THE INS & OUTS OF INTERFACING by Dan Ramer on June 5, 2000

I have yet to meet a home theater enthusiast who hasn't at one time or another been frustrated by the variety of signal formats and connectors that cause annoying incompatibilities. Take video for example. We have RGB, component, S-Video, and composite. Then there's interlaced video and progressive scan. RGB might use either BNC connectors or RCAs, as does component video. S-Video utilizes the familiar, specialized, multi-pin connector, but mysteriously turns into a YC-358 connector on a professional product. And just because the connectors are the same doesn't necessarily mean the signals are compatible. The three RGB signals are not the same as the three component signals. And DVD's three component signals are not the same as HDTV's three component signals. So let's briefly take a look at a few problems and some possible solutions.

Video Conversion and Switching

One of the most helpful video processing vendors on the planet is Extron. I first discovered Extron several years ago when I was involved in a serious image-processing project. We captured images using highly specialized cameras with shutter speeds measured in the billionths of a second. This video was recorded on digital videotape and returned to our supercomputer facility where the data was dumped to hard disk. The images were then analyzed to reveal an object's performance while our developers oversaw the complex process on a Silicon Graphics Workstation. We were obligated to report our project's progress to some very high-ranking government officials from Washington. To demonstrate our system, we hung a Barco graphics-grade projector from our conference room ceiling, two hundred feet away from the Image-Processing Lab. But how do we get the signals from the workstation to the projector? Extron to the rescue. Extron products buffered the signal coming from the workstation, drove the local 21-inch display, independently drove the long RGB-HV video lines to the conference room, and even allowed us to adjust the frequency response of each signal to equalize the cable losses. Very impressive.

When I decided to add HDTV to my home theater, I found that the only set top box that passes transmitted pictures without format conversion and that permitted digital recording of the MPEG-2 bit stream was made by Panasonic. But Panasonic's HDTV outputs are in component video form, not the RGB-HV signals I need for my projector. And how was I to switch between the output of my Faroudja Line Doubler and the HDTV set top box? Once again, Extron to the rescue. Twice.

To convert HDTV component video to RGB-HV, Extron offers the CVC200. This capable little box can convert eight different component video formats (including both DVD's and HDTV's 720p60 and 1080i30) to RGB-HV.With a bandwidth of 30 MHz, it is more than adequate for my needs. And it can double as a converter for reviewing progressive scan DVD players, permitting me to drive my projector from those sources as well. Extron also offers a family of RGB and RGB-HV switches for two, four, and six sources. I chose the SW2ARHVxi. This box offers a 350 MHz bandwidth and crosstalk and signal-to-noise ratio performances that significantly exceed any signal I might care to throw at it. It will also double as a component video switcher (without an operable auto-switch feature that relies on the presence of sync information) for my DVD player reviews. (Note that these products use 75-ohm BNC connectors.)

Extron's performance is beyond reproach, but I will offer a caution to any of you that might purchase the switcher. To accommodate a variety of signal formats that might require a tee connection (like RGsB), the vertical input is not terminated in 75 ohms. This might upset one of your signal sources, resulting in a vertically rolling image. My Faroudja was not at all happy until I put an external 75-ohm termination on the vertical input of the Extron. To browse through all of Extron's products and for a complete library of their Adobe Acrobat manuals, visit Extron. Great professional products.

Digital Audio Media Conversion

Here's another problem that caught me by surprise. Panasonic features an optical digital audio output on many of its products and omits a coaxial connection. But my B&K AVP3090 decoder/preamp is coaxial input only. What to do? Visit Midiman for a variety of digital audio products (again with available manuals) including a delightfully simple little box called the CO2. This bi-directional converter accepts both an optical and a coaxial bit stream and converts each to the other flavor. Its bandwidth is sufficient for the highest available bit rates implemented by both DTS and Dolby Digital. Just plug in your cables, connect the external power transformer with its integrated low-voltage cable, and you're done. I connected the optical output from the Panasonic set top box to the Midiman; then I connected the Midiman's coaxial output to the B&K's coaxial digital audio input. Terrific results. I also connected a review sample of a Panasonic DVD player to my B&K in a similar way. I tried DTS DVD, DTS CD, Dolby Digital DVD, and conventional CD. Flawless. Under $80. Highly recommended.

Composite to S-Video Conversion

Do you have a composite source - such as a video game - that you wish to connect to an S-Video input. Don't despair. Try visiting Camelot for both passive composite-to-S-Video conversion and active composite-to-S-Video conversion and switching. Such conversion requires the separation of luminance and chrominance from within the composite video. The black & white information and the color information are then sent down the independent conductors of S-Video. Passive separation will unavoidably reduce the luminance bandwidth, lessening the detail. Active separation, properly done, can maintain the luminance bandwidth and the picture detail. So passive conversion is more suitable for games and VHS, and active conversion is suitable for more demanding applications.

External Closed Caption

Do you happen to need closed caption, but you own a front projector without a tuner or CC decoder? Try the MYCAP Junior from MYCAP USA in Irvine, California. Once again, this is a simple product for less than $100; it adds an external CC capability - for all of the CC formats - to any system in which a composite video path is available. That, by the way, is the downside - it only works with composite video. But if you need CC capability, it might be worth the compromise.

Parting Thoughts

There are a surprising number of solutions out there to solve sometimes obscure - but always annoying - problems. As the ever-growing market for home theater matures further, I'd expect to find a product to solve any interfacing challenge. And then there are the serious signal processors, like line doublers and interpolators. But that's a subject we'll have to save for some other time.