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Old 07-12-2007, 03:20 AM
gerax gerax is offline
Join Date: Nov 2001
Location: Modena, Italy
Posts: 1,615
Default AC3 Encoding wrecked my mixes


I've been working on a music DVD of a live gig. I recorded it in surround with a 5.0 Decca Array + spot mics. I edited and mixed to 5.0 stems. Sounding good. I went to mastering and we did everything in the digital domain using TC6000 as a surround management tool (compression, limiting), and for simultaneous downmixing to LT-RT. Surround on the PMC and Blue Sky 5.1 monitoring sounded good, LT-RT a bit muffled...but still listenable.
Then it came AC3 encoding (DVD Studio Pro), and when the client got his finished DVD back from the picture editor with encoded audio...all hell broke loose...
The material being played is acoustic music (string quartet) and it's really delicate and harmonically complex. According to my client it really sounds like a bunch of wasps are flying out of his 5.1 set and Genelec 8020...while the original mixes sounded good.
I quickly took a listen to the LTRT and it admittedly sounds a bit strange...I haven't had the chance to check the 5.1 AC3 encoded mix yet, but I fear something's wrong with it too.
Is that possible that AC3 really worsens the quality of audio that much?
We are planning to include DTS too as a higher quality 5.1 audio, and I'm thinking about doing myself a straight 16bit 48KHz PCM stereo mix from the surround session, and use it as the stereo source for the final DVD.

Any thoughts on this?

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Old 07-12-2007, 04:04 AM
Stuart P. Stuart P. is offline
Join Date: Nov 2001
Location: North Hollywood, CA USA
Posts: 841
Default Re: AC3 Encoding wrecked my mixes

What was your dialog normalization setting on the encoding? For that matter, what were all of the settings on the Dolby encoding? Sounds like a mistake was made. The encoding process shouldn't be so destructive as to cause the result you've described.

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Old 07-12-2007, 06:12 AM
Richard Fairbanks Richard Fairbanks is offline
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Location: New York, NY
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Default Re: AC3 Encoding wrecked my mixes

Some thoughts --

There are choices for the amount of data reduction that is used for AC3 encoding. More reduction results in smaller files on the DVD and more quality loss. What data rate did the authoring house use?

How long is the program? It is possible that both the 5.1 and "LtRt" (or stereo) versions were encoded as AC3 to save space. It is true that a Dolby encoder (DP563 with digital ins/outs) will make a very minor change of timbre in the LtRt, but the effect is mostly very subtle.

I do not think there is any way to make a true LtRt from TC6000. You can downmix to LoRo but an LtRt requires an external encoder (as far as I know). Someone correct me if I am wrong, I haven't actually messed around with the downmixes in mine.

As to AC3 encoding, I have heard really annoying arifacts in channels with low levels when other channels are louder, even when higher data rates are used. In dialog type of programs, I have never noticed the artifacts when all channels are playing, only when listening individually. When all channels are roughly the same level the effect is much less noticeable. I imagine it would be a real problem with lower data rate AC3 encoding of wide dynamic accoustic music although I do not have any direct experience with AC3 and that type of material. Using the highest data rate that DVD playback allows should mostly resolve the issue, though.

DTS sounds differently. It can be better, but it typically uses a faster data rate (larger files) to do it.
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Old 07-12-2007, 08:12 AM
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minister minister is offline
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Location: Minneapolis, MN
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Default Re: AC3 Encoding wrecked my mixes

It sounds like a problem with either the encode or the decode.

Who encoded the AC3? Can you verify settings?

HOW EXACTLY is this being played back? There are some AWFUL Dynamic Range Setttings in playback devices that are designed to "enhance the consumers listening experience" that actually make things sound terrible!! For instance, on my DVD player "Normal" is SOME decoding compression; "Wide" is off.

Clipping tis the new Black; Wide is the new off.
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Old 07-12-2007, 11:32 AM
hef hef is offline
Join Date: Jun 2000
Location: nyc
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Default Re: AC3 Encoding wrecked my mixes

i recently did a 5.1 direct to dvd which was being encoded by the distributor for release. However i needed to make a 5.1 AC3 DVD for a screening at a theater. it came out fine. i used Compressor to make the AC3 file, and used the highest bit rate. i put it in "DVD Audio" setting- (for some reason when i tried the "DVD Video" setting the disc wasn't right. and i used the highest bit rate. Dial Norm -27.

also, in this movie there is a lot of strings and orchestration (all recorded 5.0), which sounded pretty good at the theater and on a home setup. Even the composer thought so at the screening.

its surprising that 6 files each over 400 mb compressed to one smaller file can sound as good as it did. but it did.

you need to listen to the AC3 encoded DVD yourself on a system setup correctly- because it could be your clients setup thats messing it up. or obviously it could be your just using a really low bit rate.
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