Electronics > Projects, Designs, and Technical Stuff

TV output

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Hi all,

Outputting TV signals was pretty easy until recently when things started to get a bit more complex and digital. And there lies the question - if you want something a bit more up to date: widescreen handling, HDMI, lower expense, things get hairy.

Just wondering has anyone done any project with more recent components. Like the Analog devices AD717x series amongst others.

As an example of a straightforward NTSC/PAL output - one method is to output digital RGB and then convert to composite through an A to D. Even 8 bits per pixel - 3:3:2 video RGB output - feed that to an R-2R DAC, and then fed to the three analog inputs of an AD725 will look just fine as far as it goes.

The big problem here, at least as far as I'm concerned, is that an AD725 is rather expensive. $10 or so. Plus you need to do the D to A which is slightly annoying. And what about widescreen signalling? I think you can get better value for money with more features with newer parts.

But on the other hand, the modern parts look rather complex requiring YCrCb clocked digital inputs. And converting to HDMI looks rather scary :-)


For the experimenters out there, I note Rossum of "Rossum's posterous" had a little project on an ARM LPC which used an R-2R ladder for the luma and a single bit for the chroma to produce NTSC S-Video. The chroma bit was the output from the SPI port - so it's a 1 bit DAC. Very interesting - I'd never seen that before. The SPI on the ARM has a 16 byte fifo which helps.

Bet the source is a bit complex though...

VGA output is pretty easy, and many of the newer displays have some sort of VGA input.  Wider screens mean faster bitrates, though, making those SW bit-banged hacks more difficult.

Actually that's a point - looked at the back of my parent's TV and it has 2 HDMI and a VGA input. Interesting point that. The advantage of bitbang hacks is that it is cheaper. An ARM LPC1100 is much cheaper than an AD725. Which is just silly when you think about it.

As for widescreen, I think you can just send the appropriate info on line 23. Hey, there's a wikipedia entry on it - that makes me hate my local idiot broadcasters even more for sending the widescreen signal when it's 4x3 and not sending it when it's 16x9. Idiots.

The "more pixels" problem still applies for anamorphic widescreen - which analog WS will be. If you have text at 4x3 and send the WS signal, the TV goes wide and the text looks like crap. You have to boost the horizonal pixel rate (or make a whole new non mashed font)...

For outputting HDMI/DVI video, you might want to look at chips like AD9889B. They generate a HDMI signal from RGB input. Some detect the frame size and refresh rate automatically, others need SPI/I2C bus to tell them what to expect in input and what to output. That AD9889B can also send audio along the HDMI if you give it an I2S or S/PDIF stream.

I recently sampled TFP410 from TI which is a DVI transmitter, but as we know DVI should work just fine with HDMI connector too. Haven't had a chance to design a board for that one yet.




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