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 Post subject: ac3 question.
PostPosted: 31 Dec 2015, 15:47 
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Been wondering, what kind of sound format is demodulated AC3? Does it become PCM after being demodulated?
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 Post subject: Re: ac3 question.
PostPosted: 31 Dec 2015, 15:52 
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Analog fm signal turns into a digital bitstream signal of 384kbps. Far from pcm. Dolby uses its own codec but it's similar to aac.
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 Post subject: Re: ac3 question.
PostPosted: 31 Dec 2015, 15:58 
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substance wrote:
Analog fm signal turns into a digital bitstream signal of 384kbps. Far from pcm. Dolby uses its own codec but it's similar to aac.


aye, ok, i've been trying to find a solution to downmix ac3 and/or dts to stereo, was hoping the crystalio could do it, but it only handles PCM, and found this device on ebay
but not sure if it will work or not http://www.ebay.com/itm/WyreStorm-EXP-C ... SwcF9UWsEX
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 Post subject: Re: ac3 question.
PostPosted: 31 Dec 2015, 16:47 
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Any tv should downconvert it to stereo
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 Post subject: Re: ac3 question.
PostPosted: 31 Dec 2015, 18:03 
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substance wrote:
Any tv should downconvert it to stereo


possible, but i want it hoked up to my amplifier :)
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 Post subject: Re: ac3 question.
PostPosted: 31 Dec 2015, 18:06 
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You could look for an older surround processor. Something from the late 90s early 2000s should be able to process DD and DTS and down mix to Stereo for your amp. Shouldn't be too expensive.
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 Post subject: Re: ac3 question.
PostPosted: 31 Dec 2015, 19:37 
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Dolby Digital (AC-3) is the same codec with different bitrates on LD, DVD, and BD but it may or may not go through two different masterings methods.

In the LD days, it wasn't backwards compatible to play on 2ch systems. For that, they included the Dolby Surround encoded PCM channels for systems without AC-3 capability. It is in most cases the folded down version of the AC-3, matrixed into 2ch.

In DVD days, Dolby promised full backwards compatibility. If thr DVD has AC-3 as its only available soundtrack, the DVD player can internally fold it down to 2ch stereo. This requires the second mastering method which has a slight different mix which can be easily folded down. First of all, dynamic range and the rear channels separation are compromised. The rear channels are recorded out of phase so that when combined into 2ch, they don't sound amplified. This is one of the main reasons, AC-3 on DVD doesn't sound as good.

Now well mastered DVDs(like Criterion) use two soundtracks, one for the 5.1 mix and one for the 2ch folded down mix. These do not compromise dynamics and surrounds separation therefore should sound as good as the LDs. They are far and between unfortunately. Most studios choose the 5.1 mix that can be folded down.

After all this is said, I don't recommend folding down AC-3 on the laserdisc. It will not sound right. The mix wasn't designed to be down converted. The PCM channels offfer the down converted soundtrack in much higher fidelity anyway. There is no benefit, in fact you are compromising both the mix and fidelity. PCM channels are encoded at 1.2mbps vs AC-3 at 384kbps to remind you.
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 Post subject: Re: ac3 question.
PostPosted: 31 Dec 2015, 21:33 
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Is a DD or DTS capable receiver able to "properly" downmix when set to stereo? Ive been using a 2.1 setup mostly due to room constraints and the fact that my wife doesn't like surround effects. I just bought a third matching front speaker to use as a center channel. Would it be best to tell the receiver that i have surrounds and not connect anything (losing all surround channel information) or tell it i only have LCR and hope the surround gets mixed in properly?
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 Post subject: Re: ac3 question.
PostPosted: 01 Jan 2016, 08:15 
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In DVD authoring, the engineer is adviced to apply a 90 degree phase shift to surround channels. In downmix, both surround channels are added and sent to left front. The same summed surround signal is shifted oyt phase and sent right front. This method allows proper pro logic compatible stereo downmix. This 90 phase shift is audible in 5.1 mode. That's why ac-3 on dvd sucks. Music dvds have 5.1 mix without phase shift and a separate 2ch soundtrack. Ld ac-3 doesn't have the phase shift at all because it wasn't envisioned backwards compatible. Lfe channel is not summed to the downmix in anyway.
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 Post subject: Re: ac3 question.
PostPosted: 01 Jan 2016, 13:55 
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gettian ampe is probably the best wat, i user one of the high class NAD ampis and they are cool
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 Post subject: Re: ac3 question.
PostPosted: 01 Jan 2016, 14:43 
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The player itself already outputs stereo. I don't see why you want to the AC3 mix converted when you already have stereo PCM coming out the TOSLINK.
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 Post subject: Re: ac3 question.
PostPosted: 02 Jan 2016, 01:20 
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signofzeta wrote:
The player itself already outputs stereo. I don't see why you want to the AC3 mix converted when you already have stereo PCM coming out the TOSLINK.


people have said the ac3 might have better bass than the pcm tack, thats why :)
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 Post subject: Re: ac3 question.
PostPosted: 02 Jan 2016, 01:46 
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The LFE track is not added to the down-mix. If you use an AV reciever, when you disable the subwoofer, all the subwoofer information is sent to fronts. This is the only you get the LFE in the downmix.

