How to use eac3to to edit AC3 to avoid transcoding

[borisanddoris]

The way I get my edit points is to line up the tracks and listen for a massively audible different that indicates how out of sync it is. It's a new way I've been doing it, and has been very effective and quite quick. I then zoom in and can see the difference. Using this method, I can usually pinpoint anything greater than 21ms and note an edit point. Usually less than that is imperceptible but I've had some around 18 that just felt off. On a good project, the delays are in even frames or very close to that, and infrequent. I used to use the frame approach: match 1 frame at a time but I found that sync was not nearly as consistent throughout. Not bad, but it could be better so I switched to this method and am really happy with the results. Again, knowing reel changes, side changes, potential scenes that were cut can give you clues on where the major edit points will be. Did one recently that had absolutely no edits needed for the first reel, but the next two had nearly a dozen total! last two reels only had 1 edit point needed. It's fascinating.

[alleycat]

Anyone ever had it where the center channel stays in sync but the front/right goes out of sync? I assume I'll have to transcode to wavs in order to edit the individual channels and the rejoin as a new ac3?

[zoidberg]

Is this a sync problem or just the way it's mixed? If you wanted to move the tracks relative to eachother then yes you would have to transcode. I've not done it but I think you can splice transcoded/re-encoded sections back into the original provided you stick to the codec frame structure and use the same encode settings.

In the past when I've synced older LD mixes to blu rays with newer remixes I've found the remixes moved the dialogue/score/FX around so much relative to eachother its borderline impossible to sync by audio alone. In those cases you have to use a video reference.

[borisanddoris]

Many new mixes do mess with sync on music and effects. I always sync to dialogue which leaves the rest looking great. I think the idea in the new mix is to fix things that may have always been out of sync on M/E but the older mix is typically just fine.

[dillon]

not sure if ur just using audacity but in any video editing program I go with a single 5.1 embedded channel on the timeline so nothing gets shifted

[borisanddoris]

I should say if music and effects are out of sync and dialogue isn’t or vice versa, it’s a clear indicator that the audio was remixed, possibly heavily. That too can be helpful for historical purposes.

[alleycat]

Thanks everyone - not sure what happened but when I went back to working on it the next day, the issue was fixed. I normally turn on sync lock to avoid channels from the same track getting out of sync so not sure what happened, but anyway it's fixed now. With this latest batch I've just been using Audacity to view the track to look for differences, then use eac3to to make the amends.

[NeonBible]

I do want to add one note to this excellent guide:

I utilize the -dontpatchdts option when working with 20-bit 44.1khz DTS LaserDisc audio in order to preserve the stream as is. eac3to will automatically pad these to 24-bit if you do not use that option. Not sure if it affects the audio, but I always make it a goal to keep these streams as original as possible with no extra processing included. I've yet to run into a DTS LaserDisc with dialnorm but I guess its possible.

Does this also apply to DTS tracks taken from DVDs?

[zoidberg]

Lossy codecs like DTS DCA (or AC-3) do not have a true bit-depth like PCM, the 20bit figure refers to the equivalent dynamic range made possible by the LFE channel. I don't think eac3to pads the bitstream, I'm not even sure that's possible for a lossy codec, it merely altars the metadata (which is what mediainfo reports).

[borisanddoris]

I always use -dontpatchdts for LD DTS just because it’s one less alteration. Whether it makes a difference I don’t know but it tells me that this definitely came from an LD.