Mixed VGA and Video - what are the options?

Most VLOBLIVE events require both video sources - VCRs DVDs cameras etc. and also PC graphics sources. Somehow both of these types of signals have to end up at the projector.
Here's a quick lowdown of the main ways to do it and why you might choose each one...

There are three main approaches to setting up a mixed VGA/Video system, and each has its advantages and disadvantages.
I will try here to describe them and say when you might use one or the other.

First let's explain what I mean by VGA and Video, as the terms are often interchanged.

VGA means analog or digital progressive scan PC graphics format (regardless of resolution) using separate Red, green, and blue signals. This is sometimes known as RGBHV. This is normally what goes between your PC or Mac and your monitor and is most familiar as the 15 pin D-type connector.

Video means any form of interlaced video - composite, svideo, component etc, but basically it is video signals at standard TV resolutions (Let's ignore progressive scan HD video for now).

The main point to note is that they are not compatible. They run at different frequencies, they have different numbers of lines, they 'draw' the image in different ways, etc. etc.

So if you are going to start with both VGA and video signals, and end up at a projector, you have three main options.

1. Keep them separate all the way to the projector.

In this option, the video is switched or mixed separately from the VGA and both signals arrive at the respective projector inputs. Any switching between them is done at the projector, or via the remote control, or via a serial port etc.

In reality of course, the video has to be scan-doubled up to the native resolution of the projector LCD or DLP panels so the conversion is 'invisible' as far as the setup is concerned.

Advantages:
+ No special converter boxes required
+ easy to configure
+ full VGA quality maintained for graphics

Disadvantages:
- twice as much cable required (VGA and Video)
- video scaling is dependent on internal scaler in projector - may not be very good!
- often slow to switch inputs on the projector (has to re-sync). Can be several seconds!
- can't fade between graphics and video

When to use:
When you are MOSTLY showing graphics and only the occasional video
When you only have one video source

2. Video all the way

This approach stems from a 'broadcast' model where everything gets turned into video, and then mixed, tweaked, and displayed as video.

This then requires that the PC graphics get converted from their native format (SVGA, XGA etc) into standard video resolution, and also the timing is changed to suit the interlaced TV format.
To do this you typically use a box called a scan converter, or some graphics cards may have this function built in and provide a TV out port.

Once the VGA is converted to video, you can mix, switch and route it just like normal video.

Once the video signal gets to the projector it is converted BACK into VGA to match the native resolution of the projector, and this requires another conversion step. Inevitably the quality suffers a bit at this stage, and there may be a delay introduced of a few frames (not a big deal for playback, but pretty noticable for live camera feeds). How much the quality suffers depends on how good the converter chips in the projector are.

You can compare this, if you connect a PC with a TV out graphics card AND a normal VGA output to the same projector. You can then switch between them at the projector, while displaying the same signal, and you will notice a significant drop in sharpness and quality for the video signal.

Advantages:
+ common cabling throughout
+ only one cable needed to projector (cheaper than VGA cable)
+ can use standard video mixers, switchers etc. including ex-broadcast gear cheap from ebay
+ easy to do graphics overlay (words over pictures) using Keying on mixer

Disadvantages:
- loose quality on the PC graphics - how much depends on scan converter quality
- rely on internal video converter chips in projector
- good quality scan converters are expensive (cheap ones are rubbish)

When to use:
If you are doing mainly video (IMAG or playback) and only occasional PC graphics, or if your PC graphics are mainly song words overlayed on live video.

3. VGA all the way

This approach stems from the 'presentation graphics' model where everything is cabled as VGA, and interlaced video is a weird language that needs to be translated as soon as possible.

The most common way to achieve this approach is to use what is often called a 'seamless switcher' or a 'presentation switcher' which accepts both video and VGA inputs and scales them all to match the native resolution of the LCD projector.
Ultimately this keeps the best possible quality on the VGA signal, while maximising the conversion quality of the video. The conversion chips in a seamless switcher are significantly more sophisticated than those in a projector, so the results are generally a lot better. Having said this, the cost of even the cheapest seamless switcher is about the same as most sane people would pay for the whole video projector, so it is no real surprise.

Advantages:
+ best possible quality for all inputs
+ no conversion required at projector
+ only one cable needed to projector (albeit a more expensive one)

Disadvantages:
- VERY expensive for seamless switcher (but there IS a cheaper way using a software solution)
- cabling is all more expensive
- even more delay introduced for live inputs (cameras)

When to use:
If you need the best possible quality of graphics AND you regularly use video, and you have plenty of money.

Posted: Fri - May 21, 2004 at 10:09 PM |

VLOBLIVE :: 1. Preparation

Mixed VGA and Video - what are the options?