Pro Logic II VS. Dolby Digital in handbrake

Discussion in 'Apple TV and Home Theater' started by swimfan, Apr 8, 2008.

  1. swimfan macrumors newbie

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    Mar 30, 2008
    #1
    i'd really like someone on here that REALLY knows what they are talking about to clear this up. there's are my observations:

    there's hasn't been a lot on this subject, but in my experience, there isn't much of a difference between the two in handbrake encodes. correct me if i'm wrong, but handbrake "downmixes" the 6 discrete channels of AC3 to a PLII mix. now, PLII is a 6 channel format also and that "AC3 downmix" is faithfully re-created in PLII, right? when i set my receiver to PLII "movie" i get the same sounds coming out of the same speakers as with AC3. dialog in the center and sound effects in the left, right, rear left, rear right, and subwoofer. from what i understand, PLII is a full range 20hz-20khz format with full 360 degree placement.

    now PLII has been sold as a way to get 2 channel mixes into a pseudo surround sound. like a CD mixed in stereo to sound like it's in surround. now i remember when the gamecube came out and it only supported PLII, the game designers were jazzed cause they had full 5.1 control in the format and it wasn't a whole lot different than DD 5.1. has anyone done any real tests between AC3 and PLII to hear a difference?
     
  2. Cave Man macrumors 604

    Cave Man

    #2
    DPLII sounds very good, but it doesn't sound as good as DD.
     
  3. swimfan thread starter macrumors newbie

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    Mar 30, 2008
    #3
    ok, but why exactly? both formats are lossy, correct? they should offer similar frequency ranges through all speakers, correct? the only thing that makes sense to me is that DD has slightly better separation between speakers.

    does anyone here, KNOW that it sounds better, or do they just THINK it sounds better? let's get some actual specs going here.
     
  4. dynaflash macrumors 68020

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    Mar 27, 2003
    #4
    I second that. Until eddyg managed AC3 passthrough in an mp4, DPLII was as good as we could get. A huge improvement over regular stereo and also the advantage of being portable ie. because it was carried in two channels, it would play on an iPod, etc.

    However, the AC3 5.1 passthrough in HandBrake is a perfect copy of the dvd source audio, note it also maintains the same bitrate as the original ac3 track which is typically 448 kbps for a 5.1 ac3 track (hence the increased file size using ac3 passthru). HandBrake uses the faac encoder for aac which has a limit of 160 kbps for a stereo track which includes dpl2. So not only do you get the benefit of the true 6 channel separation from using ac3 5.1 but you get the higher bitrate as well.

    One other note on dpl2. It is actually 5.0, there is no interpretation of the LFE channel. Though almost all receivers transfer the bass to the subwoofer based on the frequency cross over so it not so noticeable. AC3 5.1 pass thru indeed preserves the LFE channel. Note however the "Pass Thru" part of the name. It only passes through the source audio. So if you choose a AC3 2.0 source track you only really get two channels. Though almost all main features on dvd's use ac3 5.1.

    All of that said, for all around compatibility the HandBrake dpl2 is a very nice compromise to be sure.
     
  5. swimfan thread starter macrumors newbie

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    Mar 30, 2008
    #5
    bit rates aside, how accurate is the separation between the speakers between the two formats? are we mainly talking about a difference in bit rate here, or is there a noticeable difference in separation? i'm talking more about the 5 main speakers, not the sub.
     
  6. mrgreen4242 macrumors 601

    mrgreen4242

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    Feb 10, 2004
    #6
    OK, I'm going to preface this and say I'm not an acoustics engineer or a audio software developer, or anything; but I have done research on this, both reading online and real world tests.

    1) Bitrate aside, DD will sound better than PLII. This is "common knowledge" online and something I have verified personally using a 5.1 speaker setup with a DD and PLII compatible receiver.

    2) PLII is surround. Sort of. It uses just 2 channels of audio and then uses a phase shifting system that when decoded by a compatible receiver simulates surround/rear sound. When you listen to it "raw" (just the two channels without any decoding) it will sound like stereo - the front and back channels will be mixed into the left/right channel of the appropriate side. The center is spread across both equally.

    The center channel uses a similar encoding technique, that lets the decoder split it out. It's not the same phase shifting, but some sort of difference detection, I think.

    3) The sub channel in DPLII is busted... at least in the libraries Handbrake uses (and any other non-Dolby software encoder, apparantly). This is confirmed by the HB developers who have talked to Dolby engineers, apparantly. Something about not being able to mix the sub channel in without it causing low frequency drop out in the other channels, or somesuch. Point being, it doesn't work (in Handbrake) so it's actually 5.0.

