Pro Logic II VS. Dolby Digital in handbrake

[dynaflash]

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.

Correct. For HB 0.9.2 eddyg did a bang up job of figuring out how apple was putting the ac3 passthru into an mp4 *immediately* after AppleTV Take2 was released. He worked for two days straight reverse engineering the Intro movie in the take 2 image which uses the exact same scheme as their HD movies. Including the appropriate atom. 0.9.2 was delayed just so we could get this in.

Once he had that done for libhb and the cli. We desperately wanted to get it out so in the macgui, I had to work with most of the interface we already had, which has always used the "Codecs" dropdown to couple the video codec with the audio codec (ie. h.264 / aac) . The other issue is the the macgui has always only provided for two audio tracks in the interface. We would now be using them both up. The solution I went with was to basically cheat and make a modified h.264 / aac codec setting called h.264 / aac +ac3 that tells hb to set the audio the same as before for the aac track, but to slip the unaltered ac3 passthru track of the same source track you choose for aac into the encode. So it is pretty much automatic however its not totally transparent to the user. This method also left the second track in the macgui open so a user could choose another language for track two (which in this case becomes track 3 in the final encode) .

Having said all of that, don't get too comfortable with this shortcut. HB no longer contains the Codecs popup in our svn and you can mix and match up to four different audio tracks in an encode. AC3 is no longer "slipped in behind your back" either, its chosen just like any other track. It also shows the bitrate and samplerate used right in the window so you can see it is using the input track bitrate for the output (of course, since its pass thru you cannot modify it with ac3).

Hope that helps clear that up as far as handbrake goes.

 

[mrgreen4242]

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?

That's a lot of marketing speak. There really is an audible difference from DPL to DD. There's more separation, clearer surround sound, more bass... it just sounds better. I can't hear the difference from 96kbps AAC and CD most of the time, and never at 128kbps, but I can tell DD from DPLII (on a 5.1 setup).

 

[GermanSuplex]

Sorry to bump this thread but this is pertinent to what I need to know:

When a DVD contains both stereo and 5.1 Dolby Surround, which should I use for the AAC Pro Logic mix? Is the stereo track easier for Handbrake to mixdown to AAC PLII and give better results, or it would it be best to use the 5.1?

Second, if a DVD contains only Dolby Surround (no 5.1 audio), and assuming I don't have great ears, would simply doing an AAC PLII track and skipping the second pass-thru track be acceptable? This will still give me that pseudo-surround, just at slightly less quality than the original AC-3 track, correct?

Thanks in advance. I've been reading a lot about this but the more questions I have answered the more seem to come up.

 

[Cave Man]

I'd use the digital audio track for everything (AAC and AC3).

 

[GermanSuplex]

What I mean is that when a movie has a AC-3 2-Channel track and an AC-3 5.1 Channel, which would be the best to use for the AAC track?

 

[kiranmk2]

I'm of the opinion that if a Studio has made a mix especially for stereo/surround (ie 2.0) then that's what should be used for the dplII AAC track.

 

[eddyg]

Sorry to bump this thread but this is pertinent to what I need to know:

When a DVD contains both stereo and 5.1 Dolby Surround, which should I use for the AAC Pro Logic mix? Is the stereo track easier for Handbrake to mixdown to AAC PLII and give better results, or it would it be best to use the 5.1?

Use the 5.1 DD mix for the AAC DPL II mix so that it has as input all the tracks to mix. The better the input the better the output.

Second, if a DVD contains only Dolby Surround (no 5.1 audio), and assuming I don't have great ears, would simply doing an AAC PLII track and skipping the second pass-thru track be acceptable? This will still give me that pseudo-surround, just at slightly less quality than the original AC-3 track, correct?

Yes, I wouldn't bother with both in that case, however I do just because it's easier to be consistent, and I like th unaltered AC3 going to my receiver.

Cheers Ed.

 

[NightStorm]

Sorry to bump this thread but this is pertinent to what I need to know:

When a DVD contains both stereo and 5.1 Dolby Surround, which should I use for the AAC Pro Logic mix? Is the stereo track easier for Handbrake to mixdown to AAC PLII and give better results, or it would it be best to use the 5.1?

