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 Post subject: AC-3 RF specification?
PostPosted: 24 Jan 2021, 02:46 
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Is there a specification available somewhere for precisely how the RF modulation of the digital AC-3 signal works? Got a friend who's in the electronics industry and is personally interested in things like signal processing and RF-related circuits, and he's curious how hard it would be to build an AC-3 demodulator in this day and age. But obviously without knowing how the modulation format works (and since I don't have an AC-3 demodulator myself for him to play around with!) that would be rather impossible.
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 Post subject: Re: AC-3 RF specification?
PostPosted: 24 Jan 2021, 03:46 
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So hard to explain it so I think it's best to read this....

https://www.avsforum.com/threads/laserd ... 141/page-2



Also, I really doubt your friend would be able to make his own Demodulator since the main requirement is the Pioneer, PM-4007, DSP chip that is the heart of the decoding process. This chip is essentially not available @ this point in time.


Last edited by ldfan on 24 Jan 2021, 04:42, edited 2 times in total.
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 Post subject: Re: AC-3 RF specification?
PostPosted: 24 Jan 2021, 03:49 
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Very interesting! And DSP chips still exist, and much more powerful ones than were available in the 90s, so I would be surprised if (even if it wouldn't have been possible to do with off-the-shelf parts back then) it would be completely impossible today. Whether or not it's possible for practical purposes though is another matter... it's possible it would be more of a programming problem than an electronics one if it's "just" a case of using a modern DSP chip.
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 Post subject: Re: AC-3 RF specification?
PostPosted: 24 Jan 2021, 04:55 
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I think the main hurdle would be the legal aspect.
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 Post subject: Re: AC-3 RF specification?
PostPosted: 24 Jan 2021, 05:08 
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cplusplus wrote:
I think the main hurdle would be the legal aspect.

With Dolby you mean? This is where I'm unsure. How Dolby/AC3-specific does the demodulation process have to be? I was under the impression that it was just a matter of converting the radio signal to a bitstream, and that there was nothing in particular about Dolby's encoding that went into this stage (that decoding the Dolby Digital stream to actual 5.1 channel sound came next in the chain). Am I mistaken?
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 Post subject: Re: AC-3 RF specification?
PostPosted: 24 Jan 2021, 07:43 
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A Dolby Digital signal and 5.1 are the same thing at the same time. RF goes in and patent depending digital audio comes out the other. Anything that outputs Dolby has to be licensed. That’s why demodulators exist in the first place. They wanted that fee to not be included in already $$$ LD players. These days I think the fee would be pennies on the dollar to what it was in 1995 so if probably wouldn’t be a huge problem. I also don’t think much would happen if someone made 100 RF demodulators and sold them on eBay since from what I can see it’s pirate central.

These days there are so many ways to process a signal like that it’s it’s crazy. The only issue is knowing the exact specification, which I think nobody does. Happycube has done some research on this though.
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 Post subject: Re: AC-3 RF specification?
PostPosted: 24 Jan 2021, 08:36 
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I don’t understand the hype about the RF demodulators and ac-3. I mean we held onto our LD players because they offered uncompressed PCM over lossy AC-3 on DVD. Not much so here but on the rest of the net there is a massive craze over RF demodulators and the ancient AV receivers with RF inputs. AC-3 legit sounds so thin and soulless on a half way decent system that there are maybe a handful titles on LD that I would stand for it’s better effects separation.
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 Post subject: Re: AC-3 RF specification?
PostPosted: 24 Jan 2021, 09:48 
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substance wrote:
I don’t understand the hype about the RF demodulators and ac-3.


In many cases, it's the mix.

Many of these initial release AC-3 titles on LD had the theatrical mix and they can be incredibly immersive vs many DVD's remixed for the home. PCM tracks are also great as well in respect to how they deliver a very full bodied sound and tight bass but they are just two channel audio (but they can still deliver an impressive surround mix especially with Dolby Pro-Logic II).

In any case, my personal feeling is that I want my LD playback systems to be as flexible as it can be so that I can have access to all the audio options offered throughout the formats run.


Last edited by ldfan on 25 Jan 2021, 07:04, edited 1 time in total.
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 Post subject: Re: AC-3 RF specification?
PostPosted: 24 Jan 2021, 10:55 
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The steering is very impressive with AC-3 and the sort of aggressive LFE use I personally don’t like really depends on AC3 or DTS. In general I agree, the PCM is preferable in many cases because the actual sound quality of what’s coming out of the front speakers is way better but there are those titles like Abyss and Royal Space Force where the AC-3 really stands out in the steering and also the dynamics of the mix.
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 Post subject: Re: AC-3 RF specification?
PostPosted: 24 Jan 2021, 19:14 
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rcarlson wrote:
This is where I'm unsure.