    4) Many (most?) DVDs carry a DPLII encoded soundtrack (which is 5.1) and Handbrake is supposed to copy those out correctly, but it's hard to say.

    5) In the end, DPLII is surround, but it's going to sound "muddy" compared to DD. The channels have a certain amount of bleed over one another and the surround is never quite as distinct as DD. This is from my own personal testing of raw DD, downmixed to DPLII, and content encoded in DPLII professionally.

    It's a decent option that's nice to have, but it will not, under any 6 speaker configuration, replace real, discreet 6 channel codecs like DD.

    Hope that helps some... if I was at home (not at work) I'd include some sources and further reading, but you can learn most/all of this in the Handbrake developers forums with a little searching.
     
  7. swimfan thread starter macrumors newbie

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    Mar 30, 2008
    #7

    Starting with a 5.1 Mix
    The Dolby Pro Logic II encoding process was developed to allow consumers to enjoy a 5.1-
    channel experience on delivery systems that have a two-channel limitation. Because of this, it was
    optimized for taking a 5.1 mix created for Dolby Digital delivery, and simply running that mix
    through the encoder to create the Lt/Rt stereo downmix. In most cases, the decoded
    Dolby Pro Logic II mix will sound very close to the original.



    that's from dolby.com. and i do believe that "very close" to actually be, very close. i would however really like to see some actual specs. i cannot find them anywhere. the bit rate can't really be the determining factor. anyone that says that can hear a significant difference between a "lossless" 44.1/16bit CD and a nicely encoded 160 AAC file is fooling themselves. so, downconverting the already lossy DD 5.1 to 160 AAC in terms of sound quality should make no significant difference, correct? that said, wouldn't the equipment being used have a much more of an impact on sound quality over the difference between DPII and DD 5.1? i'm guessing the vast majority of us are running on sub $500 receivers and probably even worse speakers. then you've got to take into account the room that playback is happening in. all of these things added up in a "casual" movie watching experience say to me that you would never notice the difference. i use the term casual, because how serious are we running re-encoded DVD's in 480p resolution in the first place?
     
  8. killmoms macrumors 68040

    killmoms

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    #8


    Ha! Okay, guy. :rolleyes:
     
  9. Cave Man macrumors 604

    Cave Man

    #9
    From my personal experience, DD gives noticeably better channel separation, deeper bass (from the LFE) and substantially more dynamic range. The last one is the most noticeable.
     
  10. swimfan thread starter macrumors newbie

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    Mar 30, 2008
    #10

    http://pcworld.about.com/news/Oct022001id64123.htm

    this is just one example. there's dozens of these test online and they are using listeners that are actually spending the whole time listening for a difference. sitting in your living room eating popcorn, hanging out with the family, etc. i'm willing to bet you can never tell the difference. that's my feeling though. i'd still like to come back to the reason i started this post and find someone that has the actual numbers to back up their opinion.
     
  11. NightStorm macrumors 68000

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    Jan 26, 2006
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    Whitehouse, OH
    #11
    Technically, you get a higher bitrate-per-channel using AAC @ 160kbps DPL2 than with an AC3 soundtrack at 448kbps. ;)

    Bottom line, if the soundtrack was made with DPL2 in mind, it can sound pretty good. It is when you factor in that Handbrake (and other programs) are downmixing based on algorithms and dropping the LFE information that you start to hear the differences. The result can be (and is) a lot different than a soundtrack that was mastered from the original source and optimized for Dolby Prologic II.
     
  12. Cave Man macrumors 604

    Cave Man

    #12
    And my experience is that DD is better. I've done it blindly with the same source (a THX intro scene that many movies come with). I can tell which one is in DD and which is in DPLII just by listening.
     
  13. dynaflash macrumors 68020

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    Mar 27, 2003
    #13
    Okay mr. smarty pants, but you are talking two channels vs a true six channels. So the dpl2 at 160 causes the receiver to have to spread that out over 6 speakers. ;)

    swimfan, you are dead set that dpl2 is just as good. So go ahead and use dpl2 then. Why do you need everyone else to agree with you ? Luckily you have either option. If you think dpl2 sounds just as good, then by all means go for it. Enjoy. :)
     
  14. cohibadad macrumors 6502a

    cohibadad

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    Jul 21, 2007
    #14
    If I perceived no difference between AC3 5.1 and DPLII I would use DPLII. Unfortunately, the difference is obvious in the many encodes that I have done. I agree with Cave Man. Channel separation seems the most noticeable. Bass after that. It would seem safest to anyone not willing to test it for themselves to use the AC3/AAC option in HB. But to OP, if you notice no difference there is no blame for using DPLII. It is an excellent format IMO.
     
