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OK. Here is a block diagram of SDI - very high level stuff, but should get the point across.


1. The MPEG stream is read from the DVD and sent to the MPEG decoder.

2. The MPEG decoder uncompresses the MPEG stream and outputs a parallel digital stream called bt.656 (or bt.601 with separate EAV/SAV...but that's another story).

3. The bt.656 stream is piped to a digital-to-analog converter

4. The analog signal (YPrPb in this scenario) is connected to the inputs of an external line doubler/scaler.

5. The analog signal is then converted back to bt.656 in a analog-to-digital encoder.

6. The bt.656 stream is piped to the deinterlacing chip (Sil504, Fli2200, etc.).


You can also see in the diagram where SDI comes into play. Basically the parallel bt.656 stream is fed into a serializer then sent over a 75ohm coaxial cable (for very very long cables lengths if necessary), into a deserializing chip, then out to the deinterlacer in bt.656 format. The net result is that you have just logically placed the scaler inside your DVD player!!


In other words, if you had an SDI input on a line doubler and connected it to a DVD player it would be the same as you buying a progressive DVD player - it works the same way inside a progressive DVD player...in most cases. There are more and more chips coming that integrated the whole process into one chip - this makes SDI conversions next to impossible.


The major benefit of all of this is you are eliminating the purple boxes - the D/A and A/D required without SDI. Another benefit of SDI is that there is really no rolloff issues (unless the output stage of the line doubler/scaler introduces it).


I was talking with Joe Kane a few days ago and I mentioned I had just received my first Superbit DVD. He commented that I already owned one - Video Essentials. You see, VE and Avia both are technically Superbit already as they both use the full 6.75Mhz available on DVD. So our 6.75Mhz patch test is even more relevant these days with the advent of Superbit DVDs. SDI goes a long way toward making sure you see the full 6.75Mhz - of course there are other problem areas in the entire video chain, but we'll get there in another discussion.


Hope that helps.
 

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Tom, a question:


In the audio world, the use of digital outputs has been around a long time, and to reduce various issues (jitter, etc.) introduced by a digital output, the cable and an input, various strategies have emerged, just as FIFO buffers, reclocking the digital signal at the digital input, etc. That would be the one difference I could see between an internal digital connection and an external one.


Are there similar artifacts or issues relating to the output of the bt.656 digital video stream through the cable to the input on the scaler/deinterlacer? And if so, have you considered ways to address that?


Just curious. Perhaps it's not an issue given the nature of the video signal and certainly people rave about it.


Thanks for the diagram - very informative.


Cheers
 

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So, what makes SDI better than a HTPC dvd setup (software dvd player)? Are there D/A conversions in the dvd drive to software setup? I was under the assumption that there was not; and if you had a DVI video card and DVI projector/plasma that you could get a purely digital picture straight from the DVD to your screen.

Bob C
 
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