Mr. Blandings Part 7 - November 2003
Transcoders
by
Nov 15, 2003
Nov 15, 2003
In Part Six, I described upgrading my system with a Denon DVD-9000 player, which caused a domino effect that forced me to rework my theater’s video path and processing. Since I had both a progressive DVD player and an HDTV set top box, I wanted to switch high bandwidth component video and send the switched signal to my existing Extron transcoder. (A transcoder is required to produce the flavor of video required by my NEC CRT front projector.) My B&K digital audio decoder/preamp has component video switching, but according to its product manual the frequency response of the B&K switch is only 10 MHz. That may be more than enough for DVD but is woefully inadequate for HDTV. (Months later, I would discover that the manual was incomplete and B&K’s internal component video switch does indeed support HDTV.) I couldn’t find a reasonably priced, high bandwidth, 2-channel component video switcher, so the only choice that seemed viable at the time was adding an inexpensive transcoder and switching the two transcoder’s RGB signals through my existing Extron SW2ARHVxi RGB-HV switch. I acquired a Key Digital Systems KD-CTCA2, but as I described to you eight months ago, it had several technical problems that made it impractical to insert it into my video chain.
When I was working with the KD-CTCA2, I contacted Key Digital Systems’ Mike Tsinberg to try to resolve the problems I discovered. He was very helpful and guided me to one or two workarounds. Unfortunately, some problems could not be resolved. Among them was the need to modify the color temperature of my projector to compensate for the color channel imbalance of the KD-CTCA2. This made converting DVD component video with one transcoder, converting the HDTV set top box’s component video with another, and switching between the two sets of RGB signals unacceptable. The reason is that for DTV in 480p (as Fox is currently transmitting), my projector would automatically switch to the same display memory as for DVD; the color temperatures of DTV and DVD would then appear to be quite different. I spent months exclusively using the Extron transcoder and moving cables every time I wanted to change video sources; this was inconvenient at best, downright annoying at worst.
A Redesigned Transcoder
Mike Tsinberg was quite gracious about the flaws I discovered and he set to work to fix them in his next generation of transcoder, which is now available. I recently acquired Key Digital’s new KD-CTCA3 transcoder and I’m delighted to report that every problem I found in the KD-CTCA2 has been corrected. Even the packaging has been improved; the new transcoder is almost as tiny as the last version but is now in a more sturdy case. I began my evaluation by putting the transcoder on the bench and here is what I found:
| KD-CTCA2 | KD-CTCA3 | Extron CVC200 | ||||
| Red Output, 100 IRE | 770 mv | 111% | 630 mv | 97% | 670 mv | 97% |
| Green Output, 100 IRE | 710 mv | 100% | 650 mv | 100% | 690 mv | 100% |
| Blue Output, 100 IRE | 720 mv | 103% | 660 mv | 102% | 680 mv | 99% |
| Red Offset | 960 mv | 10 mv | 3 mv | |||
| Green Offset | 165 mv | 10 mv | 5 mv | |||
| Blue Offset | 840 mv | 10 mv | 2 mv | |||
| Passes Bi-level Sync | Yes | No* | No | |||
| Passes Tri-level Sync | Yes | No* | No | |||
*At the transcoder’s RGB outputs (each color channel was a little different), I observed a narrow, -240 mv pulse approximately 0.4 usec wide that immediately preceded the input component video’s sync pulse. It may be crosstalk from the sync suppression circuitry, but these negative pulses had no visible effect on the video as I compared the appearance of the Extron with the new Key Digital.
Now, both the Extron’s and the new Key Digital’s color channels are within +/- 3%, providing very fine color accuracy. The Key Digital’s offsets have been reduced to essentially zero, sync signals have been suppressed almost completely, and amplitudes have been reduced to more compatible levels.
The new Key Digital transcoder eliminates all but two DIP (dual inline package) switches; it was designed to operate in an automatic mode, sensing the scan rates of incoming video. The only user control choices are: RGB-HV output mode (factory default) or RGsB (sync on green) output mode; and, HV drive (factory default) or composite sync on the H output. The factory defaults are precisely what my RGB-HV projector requires, which made installation a snap. Since the KD-CTCA3 measured well on the bench, I was not surprised to find that the transcoder looked great in my theater.
