Dolby Digital 5.1 on a stereo projector?

[Ed Gordon]

I have been experimenting with converting two channel audio into 5.1 audio. I started with a stereo audio file. I converted it to a new mono file which is used for the center channel. I have not attempted to create an LFE (bass) channel. For the rear channels, I duplicated the front channels (left and right). After a few attempts I got 5 channel LCPM audio which my amplifier recognizes. After some fine tuning, I was able to produce a multi-channel file that my amplifier now indentifies as "Dolby Digital 5.1". The resulting audio is much improved; it is wide and expansive.

I was wondering if there is any way to encode the 5 channel audio into 2 channels that could be recorded with a stereo projector. Here is the MediaInfo report on the audio file I created:

Audio
Format :	FLAC
Format/Info :	Free Lossless Audio Codec
Duration :	18 s 1 ms
Bit rate mode :	Variable
Bit rate :	3 815 kb/s
Channel(s) :	6 channels
Channel layout :	L R C LFE Ls Rs
Sampling rate :	96.0 kHz
Bit depth :	16 bits
Compression mode :	Lossless
Stream size :	8.19 MiB (100%)
Writing library :	libFLAC 1.3.2 (2019-08-04)
MD5 of the unencoded content :	25A6D4CE34E1E7D24132E2508C0E77D2

And here is the MediaInfo file I used to create the new 5.1 file:

Audio
Format :	FLAC
Format/Info :	Free Lossless Audio Codec
Duration :	18 s 1 ms
Bit rate mode :	Variable
Bit rate :	1 403 kb/s
Channel(s) :	2 channels
Channel layout :	L R
Sampling rate :	96.0 kHz
Bit depth :	16 bits
Compression mode :	Lossless
Stream size :	3.01 MiB (100%)
Writing library :	libFLAC 1.3.2 (2019-08-04)
MD5 of the unencoded content :	FF1BEEBB09D18CBA835993E65E0247C9

At this point I assume I would have to have a hardware based Dolby encoder. A Wikipedia article states:

The Dolby MP Matrix was the professional system that encoded four channels of film sound into two. This track used by the Dolby Stereo theater system on a 35mm optical stereo print and decoded back to the original 4.0 Surround. The same four-channel encoded stereo track was largely left unchanged and made available to consumers as "Dolby Surround" on home video.

Any suggestions on how to do this?

PS: the software I used to create the multi-channel file is Audacity

[Osi Osgood]

Audacity is awesome! I've done most of my mixing on Audacity. I wish I could give any advice, but your experiment is facing!

[Brian Fretwell]

Well for Dolby surround the centre channel is recorded in phase at on the left and right tracks and the surround is on both tracks but 180 degrees out of phase. One of them is recorded at 50% level, but I can't remember which.

[Steve Lee]

Hi Ed,
It is possible to purchase a HDMI Audio Extractor that also has an optical input. This takes whatever multitrack input from digital and outputs it into analog stereo. I have used one many times, and they work very well. You can find them for sale on amazon and ebay, they are reasonably priced and won't break the bank.
I have found them to be a handy device especially for re recording soundtracks on super 8.
Steve

[Osi Osgood]

By the way, that was supposed to be fascinating, not facing ... Darn cellphones!

[Ed Gordon]

By the way, that was supposed to be fascinating, not facing ... Darn cellphones!

Osi, I read it as fascinating. Ever heard of Typoglycemia?

Typoglycemia (a portmanteau of typo and hypoglycemia) is a neologism for a purported discovery about the cognitive processes involved in reading text. The principle is that readers can comprehend text despite spelling errors and misplaced letters in the words. It is an urban legend and Internet meme that only appears to be correct.

The following example of typoglycemic text was circulated on the Internet in September 2003:

Aoccdrnig to a rscheearch at Cmabrigde Uinervtisy, it deosn't mttaer in waht oredr the ltteers in a wrod are, the olny iprmoetnt tihng is taht the frist and lsat ltteer be at the rghit pclae. The rset can be a toatl mses and you can sitll raed it wouthit porbelm. Tihs is bcuseae the huamn mnid deos not raed ervey lteter by istlef, but the wrod as a wlohe.

Back to the topic...

From the little bit I have read Audacity should be able to read just about any audio format, and my testing so far indicates that it can create any multi-channel format. Brian's comments on Dolby Surround and changing phasing to create a 3rd channel to use as a rear channel should work with any stereo source routed to an amplifier that decodes Dolby Surround. I got side-tracked creating 5.1 audio, which is of no use on a stereo projector.

Steve, thanks for the info on hardware solutions.