  15. swimfan thread starter macrumors newbie

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    Mar 30, 2008
    #15
    i'm not dead set on the format by any means. the issue i'm trying to get to the bottom of is, "what are the main TECHNICAL differences." everyone seems to jump out and quickly say that it's "common knowledge" that dolby digital is a significantly better format. just cause a bottle of wine costs $200 doesn't mean that it tastes better than a $20 bottle of wine. i'm looking for the facts, not opinion. my gut tells me the formats are a lot closer than they appear to be, and that's all i'm trying to figure out. we were brought up on DD 5.1. it's been around significantly longer than DPII. sure seems feasible to me that when dolby developed DPII, they were shooting for something that came very close to DD 5.1.
     
  16. Cave Man macrumors 604

    Cave Man

    #16
    DD won't be any better than DPLII for the original "War of the Worlds". It it will be noticeably better for the remake of "War of the Worlds". I can certainly tell the difference between DD 5.1 and DPLII with my Onkyo system on my home theater.
     
  17. Avatar74 macrumors 65816

    Avatar74

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    Feb 5, 2007
    #17
    I think I can address your question. I have produced original content in Dolby Digital surround sound and have a trademark service agreement with Dolby Laboratories.

    Dolby ProLogic is a home theater-based decoding system, not a theatrical format. It was originally designed to allow home theater systems to decode the theatrical Dolby Surround analog format on motion picture soundtracks.

    Dolby Surround analog is a phase-shifted matrix surround format. The surround information is encoded by shifting the surround channel(s) 90 degrees out of phase with the main audio and multiplexing that analog wave with the primary analog wave. This produces a more complex analog signal that is effectively decoupled by a Dolby ProLogic decoder. The degree of channel separation isn't the achilles heel of this format so much as is the channel limitations (only 3 in Dolby Surround vs. 6 in Dolby Digital) and dynamic range. Phase separation is more problematic when trying to use an inappropriate decoder (e.g. Dolby ProLogic IIx 5.1-channel) for the wrong format... e.g. when the original soundtrack is only stereo, or three-channel Dolby Surround (L,R,mono surround). People who hear a distinct difference are often trying to use Dolby ProLogic II to intuitively create more channels than were present in the original Dolby Surround soundtrack. This same methodology would work just as poorly if you tried to interpolate 12 channels from a six channel Dolby Digital soundtrack... i.e. the fact that it's an analog quadrature is not the problem.

    Dolby Digital (AC-3) has a bit of a different history. It was originally developed to be the standard for ATSC digital television broadcasts. It is a discrete digital format... but keep in mind that the data is multiplexed here as well. "Discrete digital" is a bit of a misnomer because the bitstream is a singular bitstream... not six bitstreams in parallel. However, digital transmission systems use bits (binary digits)... Audio is analog information, but digital data can be used to reconstruct it. In this regard, a Dolby Digital decoder demuxes the bitstream into separate channels before the analog signals are reconstructed. This is not necessarily better than analog multiplexing... but it's much cheaper to produce digital transmission systems capable of reconstructing multichannel surround well than it is to produce analog systems that reconstruct multichannel surround well.

    In addition, Dolby Digital possesses certain features such as 20kHz lowpass filtering, RF intermodulation filtering, etc. to dramatically reduce the data requirements to reconstruct an analog wave identical to the source. Also, the dynamic range of Dolby Digital is about 103-105dB whereas Dolby Surround has a dynamic range closer to 101dB.

    Lastly, Dolby Digital uses metadata for Dynamic Range Control and Dialogue Normalization, applied at the receiver not hardfiltered into the encoded audio... which allow a home theatrical receiver to control the degree to which dynamic range compression is applied and when. Dialogue normalization was a feature designed for digital television broadcast... the idea was that the engineer would input the average loudness value, Leq(A), in the dialnorm parameter which would tell the receiver from one soundtrack to the next, how much adjustment would be needed to maintain the same dialogue level from one program to the next so that the user would not have to constantly adjust the volume when flipping channels. Both these parameters are useful in a home theater as they maximize the usage of the dynamic range at any given moment while keeping the dialogue from being drowned out by loud noises.

    EDIT: One more thing... Contrary to popular belief, neither Handbrake nor AppleTV are actually capable of creating a Dolby ProLogic II mix from an AC-3 source. What actually is the case is that most professionally produced AC-3 soundtracks possess, at the recommendation of Dolby Labs white paper on AC-3 encoding, a Dolby Surround phase shifted analog mix already matrixed into the front Left and Right channels. For the two-channel AAC, the other four channels are stripped off while the stereo matrixed Dolby Surround (that was present at the time the studio created the mix) is still there. For the AC-3 mix, the Dolby Surround analog matrix is still there in the first two channels. It is important to understand this because if a mastering lab has NOT followed spec and did not include the Dolby Surround mix embedded in the L-R channels of the AC-3 as a backward compatible measure for Dolby ProLogic decoders, then the stereo AAC will be simply that... stereo AAC. Any Dolby ProLogic decoding applied by your receiver will simply be making phony interpolations which you might mistakenly interpret as an example of Dolby Surround being vastly inferior to Dolby Digital.


