Articles and whitepapers

Video Signal Processing in the Home (1/11/2004)

By Nick Mawer, Kramer Electronics UK

Why might you want to process your video signal in the home? After all, were we not constantly told by hi-fi manufacturers to do as little to the audio signal as possible? My QED 240 power amp (old I know), even has the option to bypass balance control, leaving volume as its only control.

It may be true that video is processed a lot more than audio from the moment it is captured to the moment it hits your screen, and frankly speaking, a bit more should not hurt. In fact it is sometimes absolutely necessary. How do you watch an NTSC tape on your PAL television without processing your picture? How do you convert from one type of signal to another to facilitate easy switching between source devices? How do you improve the quality of video tapes that have been watched many times before, or are generations of copies from the original? And how do you tweak your set up to get over the fact that different manufacturers' devices treat video signals slightly differently, and get them all to look the same on your display?

Video processing

Broadly speaking, there are four types of processing: standards conversion, format conversion, time base correction, and video enhancement.

Standards conversion

Different video standards are used around the world. In the UK and most of Europe the standard is PAL (Phase Alternate Line), with many variants of the basic system in terms of video channel bandwidth and audio frequency. PAL features 625 lines per frame and 50 frames per second, although only 576 lines are visible. NTSC (National Television Standards Committee or sometimes Never Twice the Same Colour) is used in the US and Japan. NTSC features 525 lines per frame and 60 frames per second. There are two versions of NTSC as well: 3.58 and 4.43. SECAM is primarily used in France, Eastern Europe, and some parts of the Middle East.

If you want to watch an NTSC tape on a PAL television, you will need to standards convert the signal. In some instances, this can be done within special VCRs, or a standalone box may be used in conjunction with a VCR in the standard that you want to watch. The process of standards conversion includes colour decoding, where video is separated into its components - usually in a digital form - and frame buffering when converting between PAL and NTSC, as the timing of these standards differ. The final stage uses a digital encoder for receiving component video, and converting the components to composite video or Y/C in the required standard.

Format conversion

The important thing to realise is that format conversion is going on right in front of you, even if you do not realise it. For instance, if you are watching a composite video signal on a television, your signal is being format converted from composite to RGB. Not all format converters are equal, and some conversion tasks are more complex than others. This means that sometimes you will get a better picture if you use a standalone box to do your format conversion rather than relying on your display device to do it for you.

There are many video signal formats: composite; YC or S-video; YUV (Y, R-Y, B-Y), (Y Pr Pb) and (Y Cr Cb); and RGB and its variants. Converting between YUV and RGB is relatively simple. It requires an accurate colour matrix which strips the brightness information from the R, G and B signals, and adds them together to make the Y signal. The green signal without its brightness information is removed as well (if you know three parts of a four part puzzle, the other part can be reconstructed).

To convert between YC and component video, a colour encoder or decoder is needed. The colour encoder receives the component signal and creates a chrominance signal by extracting the blue and red information from the component signal, and modulates this information using the colour subcarrier signal. A colour decoder does the opposite of this. The colour subcarrier is a sine wave with a specific frequency and amplitude that effectively allows the decoder to work out what the correct colour is. The colour subcarrier frequency for NTSC is 3.58MHz and for PAL is 4.43MHz.

Choosing a format converter

This is all fine in theory, but can we use this information in selecting a product? Well not really, but we can look out for the method of filtering or separation between composite and YC. There are three basic types of filter: low pass - which is a largely superseded method; notch filtering; and comb filtering. Comb filters are more expensive than notch filters, and more accurate at separating the colour subcarrier from the genuine colour information. The Kramer FC-10D is an example of a device with a digital Comb filter.

The Kramer FC-10D Composite-YC Comb Filter/Transcoder

Time base correction (TBC)

Video tape recording is not an exact science. Videotapes develop jitter as well as colour and timing errors. This is an unavoidable fact because of the way that tapes are recorded, and the fact that tape stretches and is subject to environmental interference. These errors are magnified with every generation of recording, such that the picture may become unwatchable. One of a time base corrector's many functions is to provide a constant standard output which produces an error-free image. In a nutshell, a TBC rescues poor quality images and produces good ones.

Defining the video enhancement required

Before buying a video enhancer or processor, you should define the problems you want to address. Sometimes only moderate processing is needed such as brightness and image definition control, which may be achieved simply or inexpensively.

In other cases, a stronger tool is needed, such as for full colour correction, black level control, contrast control and more. The Kramer SP-11 video processor for example, is used by several departments of the BBC to fine-tune the colour balance of plasmas and other display devices.

The Kramer SP-11 Video Processor

Sometimes video enhancement features, such as video levels, black levels, colour saturation and colour balancing, are found on time base correctors. Many of these signal processes can also be applied to RGBHV signals. For example, the Kramer VP-11 allows the red, green and blue colour levels to be controlled individually, making the picture more or less red green or blue, and to help ensure that whites appear white (white balance). The machine also features contrast control which is ideal for data projectors. It helps to ensure that you do not have burnt out highlights at the same time as having no shadow detail. Definition or cable equalisation ensures fine detail can be retained however far away the display is from the source.

The Kramer VP-11 Video Processor

Another feature is that black level control works on DC offset, which is important as some play devices do not clamp the black level, leaving it to 'float.' This manifests itself in an image as grey where black should be. Black level clampers are available for this problem when it appears with video signals. Finally, H&V shift control are available to ensure that the picture is centred correctly.

Conclusion

With video processing, it is possible to watch video tapes recorded in different countries in the UK. If you want to watch an NTSC tape in PAL you can do it - you need an NTSC player, a standards converter, and your own TV. It is possible to get better pictures by employing specialist standalone format converters. It is possible to enhance poor quality images on tapes to get a stable, better picture, and it is possible to tweak the look of your picture.

In this world there are two often conflicting arguments:

1) You get what you pay for.
2) The law of diminishing returns.

In the world of signal processing both are true. There are low-cost, low-quality solutions on the market, but beyond a certain price point, your eyes will not be able to tell the difference, and you would need complex test and measurement equipment to see the benefits of spending more. After all, it is what your eyes see that matters.

Nick Mawer is the Marketing Manager for Kramer Electronics UK Ltd, manufacturer of distribution amplifiers, switchers, matrix switchers, processors, encoders, decoders, transcoders, and a full range of interfaces in a variety of formats, sizes and prices.

www.kramerelectronics.co.uk