Component / composite video signals - Page 1
Forum section: electronics / projects

author: Leopoldo
author_rank: Contributor
timestamp: December 13, 2014, 12:40:43 pm
content: Hi,I wanted to know what kind of signal I can get out of a composite video cable; I mean,could I use it let's say to see only the blue channel of a video by connecting the blue cable to a COMPOSITE video,not component, connector of a tv? Do they have the same voltage and are compatible? I don''t have access to a digital oscilloscope and I don't want to blow my tv RCA inputs...again. Thanks a lot!!!

author: tom66
author_rank: Super Contributor
timestamp: December 13, 2014, 12:49:22 pm
content: The component video signals are Luma (Y), Chroma Blue (Pb), and Chroma Red (Pr)It is clear there is no green signal. Y is the weighted sum of red, green and blue. Pb is the difference between the luma and blue components, and Pr is the difference between luma and red.Only the luma signal contains the necessary sync signals, so just connecting the blue signal won't work (and it would give you the difference between luma and blue, not blue.)You could calculate the blue signal with some differential amplifier.If you connect just luma then you get greyscale. This will usually also work with a composite video input if 480i/576i is used.

author: Leopoldo
author_rank: Contributor
timestamp: December 13, 2014, 01:31:21 pm
content: Is there another way to get the RGB components and sync? maybe through another kind of video cable... I tried with composite video,but it requires a lot of external components that are also quite unstable (caps and inductors) to separate the 3 colors,so I thought that component video would have solved it,and to use differential amplifier I'd have to buy high speed op amps,which cost...

author: tom66
author_rank: Super Contributor
timestamp: December 13, 2014, 02:05:29 pm
content: Page 9:http://cds.linear.com/docs/en/application-note/an57fa.pdfNot too expensive, depends what bandwidth you need. For 1080i you need 150MHz+ bandwidth. For 480i, only 10MHz maybe.Or,http://www.ti.com/product/lmh1251http://uk.farnell.com/texas-instruments/lmh1251mt/ypbpr-rgb-vid-convert-switch-smd/dp/1286773(Note only up to 30Hz with 1080 line input)

author: Leopoldo
author_rank: Contributor
timestamp: December 13, 2014, 03:34:04 pm
content: Oh. Fu**.It was seriously that easy??Can't believe it. I have to really thank you for your advice and links, I'll probably choose the LMH1251 because it's easier to wire up and almost does not require external components,which otherwise would create noise or distortion.Just another thing: in the description of the ic they say :"The LMH1251 is a wideband 2:1 analog video switch" ,but what does it mean with 2:1 ? I can't figure it out...https://store.ti.com/LMH1251MTNOPB-YPbPr-to-RGBHV-Converter-and-21-Video-Switch-P42677.aspx

author: tom66
author_rank: Super Contributor
timestamp: December 13, 2014, 04:12:36 pm
content: It has two inputs, one which supports VGA-style signals (R,G,B,H,V) and one which supports YPbPr with sync on luma/green (the typical standard)In your case you'd wire up an input to the YPbPr pins then drive pin 24 HIGH to select YPbPr input. Don't forget to terminate the signal near the input to the chip. If the trace to your chip is long you may need to control the impedance of the relevant track.Drive pin 23 to set the chip to AUTO mode (see datasheet) which will allow it to detect the relevant signal format to process.

author: Leopoldo
author_rank: Contributor
timestamp: December 14, 2014, 09:38:09 pm
content: And if I wanted to use a composite video signal? is there an IC that can convert it into RGB components? Because I am not completely sure that with component video I would always have 25 fps,and I need to have them.

author: tom66
author_rank: Super Contributor
timestamp: December 14, 2014, 10:51:13 pm
content: Quote from: Leopoldo on December 14, 2014, 09:38:09 pmThere do exist other devices, such as ones from Analog Devices, but they're considerably more expensive (about £15 in one-off quantities)The refresh rate of composite can change too. What is your video source? Many devices can be set to 50Hz or 60Hz only. But why can't you handle a 60Hz signal?

author: Leopoldo
author_rank: Contributor
timestamp: December 15, 2014, 07:57:00 pm
content: Because I'm not using the signal to create a digital or analog image for a TV,instead I'm using it to show the video through a mechanical videoprojector,and I'd prefer if I could have a still number of frames,otherwise I'd have to fix some amplifiers and non-linear components...Besides,most DVD players and other devices have the composite video output; I thought that a component signal would solve the problem but I didn't think about the frame rate; anyway,do you know an IC that could convert composite video into RGB? It'd be very helpful,because otherwise I'd have to find an old CRT color TV and take the circuit from that