video basics

[tbexon]

Hey I have very little experience with video and don't intend to be running video for any gigs so my question is completely out of personal curiosity having watched a documentary about video for a major touring band and done a bit of research but still have a couple of questions:

 

firstly how does the signal get from the cameras to the screens for instance what connectors are used to get the inputs to the vision mixer (I think that's the name) and when they reach the vision mixer are all the inputs local like on a standard analogue mixer or on a separate peice of hardware which then sends the data down a different cable (or cables) to the vision mixer.

 

second question: once they are at the vision mixer what can the switcher do with them? what 'effects' can he add I read some where that Vision mixers can also be used to create various visual effects, from simple mixes and wipes between sources to advanced composite effects. A) what sort of effects B) what's a composite effect

 

lastly: once they have left the vision mixer (from the program bus I do believe it's called) how do they get the screen? is it just one cable leading into the back of the screen like on a standard T.V or if it is something bigger say a huge LED screen is it the same principle?

 

thank you in advance for your time and helpful answers. It would be great if anyone could point me towards any online tutorials which answer my questions. apologise if this has been answered somewhere I couldn't find it nor could I find the answers to my questions on the wiki (not that I'm criticizing the mod team as you guys do a amazing job)

 

oh yeah think it's in the right place but mods feel free to move it :(

 

thanks again

[paulears]

That's a major question!

 

The answer comes in many varieties - if you look at the typical consumer video kit - it's analogue in the main, and the video signal is termed composite - it a mixture of all the things video kit need. The colour information, the detail information - essentially the luminance info (B&W) plus some timing information - the syncs. These are all merged together into composite, and sent down one simple cable, using the connectors designed for audio in the main, but adopted by consumer video needs. A phono connector, often coloured yellow for identification.

 

You can squirt this down decent cable for surprising lengths, so your camera could connect to a switcher/mixer where each source as in cameras, computers, video playback etc can be mixed and merged, and then the output via the programme bus as you mention gets sent to screen or projectors, again using composite techniques.

 

The downside to composite is that merging all the individual components tends to reduce quality a bit - it can still be good, but as each piece of kit converts this to the individual components to process and 'do things with', the repeated combining and splitting out again has an impact on definition. The next stage up is called 'component'. and there are a number of different versions, but in general, the image quality is higher again. Component mixers and amps for distribution are usually more expensive. Domestic and consumer kit uses phono connectors for cheapness, but professional kit uses a push an twist to lock connector called a BNC - these are properly designed connectors, and are electrically and physically superior.

 

Once you go digital, we move to the SDI - Serial Digital Interface - usually the same type of cable with BNC on each end, but this time it's a digital signal, not an analogue one. It comes in two main types SD-SDI, Standard Definition and HD-SDI, High Definition - to match the rapidly taking over HD video systems. There are many different types of HD format, but in most cases, the HD image is around twice the SD width and height.

 

The mixers can cut between sources, or mix between them, or do transitions that wipe, slide, push, rotate - or many other options of going from one source to another. Many can also key - this means they superimpose one source on top of another. This can be a luminance key, where perhaps a white caption is stuck over the image below, or it can be cleverer - chroma keying using a particular colour as the key - often blue or green. Wherever there is green in the picture, it gets replaced by the other source, so the newsreader can have the picture of the war zone behind them, or the presenter in the studio against a green background can be made to appear on a desert island. More advanced keying allows these things to be virtually invisible.

 

To be fair, I suspect that doing proper research into this from knowing nothing at all will take considerable time and effort. Every manufacturer has downloadable specs and brochures designed for beginners that explain features and benefits.

 

I think we can respond to specific questions once you have understood the basics, but a 'tell me everything' question is impossible to answer really.

 

 

Much of what I've said will confuse, no doubt - but I don't know how advanced technically you are.

 

Feel free to now ask specifics.

Paul

[boatman]

One of the best online resources for the technical aspects of basic analogue video is the Maxim App note 734. Once you've read that, Google for VGA, SDI (Serial Digital Interface)and HDMI (High-Definition Multimedia Interface) for more stuff.

 

There's a lot of very good video tutorials dealing with the production aspects on a much lower technical level here.

 

Hope that helps.

 

E2A: HDMI stuff.

[tbexon]

ah right yeah I'm with you on the composite thing thanks

 

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thnks boat man have read the articles on the ccu and video switcher on the media collegde website very usefull thanks :(

 

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ok thanks for that paul ears and boat man for that I think I now have more of a understanding of the basics of video and will endeavour to read those articles you recommended and sorry now realise how broad a question my original question was however my next question is once the video has been processed at the vision mixer how does it get to a really big LED screen from the documentary I mentioned before I noticed that each 'section' of the LED screen had a individual signal connector how does that work is there a distro type thing that has one input and splits the input 'picture' across the pixels in the individual screen? or have I got it entirely wrong?

 

thanks again

[boatman]

I assume that by a "big LED screen" you mean a video wall. This usually comprises a bank of 3x3, 4x4 or even more standard displays. Each display is fed from a frame store which picks out the particular section of the picture required for that display and sub-samples it back up to a full screen picture of just that section. So a 3x3 video wall requires 9 framestores. Dumb framestores will simply repeat pixels horizontally and vertically to get the required picture size but more expensive ones will use spatial filtering to reproduce a higher resolution picture.

[tbexon]

ah thanks thats a great help I am still reading all these articles it's alot more complicated than I ever thought :(

[paulears]

True, but you can't expect to learn an entire new area of the industry overnight - that's unrealistic, and we've only really scratched the surface. We've also very much simplified how it all works, don't forget - so there are huge minefields to navigate though. Reading the suggested material is a place to start - it cannot replace experience, but with video - the technical vocabulary is a good start so you should be able to understand the specs you are reading. We've not even touched on video standards and there are so many now we are dealing with SD and HD - the other thing is scaling. All the screens and kit have different aspect ratios - so 4:3 images on a 16:9 display need stretching or squashing, or shrinking, and so it starts!

[tbexon]

true fair enough I never really expected to get anything but a nudge in the right direction so as people mention issues I can then go and do some mroe indepth research into it

[sleepytom]

I assume that by a "big LED screen" you mean a video wall..

That's an odd assumption. I presume he means an LED Screen, as these are the industry standard large scale display for big outdoor events.

The "How does an LED screen work" question is a big one in itself. There are a few different systems in use but they all basically work in a similar way; video (or VGA / DVI signals) is fed into the "led processor" this then sends out the control signals to the led components, pixel mapping the incoming video signal to the LED pixels. http://www.howstuffworks.com/jumbo-tv2.htm is a basic explanation of the way and LED screen works...

 

As a general starting point wikipedia is not a bad source of information. Don't think that it provides 100% accurate, comprehensive answers but to get your head round some of the concepts involved in video systems its a reasonable starting point.

http://en.wikipedia.org/wiki/Tv_production

http://en.wikipedia.org/wiki/Video_signal

http://en.wikipedia.org/wiki/Professional_video_camera

 

The best way to learn is to get involved in productions that use video, only by coiling the different types of cable can you really learn anything about them!