The biggest trouble with the downmix is because Dolby employs a minus 3db attenuation on the surround channels when added to the fronts. There is a good reason for this. In signal processing, when you add two in phase and identical signals, you get the same signal with double the amplitude. Minus 3db attenuation compensate this however any signal which is not in phase or identical to both channels do not get amplifed when added up. Dolby down mix applies minus 3db attenuation anyway. Due to this some signal is lost half energy.

As signofzeta suggested, use the pcm channels instead. The fidelity is much higher, doesn't suffer mixing casualties. The whole point of AC-3 was much higher channel separation than Pro logic, it was never about better sound. Matrix method allows about 10-12 db channel separation. Pro logic decoder applies steering, which increases this to up to 30db. Pro logic steering also has its major flaws like guessing the direction wrong. It's basically bunch of comperators calculating the dominant channel in the recording then applies attenuation to the other channels to increase the separation. Dolby Digital with its discreet channels allow above 45 db channel separation.

I don't know how DTS does its down mix. I didn't read its tech paper on this. I don't see it done any other way so it's probably the same method.
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 Post subject: Re: ac3 question.
PostPosted: 02 Jan 2016, 03:31 
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i never knew any of this stuff :o
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 Post subject: Re: ac3 question.
PostPosted: 02 Jan 2016, 04:30 
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@substance as a hypothetical, if you had to choose LD AC-3 down mixed to 3.1 or 3.1 with surrounds chopped off, what would you choose?
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 Post subject: Re: ac3 question.
PostPosted: 02 Jan 2016, 05:21 
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I would take my chances with the downmix.
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 Post subject: Re: ac3 question.
PostPosted: 02 Jan 2016, 18:55 
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...Or third choice stack the surround speaker (identical of course) on top of the respective front :P
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 Post subject: Re: ac3 question.
PostPosted: 02 Jan 2016, 21:55 
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I wouldn't waste money or shelf space for that. Eitherway you are compromising. The mix is primarily done for theaters than we get the cooked down home theater mix then you take a little more from the mix removing surrounds. I don't even go into properly placing each speaker and calibration (and properly positioned seats and on...). You just try to make the best out of your situation. It's quite difficult to adapt a living room into a theater room. Fortunately for us, most mixes are front heavy, making rear surrounds less important. Some AC-3 mixes are so front heavy I wonder why they even bothered recording it in discreet surround. In your case, I am sure you will agree, you had the greatest improvement with the added center speaker. I don't anticipate an improvement close to that with adding rears unless you place them like in theaters and you sit at the optimum spot.
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 Post subject: Re: ac3 question.
PostPosted: 02 Jan 2016, 22:38 
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I was just kidding around with that last one. I plan to stick with 3.1 so I can at least get the benefit of good dialogue separation. If I wanted a perfect home theater environment I would have bought a different house.
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 Post subject: Re: ac3 question.
PostPosted: 01 May 2016, 07:36 
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substance wrote:
Analog fm signal turns into a digital bitstream signal of 384kbps. Far from pcm. Dolby uses its own codec but it's similar to aac.

No, it doesn't turn into a digital bitstream because it is digital to begin with. It doesn't matter in which form logically digital information manifest, as long as it meets the criteria to be treated digitally on a logical level, then it is. Just as VHS optionally had PCM tracks, in theory, one also could use a vinyl record digitally if there is enough error correction applied. Probably due to the very limited bandwidth and high distortion, the effective PCM audio wouldn't meet high standards though.

Furthermore, the quote 384 kbps stream is what is left over when one strips the stuffed PCM data which in total runs at 1536 kbps. The bitrate of the raw S/PDIF stream is even higher of course due to the additional data.

The technicial correct explanation is that you got a broadband modulated FM signal which is demodulated to a baseband signal and output as S/PDIF. Cause this is what a demodulator does: it demodulates, not converts analog into digital ones or vice versa.

cpix wrote:
Does it become PCM after being demodulated?


This is actually not so far from the truth. As S/PDIF carries AC3 and DTS both as baseband PCM, in order to properly capture and further process it, both formats may be treated as regular 44.1/48 kHz at 16 Bit Stereo LPCM first and then extract the carried lossy audio from that.
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