Same here, I was just throwing the thought out there.

The best documentation I know of for the demodulation is the PM4007A info I posted here:
Datasheet PM4007A

I wish there was something like the IEC paper that we have for LaserVision.
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 Post subject: Re: AC-3 RF specification?
PostPosted: 01 Mar 2021, 22:45 
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I came into this forum to basically post this exact thread.

I was hoping to find some information on the AC3 RF signal. Like the frequency it operates on, etc.

I'm an embedded firmware developer, and I wanted to look into building my own demodulator with a few cheap parts like say an AVR (Not sure if they're fast enough -- might need to go ARM) plus some RF and SPDIF modules. The problem is I just can't seem to find the required technical documentation on the AC3 RF signal itself.

In theory, I should be able to convert the RF signal into a bitstream and send it out over SPDIF relatively easily. In theory. I'll see if theory matches up with reality if I can get my hands on specs. :lol:

If I'm able to come up with a solution, I'd be happy to share the information and even source code with others who could build their own. Hopefully with just a dirt-cheap Arduino board and a couple other bits. I could utterly fail, as RF isn't one of my areas of expertise, but I'd at least like to give it a try.

$200+ on eBay just to get AC3 surround from a Laserdisc is insane.

I'm wondering if the details are too well-kept of a secret to pull this off.
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 Post subject: Re: AC-3 RF specification?
PostPosted: 01 Mar 2021, 23:02 
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mikechambers wrote:
I came into this forum to basically post this exact thread.

I was hoping to find some information on the AC3 RF signal. Like the frequency it operates on, etc.

I'm an embedded firmware developer, and I wanted to look into building my own demodulator with a few cheap parts like say an AVR (Not sure if they're fast enough -- might need to go ARM) plus some RF and SPDIF modules. The problem is I just can't seem to find the required technical documentation on the AC3 RF signal itself.

In theory, I should be able to convert the RF signal into a bitstream and send it out over SPDIF relatively easily. In theory. I'll see if theory matches up with reality if I can get my hands on specs. :lol:

If I'm able to come up with a solution, I'd be happy to share the information and even source code with others who could build their own. Hopefully with just a dirt-cheap Arduino board and a couple other bits. I could utterly fail, as RF isn't one of my areas of expertise, but I'd at least like to give it a try.

$200+ on eBay just to get AC3 surround from a Laserdisc is insane.

I'm wondering if the details are too well-kept of a secret to pull this off.


Please keep us posted if you can or can't do this, would love to know.
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 Post subject: Re: AC-3 RF specification?
PostPosted: 02 Mar 2021, 04:40 
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cplusplus wrote:
I think the main hurdle would be the legal aspect.


The patents on AC-3 actually expired in 2017.

rein-o wrote:
Please keep us posted if you can or can't do this, would love to know.


Will do. So far, I haven't been able to dig up anything useful.
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 Post subject: Re: AC-3 RF specification?
PostPosted: 03 Mar 2021, 01:13 
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substance wrote:
I don’t understand the hype about the RF demodulators and ac-3. I mean we held onto our LD players because they offered uncompressed PCM over lossy AC-3 on DVD. Not much so here but on the rest of the net there is a massive craze over RF demodulators and the ancient AV receivers with RF inputs. AC-3 legit sounds so thin and soulless on a half way decent system that there are maybe a handful titles on LD that I would stand for it’s better effects separation.

Everyone comes at it for different reasons. Like you, I love that there is Matrixed PCM but I also love that the format evolved to support discreet surround codecs like AC-3 and then DTS. Its just another cool feature of the format to enjoy. Fortunately for you, most AC-3 LDs also have PCM. Just like like how LBX is less resolution than full screen but it is a better overall experience, AC-3 may not be PCM but it does simulate a theatrical experience on humble home systems.
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 Post subject: Re: AC-3 RF specification?
PostPosted: 03 Mar 2021, 03:05 
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I think it’s more like...to be blunt, if your stereo sucks you’re more likely to get a bigger kick out of very precise steering and nonstop sub rumble that you’ll get from two channels done badly. If you have good speakers and amp the thinness of AC-3 at Laserdisc bit rates will be something you will be aware of through the whole movie. I also blame Dolby/DTS for hoodwinking people with codec marketing wank...come on, Dolby Atmos on a phone? A phone? Are you kidding me? (I won’t mention Dolby Vision and 4K Netflix...the visual version of the same nonsense...)