I auditioned 480p DVD, 720p HDTV, and 1080i HDTV. The KD-CTCA3 performed flawlessly. Gone was the hazy onscreen interference of the positive half-cycle of HDTV’s tri-level sync pulses passed by the previous model. Gone was the exaggeration of reds. Gone was the brightness shift caused by the sync pulses superimposed on the color channels. And gone was the incompatibility with my Extron RGB-HV switch caused by the older unit’s excessive offsets and amplitudes. I played one of my current reference DVDs, the Superbit release of Starship Troopers. I watched the PBS demo loop (1080i), lovingly shot to highlight the resolving power and color accuracy of HDTV. I checked out a football game on CBS-DT (1080i) and NYPD Blue on ABC-DT (720p). Since the Key Digital’s bandwidth of 110 MHz is twice that of the Extron, I was not surprised that 1080i’s spatial resolution superiority over 720p’s was readily apparent. For DVD, I found that the KD-CTCA3, like its predecessor, is unaffected by Macrovision, a vulnerability in the Extron CVC200 that has since been corrected. The Key Digital consistently produced wonderful images that were indistinguishable from the Extron. This is high praise indeed, for the Extron is a professional grade transcoder with a list price of $895; the Key Digital is priced at only $349.
I’d have been quite satisfied enough with the transcoder’s video performance and its ability to deliver transparent video conversion, but the KD-CTCA3 offers two additional helpful features. First, it has a second input that accepts RGB-HV. In the presence of component video, the transcoder converts that component video to RGB-HV. With no component video present, whatever signals are present at the transcoder’s RGB-HV input are passed automatically to its RGB-HV output. This is terribly handy for anyone who wants to connect either a home theater PC or an HDTV set top box that offers the option of an RGB-HV output to a home theater system. Very nice. The transcoder’s second additional feature is for those who may need access to the component video being converted; the KD-CTCA3 offers a second output, buffered component video.
Adding Source Switching
But what if your HDTV set top box only offers a component video output and your digital audio decoder’s component video switch has limited bandwidth (as I thought my B&K had). Key Digital Systems has a very nice solution for that problem as well, the KD-SW2X1 2-Channel Component Auto Switcher. Each of the switcher’s channels has 75-ohm RCA coaxial inputs for component video, analog stereo, and a digital audio bit stream, so the switcher allows total source signal switching capability. The switcher supports 480p, 720p, and 1080i, passing both bi-level and tri-level sync. The switcher’s -3 dB bandwidth is 110 MHz and it’s designed with a linear phase passband. Linear phase filters are the most benign possible in the time domain; to translate into English, linear phase filters do not ring, so they do not contribute to visible haloing. The KD-SW2X1 an auto-switcher. It defaults to passing signals from channel A; in the presence of video signals on channel B, those signals are passed to the outputs. A manual front panel control allows you to override auto-switching.
I temporarily wired the Key Digital Switcher into my system in a configuration that allowed me to compare, in real time, identical video with and without the KD-SW2X1 in the signal path. The Key Digital was transparent for 480p, 720p, and 1080i. I could still easily detect the improved resolution of 1080i over 720p. I could not detect a difference in the images with and without the switcher in the path. Total transparency; this is precisely what you want in such a device.
Now, both the Extron’s and the new Key Digital’s color channels are within +/- 3%, providing very fine color accuracy. The Key Digital’s offsets have been reduced to essentially zero, sync signals have been suppressed almost completely, and amplitudes have been reduced to more compatible levels.
The new Key Digital transcoder eliminates all but two DIP (dual inline package) switches; it was designed to operate in an automatic mode, sensing the scan rates of incoming video. The only user control choices are: RGB-HV output mode (factory default) or RGsB (sync on green) output mode; and, HV drive (factory default) or composite sync on the H output. The factory defaults are precisely what my RGB-HV projector requires, which made installation a snap. Since the KD-CTCA3 measured well on the bench, I was not surprised to find that the transcoder looked great in my theater.
