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Old 11-18-2007, 10:08 PM
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wa2ise wa2ise is offline
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Question An HDTV receiver box to insert in vintage TV IF strip?

Wonder if a HDTV decoder rigged to receive at the usual IF frequencies (41 to 47 or so MHz) could be patched in between the tuner and the IF of a vintage TV set? The box would create a new "fake" NTSC IF signal of the decoded HDTV channel. I don't know if the HD decoder would care that the received signal from the vintage TV's tuner is frequency inverted or not.

If this could work, your vintage TV would function similar to before, though you'd have to know where the HD stations really are on the dial.
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Old 11-18-2007, 10:43 PM
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You'd probably have to modify the firmware on one of the currently available settop boxes. A degree in computer hardware and software engineering would probably be helpful.
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Old 11-18-2007, 10:46 PM
MRX37 MRX37 is offline
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To me that would seem difficult to pull off, and would essentally turn the TV into a hybrid between old and new. While that would be pretty cool, my main concern is how well the new would get along with the old...

In my opinion, an HDTV box combined with a wireless transmitter hooked to it's output would do the same job, and you wouldn't have to modify the TV.
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Old 11-20-2007, 07:42 PM
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The inversion of the IF **would** cause a problem if you simply tuned the tuner of the box to 44 MHz, since the digital signal has a pilot carrier at the lower edge (upper edge at IF). Also, some DTV decoder chip sets take a 44 Mhz IF and some take a lower IF, like 6 MHz at the point where you could interface.
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Old 11-20-2007, 08:59 PM
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Quote:
Originally Posted by old_tv_nut View Post
The inversion of the IF **would** cause a problem if you simply tuned the tuner of the box to 44 MHz, since the digital signal has a pilot carrier at the lower edge (upper edge at IF).
I was afraid of that, to some extent. Though if we disconnect the HDTV box's tuner, and patch in the vintage TV's tuner instead, it might trick the HDTV box into decoding a signal.. However, the HDTV box's tuner is digitally synthezized tuned, and the decoder may want to automatic fine tune the tuner to get the pilot frequency within some limits. An old style tube TV tuner would likely drift too much. So this concept is probably not gonna fly.

Now if the decoder wants some other IF freq, then that's the end of this ballgame.
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Old 11-23-2007, 02:08 AM
Tom_Ryan Tom_Ryan is offline
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Here's an approach I use that works reasonable well. Most people have laptop computers so I put mine to some work! I got an Express Card (HP Digital Tuner) and plugged it into the card slot on the side of the PC. Then ran Microsoft Media Center software, comes standard with Windows Vista. The laptop has a composite video output which feeds an agile modulator. The modulator output is easily tuned to the desired TV channel. I found this approach to be fairly inexpensive, requiring no modifications to a TV. Small enough to be placed inside a TV if needed - at least where cabinet space permits. The only thing I splurged on was the Express Card ... $35.00
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Old 11-23-2007, 12:36 PM
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Here is a Rube Goldberg scheme that could allow a person to change channels on their antique TV set and receive the corresponding digital signal.

Items needed:
-DTV set-top box with remote control
-Universal remote control with multiple "macro-programmable" keys
-Agile VHF NTSC modulator, modified to allow external access to channel selection (usually DIP switches)
-12 DPDT relays, miscellaneous diodes, low voltage power supply, etc.

The idea is, you modify your TV's tuner to bring out one wire per VHF channel position and a common ground wire. This could be done by some kind of optical or magnetic-sensor assembly that detected what channel was selected, or maybe even using contacts already on the tuner's wafer switches. The twelve wires go to the relays, turning on whichever one matches the channel selected. One contact on each relay is wired to one macro button on the remote control that is pre-programmed to send the command to choose the desired channel on the DTV tuner. (A timer circuit may be needed to make the "button press" a momentary event.) The other relay contact turns on a diode matrix wired to the DIP switch contacts of the agile modulator, changing its output to match the old NTSC channel number of the DTV tuner's selected channel. Connect the DTV tuner to the modulator and the modulator to the TV's antenna inputs with a matching transformer. You now have an on-the-fly digital-to-NTSC converter.
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