV system. 640x480 only, and does only 16-bit color, so I have to use WinXP or older for the VGA video. ...

It is possible to get 640x480 from Windows 7. You have to go into 'screen resolution' 'advanced settings' 'adapter' and 'list all modes'. 640x480 will be on the list at the very top. You can setup your TV converter as the second display so you don't have to struggle with 640x480 as your main screen. Make sure you select to extend the desktop rather than duplicate the displays and that the 640x480 is not set as the primary display. Just drag your media player to the second display when you want to send it to the TV.

I've been using an old Thinkpad T43 with an S-video output for this, but I will have to find something better since it slightly distorts the aspect ratio. There's a slight black bar across the top and bottom, so it looks like all my TVs are under scanning.

Another option would be to use a HDMI to composite converter (available on ebay) but the quality of the scan converter may be lacking.

N2IXK
02-18-2015, 09:08 PM
The Extron arrived today, and I'm just blown away by the difference in performance and build quality. Of course, the Extron had a list price of about $4k, as opposed to the $12 special from the PRC. The upside of the phasing out of NTSC video is all kinds of high end gear available at cheap prices.

Anyway, the first scope shot is SMPTE bars played back through VLC media player on the laptop. Looking great! The vectorscope presentation isn't perfect, but it falls within the "broadcast legal" boxes. :)

Final shot is typical program material converted from VGA.

Findm-Keepm
02-19-2015, 01:00 AM
It is possible to get 640x480 from Windows 7. You have to go into 'screen resolution' 'advanced settings' 'adapter' and 'list all modes'. 640x480 will be on the list at the very top. You can setup your TV converter as the second display so you don't have to struggle with 640x480 as your main screen. Make sure you select to extend the desktop rather than duplicate the displays and that the 640x480 is not set as the primary display. Just drag your media player to the second display when you want to send it to the TV.

I've been using an old Thinkpad T43 with an S-video output for this, but I will have to find something better since it slightly distorts the aspect ratio. There's a slight black bar across the top and bottom, so it looks like all my TVs are under scanning.

Another option would be to use a HDMI to composite converter (available on ebay) but the quality of the scan converter may be lacking.
:ntwrthy: Thank you! I fought it on Vista, thought Win7 might have it, and lo and behold, you shed some light! Thanks! Now I can do VK on my 46" set :no:

Actually, I'll probably use this "new" capability to view laptop stuff on a CRT set at church. Gotta go - and start playing a bit!

Thanks Andy!!

CoogarXR
02-19-2015, 10:16 AM
Great thread. I have used plenty of VGA -> NTSC converters, and it's neat to see the visual test results.