I auditioned 480p DVD, 720p HDTV, and 1080i HDTV. The KD-CTCA3 performed flawlessly. Gone was the hazy onscreen interference of the positive half-cycle of HDTV’s tri-level sync pulses passed by the previous model. Gone was the exaggeration of reds. Gone was the brightness shift caused by the sync pulses superimposed on the color channels. And gone was the incompatibility with my Extron RGB-HV switch caused by the older unit’s excessive offsets and amplitudes. I played one of my current reference DVDs, the Superbit release of Starship Troopers. I watched the PBS demo loop (1080i), lovingly shot to highlight the resolving power and color accuracy of HDTV. I checked out a football game on CBS-DT (1080i) and NYPD Blue on ABC-DT (720p). Since the Key Digital’s bandwidth of 110 MHz is twice that of the Extron, I was not surprised that 1080i’s spatial resolution superiority over 720p’s was readily apparent. For DVD, I found that the KD-CTCA3, like its predecessor, is unaffected by Macrovision, a vulnerability in the Extron CVC200 that has since been corrected. The Key Digital consistently produced wonderful images that were indistinguishable from the Extron. This is high praise indeed, for the Extron is a professional grade transcoder with a list price of $895; the Key Digital is priced at only $349.
I’d have been quite satisfied enough with the transcoder’s video performance and its ability to deliver transparent video conversion, but the KD-CTCA3 offers two additional helpful features. First, it has a second input that accepts RGB-HV. In the presence of component video, the transcoder converts that component video to RGB-HV. With no component video present, whatever signals are present at the transcoder’s RGB-HV input are passed automatically to its RGB-HV output. This is terribly handy for anyone who wants to connect either a home theater PC or an HDTV set top box that offers the option of an RGB-HV output to a home theater system. Very nice. The transcoder’s second additional feature is for those who may need access to the component video being converted; the KD-CTCA3 offers a second output, buffered component video.
Adding Source Switching
But what if your HDTV set top box only offers a component video output and your digital audio decoder’s component video switch has limited bandwidth (as I thought my B&K had). Key Digital Systems has a very nice solution for that problem as well, the KD-SW2X1 2-Channel Component Auto Switcher. Each of the switcher’s channels has 75-ohm RCA coaxial inputs for component video, analog stereo, and a digital audio bit stream, so the switcher allows total source signal switching capability. The switcher supports 480p, 720p, and 1080i, passing both bi-level and tri-level sync. The switcher’s -3 dB bandwidth is 110 MHz and it’s designed with a linear phase passband. Linear phase filters are the most benign possible in the time domain; to translate into English, linear phase filters do not ring, so they do not contribute to visible haloing. The KD-SW2X1 an auto-switcher. It defaults to passing signals from channel A; in the presence of video signals on channel B, those signals are passed to the outputs. A manual front panel control allows you to override auto-switching.
I temporarily wired the Key Digital Switcher into my system in a configuration that allowed me to compare, in real time, identical video with and without the KD-SW2X1 in the signal path. The Key Digital was transparent for 480p, 720p, and 1080i. I could still easily detect the improved resolution of 1080i over 720p. I could not detect a difference in the images with and without the switcher in the path. Total transparency; this is precisely what you want in such a device.
With the addition of a second transcoder with two inputs and a two-channel switcher, I now have much greater flexibility to either add three more source components or connect a variety of new hardware for A/B comparisons. These two external components make it possible for my system to accommodate up to five sources, for example:
Or I could have just as easily substituted an HD-ready video game system, like the Microsoft Xbox, for the D-Theater D-VHS machine. The great thing about this arrangement is that both component video sources and RGB sources can be connected directly to my system. That’s flexibility.
Conclusion
The redesigned Key Digital Transcoder is a wonderfully economical means of transforming component video to the RGB signals that the overwhelming majority of front projectors require. It is transparent in use, trivial to install, and does not affect the quality of the images. I couldn’t wholeheartedly recommend last year’s KD-CTCA2, but the new KD-CTCA3 deserves your most serious consideration. Similarly, the KD-SW2X1 2-Channel Auto Switcher is an economical means of transparently switching between two high quality audio/video sources. If you want to add additional sources to your system, or if your digital decoder preamp or receiver doesn’t support HDTV signals, it too deserves your consideration.
(If you have any questions or comments for the author, say hello to Dan here.)
Conclusion
The redesigned Key Digital Transcoder is a wonderfully economical means of transforming component video to the RGB signals that the overwhelming majority of front projectors require. It is transparent in use, trivial to install, and does not affect the quality of the images. I couldn’t wholeheartedly recommend last year’s KD-CTCA2, but the new KD-CTCA3 deserves your most serious consideration. Similarly, the KD-SW2X1 2-Channel Auto Switcher is an economical means of transparently switching between two high quality audio/video sources. If you want to add additional sources to your system, or if your digital decoder preamp or receiver doesn’t support HDTV signals, it too deserves your consideration.
(If you have any questions or comments for the author, say hello to Dan here.)
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