Video to VGA Conversion - What's the big deal?

There are plenty of devices available that can convert VGA (and SVGA, XGA, etc.) to composite and s-video... some do it extremely well and others, ah, um, shall we say, not so well. I think it is fairly safe to say you get what you pay for. With only a few exceptions, the process is generally not reversible within the same unit, i.e. video / s-video to VGA conversion. Why?

Without exception all devices are digital, that is, they sample the analogue input which converts it to digital data, then store the data in digital memory, perform some complex mathematical filtering, and finally convert the resulting data back to analogue again. There are further complications too...

1. VGA uses a screen refresh technique called non-interlaced or progressive scan whilst video uses interlaced scan. These are major differences and have to be allowed for in the conversion process, usually with some user control over how much vertical filtering is applied as different picture content needs different treatment.

2. Modern VGA monitors allow viewing the whole picture, right to the edge. Video does not. About 10% of the screen area is never seen on a properly adjusted TV. There are various reasons for this... sync and blanking pulses have to be hidden, test patterns and teletext and macrovision have to be hidden, etc. The upshot of this "overscanning" is that when VGA is converted to video it has to be shrunk down a bit (or underscanned) to fit the whole picture into the visible area of the TV. This also needs a user control, because no two TV's will have exactly the same amount of overscanning!

With VGA to video conversion there are three streams of analogue data (red, green and blue) as well as two streams of TTL pulses for the horizontal (lines) and vertical (refresh) synchronisation. These five things are converted and mixed down to one (for composite video) or two (for s-video) signals of much lower bandwidth. The resultant video / s-video output is useful for recording to tape or for showing the VGA screen on a video projector (as opposed to a data projector).

As a rough guide, the bandwidth of the red, green and blue signals in VGA (SVGA, XGA, UXGA, etc, etc.) will likely be a minimum of 40MHz each to 400MHz each, or even higher! As well as the huge bandwidth of the VGA analogue signals, the sync frequencies are high too. Typically, VGA line sync is 31.5KHz to 100KHz or more and refresh rate is a minimum of 60Hz to 200Hz or more. This is how such fine detail can be displayed on a VGA monitor, but since video has much lower resolution, loss of fine detail will result from the conversion process.

The luminance portion of a composite video signal has a practical bandwidth of approximately 2.5 to 3 Mhz, and that of an s-video signal about 5 to 6MHz, still way short of even a single colour's bandwidth in the lowest possible VGA resolution. Obviously, the same fine detail is just not possible on a TV screen. The sync frequencies for video are rigidly fixed at 15.625KHz and 50Hz for PAL and 15.734KHz and 59.94Hz for NTSC.

OK, so hopefully, all the above should give you an idea of the complexity of VGA to video scan conversion. Now let's look at the reverse process, Video to VGA...

To do the job properly the video or s-video signal has to be split into it's red, green and blue components and the sync has to be extracted. These signals are digitised and stored in memory for some very fancy mathematical manipulation on their way to the digital to analogue converters. Interlaced lines have to be interpolated to non-interlaced, and the picture must be re-sized to fill the VGA screen, yet hide the junk around the edges. Horizontal and vertical sync's have to be altered to suit the target VGA resolution (640x480, 800x600, 1024x768, etc. at whatever refresh rate is applicable) and a user control to select which VGA "standard" is desired is likely to be necessary.

Herein lies the rub... cheap converters (generally known as Scan Doublers) do some of the job, but stop short of altering the vertical refresh rate and provide nothing but bog-standard VGA output. The reason becomes apparent when we look at the frequencies. NTSC (the big market for these things) produces a horizontal frequency when doubled of 31.468KHz (15.734KHz x 2) and refresh rate of 59.94Hz which is very close to the 31.5KHz/60Hz of standard VGA, so the VGA monitor is happy.

Unfortunately, PAL produces 31.25KHz (still close enough) and 50Hz. This 50Hz vertical refresh rate is far too low for most modern VGA monitors which have low persistence phosphors and even on the ones that will display it, the visible flicker is painful, really horrid! This is the stuff that migraines are made of. It's like staring into a strobe light!

So, el-cheapo scan doublers are barely OK for NTSC, but useless for PAL. What you really need is a device which offers full conversion of horizontal and vertical frequencies that exactly match actual VGA standards - these are usually called Up-Converters, Video Scalers or Scan Managers. They work beautifully, but they can be very expensive. We now have a good quality Up-Converter - the CDM-640 - which is budget-priced and also functions as a Standards Converter and a Multi-standard Timebase Corrector.

(c)2002 Quest Electronics Pty Limited.