Second, if a DVD contains only Dolby Surround (no 5.1 audio), and assuming I don't have great ears, would simply doing an AAC PLII track and skipping the second pass-thru track be acceptable? This will still give me that pseudo-surround, just at slightly less quality than the original AC-3 track, correct?

Thanks in advance. I've been reading a lot about this but the more questions I have answered the more seem to come up.

As has already been said, I use the AC3 soundtrack for both inputs to simplify things. However, if the source has anything less than a AC3 5.1 mix (ex. 5.0 or Dolby Surround only), I'll encode a single 256kbps AAC soundtrack with Dolby Pro Logic II selected.

 

[eclvvin]

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.

You clearly know what you are talking about. I had two questions, and you already answered the first, but like a good student I like to check my understanding.

1. It sounds like the best default setting is to passthrough AC-3. This will give most home surround sound setups a clearly better experience.

2. If while away from home I want to steam from a home based media drive, does it make sense to have Handbrake create a second audio channel? If so, what is the optimal file type for steaming to what will likely be hotel tvs or an iPad? Is it worth it to PLII? Does Stereo or Dolby Surround offer enough of a file size savings as to make it better for streaming? I have the understanding that if the iPad selects the AAC instead of the AC3, you will have better steaming.

 

[off_piste]

You clearly know what you are talking about. I had two questions, and you already answered the first, but like a good student I like to check my understanding.

1. It sounds like the best default setting is to passthrough AC-3. This will give most home surround sound setups a clearly better experience.

2. If while away from home I want to steam from a home based media drive, does it make sense to have Handbrake create a second audio channel? If so, what is the optimal file type for steaming to what will likely be hotel tvs or an iPad? Is it worth it to PLII? Does Stereo or Dolby Surround offer enough of a file size savings as to make it better for streaming? I have the understanding that if the iPad selects the AAC instead of the AC3, you will have better steaming.

Impressive. The post you quoted is over nine-years old and the last post in the thread nearly eight-years old.

 

[Cave Man]

1. It sounds like the best default setting is to passthrough AC-3. This will give most home surround sound setups a clearly better experience.

Yes (or DTS if that's what the original has, e.g. Blu-ray).

2. If while away from home I want to steam from a home based media drive, does it make sense to have Handbrake create a second audio channel? If so, what is the optimal file type for steaming to what will likely be hotel tvs or an iPad? Is it worth it to PLII? Does Stereo or Dolby Surround offer enough of a file size savings as to make it better for streaming? I have the understanding that if the iPad selects the AAC instead of the AC3, you will have better steaming.

That depends on what you're using to stream. If Plex, I'd leave it at AC3/DTS because it will transcode on the fly to AAC. I don't know if any of the streaming apps stream AC3 or DTS.

 

[porg]

Article / Section https://en.wikipedia.org/wiki/Dolby_Pro_Logic#Software_encoding_and_decoding very lapidary states:

HandBrake and FFmpeg are capable of downmixing Dolby Digital AC-3 5.1 to Lt/Rt stereo tracks compatible with Dolby Pro Logic I & II decoders.

@Avatar74: This your section from your post debunks/clarifies that in great detail:

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.

Thanks! This is very valuable information! Brought forward very credibly too. Would you be so kind and integrate your quoted section to the forementioned Wikipedia article/section? Or grant me the permission to copy/paste it there copyright-free? I would link to your post as the source, with your nickname. For best scientific/encyclopedic practice it would ofc be ideal if you yourself add it there with proper source/citation. But if you have little time, I'd prefer to add it there the "laissez fair" style instead of not being on Wikipedia at all.

Hope that a notification will still reach you now many years later! Thanks!

---

The forementioned specification is likely:

ATSC Standard: Digital Audio Compression (AC-3, E-AC-3)

• Doc. A/52:2012
• 17 December 2012
• By the Advanced Television Systems Committee

And the downmixing rulesets are handled in the sections:

• 6.1.12 Downmixing
• 7.8 Downmixing
• 7.8.1 General Downmix Procedure
• 7.8.2 Downmixing into Two Channels

 

[netnewswireuser]

dd -> real 5.1 channels

dpl2 -> fake 5 channels matrixed in regular 2 channel stereo audio

dd is much better and you should use it if you can.