As to why all noobs ask about AC-3 right off the bat, I’m inclined to believe that it’s gamers. When gamers decide to get a “new” (old) game system they want to buy the “right” one right off the bat, the one that can play all the games. AC-3 is like the PC Engine Arcade Card and on PC Engine forums there are old timers who say you don’t need the Arcade Card since almost no titles support it and even the ones that do disappoint 3/4 of the time but they just won’t hear it...

The people who buy and sell stuff constantly are the ones people listen to. My daily gospel, “You don’t need this...” is rarely compelling to noobs. They want hype, not anti-hype, no matter what the product or message is.

You don’t need HiVision either...but you can never tell anyone anything in these situations, I’ve learned that. They just got their tax return and it’s monorail time, babe!
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 Post subject: Re: AC-3 RF specification?
PostPosted: 17 Mar 2021, 01:24 
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I get the appeal of both PCM stereo and AC-3 surround. My stereo system doesn't suck. If it's a movie that makes good use of effective discrete surround, I'd rather listen to the AC-3 track, otherwise PCM all day.

The AC-3 sound quality on Laserdisc is not that bad though. You'd think 384 kbit divided across 5 channels means 76 kbit per channel, minus whatever little amount is stolen for the LFE. That would definitely suck, but they actually allocate the bandwidth dynamically based on how much is going on in each channel. The only time you're going to have very low bandwidth all around in every channel is if all of them are going nuts at the same time. The majority of the time, the lion's share of the bandwidth is being given to the front and/or center data.

Plus AC-3 isn't so prone to those terrible, annoying warbly noise artifacts that you get with something like low-bitrate MP3. So, it's not awful. No, it doesn't have the perfect clarity of PCM, but a well-done surround mix really can enhance the enjoyment of some movies.

I don't have to have it, but it's a nice option sometimes.

IMO anyway, just my 2 cents. Some people do make too big a deal of it though, I agree! I used LD for 17 years before being able to listen to the AC-3 tracks, and I was perfectly happy with it!
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 Post subject: Re: AC-3 RF specification?
PostPosted: 20 Mar 2021, 15:23 
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It’s been removed from the Benedictus main homepage, but they used to sell an autoswitching AC3 demodulator: http://www.benedictus.de/bde/rfd.htm

The ultimate in comfort would be to clone the PCB of a demodulator - info is available from service manuals. The Pioneer IC can be sourced, but beware of fakes. Then add autoswitching capability to consolidate the standard or DTS SPDIF with the demodulated AC for 1 single coax or Toslink output from the LD player. If you digitize the stereo RCA at 24/96 you could end up with a single audio cable to the receiver. Those of us who still have access to older receivers with plenty of coax/Toslink inputs are fine, the case is what to do in order to simplify connection to a more modern, anemic almost HDMI only receiver...

I have the eval board, and the Denon boxes that handle most of the above, but adding a cc decoder makes for an unnecessary clutter of single function external boxes. LD is about tactility and chunky equipment, not so much convenience :lol:


Last edited by jojjelito on 20 Mar 2021, 15:28, edited 1 time in total.
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 Post subject: Re: AC-3 RF specification?
PostPosted: 20 Mar 2021, 15:28 
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Quote:
The Pioneer IC can be sourced, but beware of fakes.


Really?
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 Post subject: Re: AC-3 RF specification?
PostPosted: 20 Mar 2021, 15:36 
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signofzeta wrote:
Quote:
The Pioneer IC can be sourced, but beware of fakes.


Really?


Yup! https://www.digipart.com/part/PM4007A You probably need to be a company, then it’s just a question of asking for quotes for your desired amount. So, given time and willingness to experiment it should be possible to build a full AC3RF demod. It’s not going to make any money due to lack of market, but you could make it an Open Source type project like the OSSC scaler.
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 Post subject: Re: AC-3 RF specification?
PostPosted: 20 Mar 2021, 17:16 
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If the whole thing is based on a proprietary chip long out of production that we don’t have real documentation on then it’s as far from open source as I can imagine.

And are there seriously pirate copies of this useless chip? That seems crazy.
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