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Discussion Starter · #1 ·
Could someone point me to a source to understand how RF-AC3 is converted to something useable on the S/PDIF input?


From http://en.wikipedia.org/wiki/Laserdisc
Quote:
...Unlike DVDs, which carry Dolby Digital audio in digital form, Laserdiscs store Dolby Digital in a frequency modulated form within a track normally used for analog audio. Extracting Dolby Digital from a Laserdisc required a player equipped with a special "AC-3 RF" output and an external demodulator in addition to an AC-3 decoder. The demodulator was necessary to convert the 2.88 MHz modulated AC-3 information on the disc into a 384 kbit/s signal that the decoder could handle. ...

Oh, I thought, sounds simple, just need some sort of demodulator, probably FM demodulator using a phased locked loop and you are done. However, after much searching, I found a block diagram and schematic for a Pioneer RFD-1 demodulator at http://www.kallhovde.com/pioneer/tri...-1_RRV2125.pdf


Studying the block diagram for the circuit given in this reference that includes a block diagram of the Pioneer PM4007A IC that does the heavy lifting, it is clear that this is not just a simple RF demodulator. It appears to use PLLs to demodulate the signal. It also has memory and appears to be reformatting the digital stream before relaying it. Also, the RF frequencies in the PLL circuit do not appear consistent with the wikipedia description (or some details in the circuit are not obvious to me from this diagram.)


Can someone enlighten me on the rf modulation scheme used and also the digital reformatting required?
 

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Wow, that's a lot of circuitry for converting analog to digital. Maybe it's hard to modulate 300+ kbs digital to FM reliably.


I was also confused by the frequencies being used.


I supposed the AC3 RF specification was never an open standard, either?


I found this stuff, but it was about modifying a player to output RF, not about decoding.

http://www.laaudiofile.com/ac3mod.html
 

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Whoa, that is more complicated than I had expected. Sorry, I'm not really familiar with the details of the process. I had always assumed it was pretty straightforward, but there appears to be more going on than I was aware of.


Thanks for posting the service manual, though. It may be helpful if my RFD-1 ever dies.



I've also done the mod Michael referenced and it doesn't include a demodulator circuit. It simply adds a small board to extract the RF modulated AC3 signal from the right analog audio track and pass it out of the player. The kit I had didn't include any technical information or schematics; just installation instructions.
 

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If your player has the AC3 out? It's pretty simple. If you have to mod it to get the AC3 out? I'd just find one on the bay with AC3 output in all honesty.


Were you wanting to build the extrenal box? I am using the one I got off of some auction site back in the 90's. Always stays on and I don't do AC3 LD's that darn often. If you buying a player with AC3 output? See if you can also find one that has two 2video outputs. This makes it much simpler when doing automation with a remote. Like on my Harmony I have an activity defined as "Play LD wAC3" or something like that.
 

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Discussion Starter · #5 ·
Well, I was thinking of building an external box (or putting it internal) and adding an AC3 output. Adding the RF AC3 output is easily doable. The external box looks problematic until I can understand the function.


My current thought is that it may be possible to demodulate and send to a computer interface to finish the job.


However, at this point, the interest may just be academic. My current guess is that the modulation may be some sort of PSK with a 2.88 MHz carrier frequency. Note that 46.08 MHz is just 16 x 2.88 MHz. Maybe someone can enlighten me further.
 

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I probably knew somewhat how it worked at one time. This would have been easier in a by gone era of the past.



There was a guy selling for $75 on Satalite Guys forums recently. It's worth that.
 

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wow.. I just checked and these still bring some cash on the bay. Surprising. If I ever get hard up for cash, I may list mine as I am generally fine with modern processors and old 2 channel ProLogic. Although for nastalgia sake, I do pull out DTS and DD LD's to screen sometimes in a row. Always reminds me of more hopeful / youthful times in my life.
 

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Yeah, the RF demodulators often go for $300+ on eBay. Anyone that still uses their LD player needs one, since no new receivers or processors have them built-in anymore. I got lucky a couple of years ago when I semi-retired my DSP-A1 and snagged the RFD-1 for "only" $80 by somebody who I guess didn't check prices before listing his "Buy it Now" sale.
 

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To tell you the truth, it's been so long now, I can't even remember the exact transaction of how / where I aquired mine. I think it's a Sony. I mean it istucked away in the equipment rack somewhere and I never even look at it.


I'd still just try to acquire one if I were the op. Make sure you understand the vast majority of LD's don't even have the AC3 on them. The few real DTS discs have the DTS encoded the audio goes right out the normal optical digital audio output jack.
 

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Not every LD used AC3-DD. Some still used LPCM. So it (the RF) had to be processed, and recognized before being sent through. Then there's error correction checksums, and so on and so on. To Process or Not To Process, that is the question.



Just buy the converter.
 

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But I thought you could have the SC27 apply processing to the analog DD signal output to get the 5.1 channel sound without the AC-3. I think it will be comparable if not better than the AC-3 option considering the plethora of 5.1 processing options on the Pioneer. I will try it tonight and report back how it sounds tho I dont have a AC-3. But I do have the same source material in DVD and Laserdisc so I am going to give it a try.
 

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It can't take it from the two channel analog output. Or not that I ever knew of anyways. I mean it gets stripped before hitting the actual analog outputs of the LD player.
 

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Quote:
Originally Posted by JOHNnDENVER /forum/post/18278037


It can't take it from the two channel analog output. Or not that I ever knew of anyways. I mean it gets stripped before hitting the actual analog outputs of the LD player.

???


The 5.1 DD discrete signal gets stripped, but you can use the anolog/digital two track DD signals and have them processed by the Sc27 into 5.1 format or 7.1 format (tho not 5.1 discrete DD)- see page 62 of the manual - into Pro Logic IIx Movie, Neo:6 Cinema etc. which should compare favorably with what the Laserdisc with AC-3 puts out (5.1 DD).
 

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As I mentioned above many brands supported the laserdisc format in previous years, and certain brands even had built-in RF modulators like Denon (5600), Yamaha and Pioneer in their AVRs. Also they sold the RF modulator as a separate component such as the Yamaha APD-1. And frequently these can be found on Craig's List and E-bagger. Here is a link to an APD-1 now on the bag..

http://cgi.ebay.com/Yamaha-APD-1-AC-...item20afccc8a7


Just my $0.01..
 

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I thought there were at least some upper model recent Denon AVR's that still had it no?


Would be cool to have HDMI full bore and the legacy AC3 input available for sure.
 
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