    Ultimately, Dolby Digital is a superior format than Dolby Surround analog, but Dolby Surround analog is certainly not worthless.

    This is kind of a basic explanation as I am off to work and didn't have time to get more elaborate... but I'm more than happy to answer other questions you may have.
     
  18. err404 macrumors 68020

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    Mar 4, 2007
    #18
    It basically comes down to how much you are willing to compromise in your files. DD is better (how much better is debatable) but also takes up a lot of space.
    Personally I use DD for of my "A" titles and AAC for my "B" titles (or Dramas/Comedies) where accurate surround reproduction is less important.

    Best:
    Video - CRF 70%
    Audio - AC3+AAC

    Good:
    Video - CRF 65%
    Audio - AAC
     
  19. Killyp macrumors 68040

    Killyp

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    Jun 14, 2006
    #19
    DD systems practically have 100% separation between channels, whilst on a PL or PLII system, there sound is 'leaked' from one speaker to another.

    I also believe there is no subwoofer channel in PL/PLII systems, the subwoofer channel is just created from the 5 main channels filtered down.

    There is then also the fact that PLII just doesn't sound as good as DD on a decent system. On the Sony system in our lounge, the difference is negligible. On the surround sound setup I used to run in my room, the difference was very very obvious.
     
  20. Gagoots macrumors regular

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    Oct 8, 2006
    #20
    This is what I don't understand:

    I have been backing up my movies in anticipation of getting an Apple TV in the near future. I have been using Handbrake, and its ATV preset. The codec is set for AC3 + PL2. Now what I DON"T understand is how the AC3 dolby digital track is altered...or IS it?
    IS it a 5.1 AC3 (which is not really discrete anyway, but thats another story)
    passthrough, or is it compressed in some way?
     
  21. dynaflash macrumors 68020

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    #21
    Nope, not altered hence the "Pass Thru" portion of the setting. Its a copy of the source audio track.
     
  22. Gagoots macrumors regular

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    Oct 8, 2006
    #22
    Thanks...but I only see "passthrough" when I have ONLY the DD selected. On the "hybrid" mode, it lists both formats..and gives you an option for a bit rate..
     
  23. slapppy macrumors 65816

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    Mar 20, 2008
    #23
    Easy test. Fire up U-571 on your 5.1 or 7.2 ( I have 2 subs ). DPLII will sound great, but you have no .1 channel.

    Fire up a Handbrake encode with full DD... better tape your breakables down, because your house will shake with this movie. If your subs and system can provide the power.

    Anyway once you hear DD vs PLII.... its practically night and day. Forget all those mumbo jumbo techno stuff. Just listen for yourself.

    Now for true separations, HB can also adds the Dolby EX flags on the encode. Boy those 5.1/7.1 receivers will make your head spin hearing clear discrete audio.
     
  24. Avatar74 macrumors 65816

    Avatar74

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    Feb 5, 2007
    #24
    It isn't.

    The short answer:

    It is AC-3. AC-3 is discrete multichannel. And it is compressed. And it is an exact copy of the original file (pass through).

    Confused?

    The long answer:

    AC-3 is a compressed format to begin with. Technically so is CD audio, but I won't go into that unless someone wants me to... AC-3 is a format that uses both compression and perceptual encoding to drastically reduce the data requirements to produce a multichannel surround mix acoustically transparent relative to a Linear PCM ("uncompressed digital") source.

    AC-3 is discrete multichannel surround in the sense that the source file begins with six digital channels of data, rather than a multiplexed analog channel of data... and then produces one multiplexed bitstream carrying six digital channels of data, which are then converted into six analog channels of audio.

    I've checked the bitstream size of the AC-3 pass through file embedded in the .m4v files produced by Handbrake 0.9.2, and it matches the source, 448 Kbps in the case of DVD AC-3. Note that the running data rate or frame rate in Quicktime often appears slightly different during playback but it's close enough that we can be reasonably certain the bitstream is 448 Kbps.
     
  25. Gagoots macrumors regular

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    Oct 8, 2006
    #25
    Thanks Av.
    I do AC3 encoding, so I know how compressed it is. It DOES NOT sound as good a full range 5.1 mix.
    That aside, I am stoked that I get the unaltered AC-3 AND a PL2 file. SO that means the bit rate selection (160 max) is just for the 2chnl.
     

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