iTumes Movies Sound Track and Dolby Digital?

Good evening,

I just downloaded Toy Story tonight, and am playing it through iTunes 7 and a set of Logitech 680 speakers through my optical audio out port. Looks like the file is in Dolby Sourround (stereo) --

Apparently, you must set your receiver (in my case, my Logitech 680 speakers) to "Dolby Movie Mode" in order for the receiver to decode the sound and direct it to the correct speakers (and ultimately obtain sourround sound).

Maybe i'm mistaken, but I think thats how it works.

I wish there were an embedded DTS track or something- I don't like having to set my speakers from "Dolby Movie Mode" and back to "Stereo mode" all the time.

According to Logitech's site "Dolby Pro Logic® II decoding creates a 5.1 soundfield from two-channel sound sources".

Can anyone confirm I'm not doing anything wrong to listen to this audio in perfect 5.1 ?

Power Mac G5 Quad  30" Display  20" Display  4GB Nano  Sony HVR-Z1U HDV  Mac OS X (10.4.7) 4 x 1.2TB Lacie HD  Logitech Speakers, Mouse  Wacom 12" x 12"  Titanium 667

In order to listen to music in "perfect 5.1", you need a source with a 5.1 Dolby Surround or DTS audio stream. It is not possible to up-mix a stereo recording into 5.1 (although some devices employ various psychoacoustic tricks to simulate surround sound, some with more success than others).

The bottom line is that if the movies downloaded from iTunes are not in 5.1, there's no way to play them in surround sound.

.. Apple Claims that all movies on the iTunes Movie store are downloaded in "Dolby Surround". (as said in the webcast)

My copy of Toy Story reports 2 channels of audio as well. I think this is normal for a Dolby Surround Track since a Dolby-enabled receiver will decode a 2 channel mix and turn it into a 5.1 sound.

I could be wrong, if I am, someone please correct me.

I visited http://www.dolby.com/consumer/technology/tech_overview.html and found out that: "Dolby Surround technology encodes four channels of audio (Left/Center/Right/Mono Surround) onto just two audio tracks for media such as TV broadcasts and feature films on VHS. On systems containing Dolby Pro Logic® technology, the audio is reconstructed to its original four-channel surround sound."

I'm just not 100% sold on the fact that the audio that Apple is indeed including in their movies are encoded in Dolby Surround or if it's simply normal Stereo.


[PS - Thanks for the Reward Points, dnar]


Power Mac G5 Quad  30" Display  20" Display  4GB Nano  Sony HVR-Z1U HDV  Mac OS X (10.4.5) 4 x 1.2TB Lacie HD  Logitech Speakers, Mouse  Wacom 12" x 12"  Titanium 667

What I gather from this (and my own download of Sixth Sense), what Apple has done is to encode with Dolby Surround, not Dolby Digital 5.1. IIRC what Dolby Surround does is to give you 4 channel sound encoded off the stereo signal - L, C, R and Rear all from a matrix - basic Dolby Pro Logic. If you have Pro Logic IIx in your receiver, you'll get a little more (my Yamaha does an OK job with Pro Logic IIx and using DTS Neo 6 driving 5 channels) - No LFE (sub) though...

Suboptimal in my opinion and I will not buy anything more from iTunes movie store until they start shipping Dolby Digital 5.1 (at a minimum). The semi-instant gratification doesn't overcome the loss of audio quality, so I'll keep buying DVDs.

Be interesting to see if Apple does upgrade the audio - particularly when the iTV ships.

My copy of Toy Story reports 2 channels of audio as
well. I think this is normal for a Dolby Surround
Track since a Dolby-enabled receiver will decode a 2
channel mix and turn it into a 5.1 sound.

As I said above, you cannot turn a 2-channel mix into 5.1 surround. You need five channels of audio to get 5.1. The best that can be done is to apply various effects to the 2-channel signal in an attempt to simulate surround sound.

I dissagree. If indeed Apple is distributing their movies in Dolby Surround, the end result can be played back in surround sound. (4 channels - Left, Center, and Right, plus a mono surround channel that's usually split between two rear speakers.)

Apple must therefore be encoding their movies from a 5.1 source, into a Dolby Surround track. The end result is a 2 channel sound file - which is sent to us via the iTunes Movie store. Then, it must be decoded at the consumer level to be played back in a 4 channel system.

I dont think this is "simulated surround". Yes, it is down-mixed, but it can be brought back to a 4-clannel mix with the right receiver.

A tech document from Dolby says: "Material encoded in Dolby Surround is fully compatible for playback on mono and stereo systems: upon playback on systems containing Dolby Pro Logic technology, the audio is reconstructed to its original four-channel surround sound." So there you go, right from dolby, they say it is reconstructed back into the ORIGINAL four-channel surround mix.

More info about the encoding process here: http://www.dolby.com/professional/proaudio_engineering/solutionssurround.html

I still wish Apple would include a DTS track -- or something with a little more "oompf". But this wont stop me from buying movies from the iTunes Store. The lack of HD movies will. 😟

Message was edited by: Stéphane Sylvester (added last paragraph)

Dolby Digital and Dolby Pro Logic are two different things.

I know of no DVDs that are mixed in Dolby Pro Logic; DVDs have either a PCM, Dolby Digital, DTS or MP2 soundtrack. Something has to be encoded in Dolby Pro Logic during recording in order to play back with four channels of audio. If Apple are encoding their movies in Dolby Pro Logic, they have got the original audio tapes for the films, and had them remixed specially.

All of this would be rather pointless however, as iTunes cannot play Dolby Pro Logic audio streams. So, it seems unlikely that Apple's movies are encoded in it.

I agree... I won't be purchasing any movies from iTunes without at least a 5.1 audio track. Unfortunately I can't find anywhere on the feedback pages to tell Apple of my dislike of their choice to not include this, the only one I can find is to request things.

Hopefully when they release the iTV or when HD becomes the standard for their downloads they'll solve this issue and I'll be able to begin to support them in their endevors, until that time I'll just have to continue with purchasing DVDs.

Typically the stereo PCM signal on a DVD is actually a Pro Logic or Pro Logic II encoded signal.

The generation of the 4 channels (L-C-R and Rear) is not done magically, there is additional information overlaid on top of the stereo signal, so the receiver then extracts the additional info. It's actually only one additional channel that is overlayed - the rear mono signal, since it is trivial to steer a stereo signal to Left Center and Right channels.

If you have a Pro Logic II capable receiver it will synthesise left and right rear as well, and if you have IIx it will also steer a center channel.

It actually works pretty well, but is not as good as true 5.1 - which means that the movie is significantly less than what you get on a DVD, for about the same price. The only advantage I see is the on-demand side of it.

iMac, MacBook Pro, Mac Mini, iBook Mac OS X (10.4.7)

Typically the stereo PCM signal on a DVD is actually
a Pro Logic or Pro Logic II encoded signal.

A stereo PMC signal is a stereo PCM signal. Digital Pro Logic is encoded in AC-3, as is the Dolby Digital 5.1 portion of the DVD. Most DVDs do not contain a PCM soundtrack anyway, as it would take up too much space.

I'll try to summize what the realty is.

- iTunes movies are in Dolby Surround
- Dolby Surround works by encoding 5 channels of sound in 2 channels.
- the .1 channel is not encoded in any Dolby format but is instead filtered from the 5 channels and sent to a subwoofer, if one is present.
- On 2 channel systems, you only get L/R
- if you run L/R or digital to a receiver that can decode Dolby Surround AND you have 5 speakers connected, you will get 5 channel surround.
- If you have a subwoofer connected, your receiver will route the low frequency sounds to it.