Simple VGA to YPbPr cable? or decieved masses? - Page 1
Forum section: electronics / beginners

author: Cliff Matthews
author_rank: Unknown
timestamp: April 05, 2015, 12:13:10 am
content: I have a lot of confidence in modern electronics,perhaps too much. Trying to make a cable for a friend, I soldered something together based on a quick search.https://www.google.ca/search?q=diy+vga+to+pr+y+pb+cable&safe=off&tbm=isch&tbo=u&source=univThis pinout is present on a lot of AV web forums and even Amazon sells these things - but do they work? (for me, no).Looking at it, I kind-of thought there had to be at least capacitors to throw-in sync pulses somewhere, but I somehow trusted that modern electronics could make up for pulses based on timing gaps in the other signals. Is this the case? I mean, they're selling tons of them right?I've tried this cable on two flat-screens and keep getting colored and sharp diagonal video lines (void of any sync I suspect) and soon I'll loose face giving it to my friend to try on an older Phillips Plasma TV with no VGA input. Would there be any hope throwing a couple X? picofarad caps from the H and V TTL lines to the Y signal?I think this circuit may be over-kill and requires power - can anyone recommend a good passive (in the cable) solution?

author: rx8pilot
author_rank: Super Contributor
timestamp: April 05, 2015, 12:24:46 am
content: I never paid much attention to VGA signals, but....I think VGA is RGB+S where YPrPb is a component standard where sync in embedded in the Y channel and the Pr and Pb channels are half bandwidth. It is possible that some VGA outputs could have unused pins that have component signals? The VGA-BNC cables I have have 4 signals - RGB + sync.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: April 05, 2015, 12:25:02 am
content: VGA means nothing more than "it's not EGA/CGA". Without knowing exactly what the timings are, it's hard to say what the other device will do.Essentially Y Pr Pb refers to a colorspace and how it's encoded. The rest is up in the air.Flat screen means TV? A TV would either work in 15KHz or if you're lucky it accepts scan doubled video.I'd guess that the cable is meant to work with a certain video card that converts the color space internally and provides the proper timings for a TV.Also, in 2000, there where hardware MPEG decoders to play back video, and that HD-15 connector might not refer to a VGA video card at all.Out of the box, VGA is non-interlaced, old-school TV video is interlaced, it will not interconnect without scan doubling hardware, which requires much more than a handful of gates and opamps.Something else is going on here. You can not go from VGA to video "passively" unless the video card has specific hardware to do so.

author: Ian.M
author_rank: Super Contributor
timestamp: April 05, 2015, 12:43:22 am
content: ... or its not actually VGA, i.e. the specific video card supports outputting component video with sync on Y on the VGA RGB pins.Once one company does something like that, the chinese cable manufacturers clone the official cable and it doesn't take long for it to be sold as generic with no information about compatibility.

author: Cliff Matthews
author_rank: Unknown
timestamp: April 05, 2015, 01:12:54 am
content: The flat-screen in question was $1000 just 6 years ago - I thought at that price it should have had a VGA input...http://www.amazon.com/Philips-42PFP5332D-42-Inch-Plasma-HDTV/dp/B000NV7PUO/My friend is showing PowerPoint slides for kids from an old laptop and the TV specs say it has 1024 x 768p resolution.From what I see, the active circuit floors the op-amp input low on vertical pulses and does a little less for each horizontal pulse.If I set the laptops 2nd screen output to something rustic (like 800x600), can I try bridging two different caps across to the Y line without harm?Should I expect to see any results if the TV receives some benign aberrations (sync pulses) on Y?

author: amyk
author_rank: Super Contributor
timestamp: April 05, 2015, 01:30:16 am
content: That looks like a passive R -> Pr, G -> Y, B -> Pb adapter and will only work with the few video cards that can output component video with sync-on-Y through their VGA output.

author: Richard Crowley
author_rank: Super Contributor
timestamp: April 05, 2015, 01:37:28 am
content: What is the actual question here?VGA is RGB, it is NOT Y-Pr-Pb ("component"), and there is no passive way of converting RGB to Y-Pr-Pb (or vice-versa)VGA also has separate H and V sync. Y-Pr-Pb wants embedded sync on Y. There are some fiddly and unsatisfactory ways of doing that passively.It appears that there is a fundamental mismatch between the source equipment (identity not revealed) and the destination (also a mystery).For that matter it is not revealed whether the Component end is the source or the destination (and vice-versa).

author: Cliff Matthews
author_rank: Unknown
timestamp: April 05, 2015, 01:48:46 am
content: Amyk, so what I saw as diagonal video was worse than just being out-of-sync? Someone else mentioned color space.What I was hoping for was simple like this:http://www.epanorama.net/documents/vga2rgb/interfacing.html#vga_3bncThere's just a FET and a 74HC chip, it looks like it could run for days on a small battery...

author: Ian.M
author_rank: Super Contributor
timestamp: April 05, 2015, 01:51:14 am
content: The display's component input may not support any resolution or frame rate the laptop's video card can output even with an active converter in between.The Amazon link says the display has HDMI.If the laptop is old to the point of obsolescence, I suspect a USB HDMI second display adapter is not a particularly good idea (driver compatibility and system stability) and I expect a VGA to HDMI converter box would have the best chance of working.

author: Cliff Matthews
author_rank: Unknown
timestamp: April 05, 2015, 01:53:20 am
content: Source = older IBM laptop with VGA 15-pin on the side.Destination = 6-yr old Phillips 42" Plasma TV (sadly, with only HDMI, composite, and component inputs)

author: andtfoot
author_rank: Unknown
timestamp: April 05, 2015, 02:01:35 am
content: Having multi-format inputs shared on a connector on video gear is quite common in at least the professional AV world. Outputs... less so. (it's usually harder to control or change the source than the display/sink)One of those passive breakout cables would normally connected to one of these multi-format inputs. Examples of these inputs from a couple of the bigger players:Crestron,Extron, andAMX

author: Cliff Matthews
author_rank: Unknown
timestamp: April 05, 2015, 02:02:55 am
content: Ian, I searched ebay forVGA to HDMI converter boxand foundhttp://www.ebay.com/itm/1080P-Audio-VGA-to-HDMI-HD-HDTV-Video-Converter-Box-Adapter-for-PC-Laptop-DVD-US-/181306560583what you mentioned - China does it again! GeeWhiz! And at $16.39, it's a price my friend can handle!

author: Cliff Matthews
author_rank: Unknown
timestamp: April 05, 2015, 02:37:28 am
content: Thanks guys, I guess I've done enough fruitless soldering. Now I can save my capacitors for something that will actually functionI supply the cables and for under $20 there's a useful, multipurpose, hardware-only solution with better quality video and audio too.

author: Cliff Matthews
author_rank: Unknown
timestamp: April 05, 2015, 06:40:01 pm
content: A final note to those who are "ebay-shy" and would like this "piece of kit" - I found it for$17.68 at Amazon:http://www.amazon.com/1080P-Audio-Converter-Adapter-Laptop/dp/B00K3OJPZOHope this journey helps someone else!- Cliff