Generate composite and s-video from luma and chrome

Thread Starter

supertony

Joined Jan 15, 2013
33
I could not find any good resource about circuits for generating composite video from the luma and chroma along with explanation how it works and why they are build like that. I saw few with bjt and fews components...any idea? thank u
 

drjohsmith

Joined Dec 13, 2021
377
Re the research you have done,
just want to check our common reference

what do you know about Luma / chroma and composite video
whats the difference

whats the timing difference between them

Do luma / chroma and composite have synchronisation
how is the colour information encodes in the two systems ?
 

Thread Starter

supertony

Joined Jan 15, 2013
33
Hi. I will try to better expose my case. Yes I learned how the composite signal is generated. Basically I have an old chip that generate a sync+lum signal and color which are feed to an RF modulator which generate an s-video signal/composite.

This is the current circuit ( PAL version)
http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/c64/Modulator_MD6_VA3403_UE3603.jpg

I would like to :
- understand how this circuits works.
- if it's possible from these sync+lum and color to generate RGB/Vga/hdmi

I am not searching for a solutions ;) just hints. I was able on to find a book about op-amp how to generate composite from RGB ( block diagram).
I started reading this one https://www.tinaja.com/ebooks/tvtcb.pdf hope will find some good info ;)

Thank you.
 

MrChips

Joined Oct 2, 2009
25,919
If this is just an educational exercise, I would start with just reading about VGA. It has the five required signals, R, G, B, H, V, already separated.
 

drjohsmith

Joined Dec 13, 2021
377
If this is just an educational exercise, I would start with just reading about VGA. It has the five required signals, R, G, B, H, V, already separated.
"All" colour are described in a "colour space"

This can be amongst other
RGB, Red Green Blue
RGBK, Red Green Blue Black
Luma / chroma

There are conversion "matrix" between the different colour spaces,
e.g look here

https://en.wikipedia.org/wiki/YCbCr


The other "information" that all transmission systems need, is where each "colour" goes,
this is "commonly" a start of page / frame / line , for a "pixel"


How this information is encoded , is system dependent,

So PAL TV of old, transmitted Luma as an amplitude above "black" reference
Luma , as a phase shift
Timing, as a level / edge
Black level as a DC level at a certain point in the timing,


Now when it comes to digital systems,
they can just digitise the analog information, as in SDI
or they can packetize, such as MPEG

Good luck
 
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