I need to burn a video to a 4.37GB DVD-5, and I want the lowest AC3 bitrate I can use without losing that much quality. I don't know the difference in efficiency between Opus and AC3, but I figure either a 256kbps or 384kbps AC3 should be comparable to a 116kbps Opus, right? This is stereo not multichannel btw.
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Use your ears, I used to use 256kbps.
The audio has already taken a loss by it's conversion to aac from whatever the original source was;
you're just trying to preserve what is left there
The bitrate of the original audio is pretty much irrelevant. The source audio is decoded, effectively as a 16 bit, 24 bit or 32 bit wave file (depending on the program doing the decoding) and that's all the encoder sees. It has no idea what the source was, so the bitrate you choose is really only related to the quality of the (re)encode, regardless of the source.
192 kbps would probably be fine, but of course use a higher bitrate if quality is the priority. If my math is correct an hour at 192 kbp2 would result in a roughly 76MB file, and at 256kbps it'd be 115MB. Compared to the size of the video stream, the difference doesn't matter.
For the LAME MP3 encoder, the -V2 variable bitrate preset is considered "transparent", and it probably "averages" around 192 kbps for stereo music. Maybe a bot more, but the consensus seems to be.... increasing the bitrate beyond -V2 really only uses more bits.
A good AAC encoder such as QAAC (it uses the Apple ACC encoding engine) probably hits "transparent" at around 128 kbps, but they're not all created equally, and for the open source AC3 encoder (are you encoding with ffmpeg or aften?) my understanding is the encoder was created from scratch to output spec compliant AC3, but it's probably not capable of the quality of the Dolby encoder at the same bitrate, as the Dolby encoder probably has additional compression tricks up it's sleeve, so for open source AC3 encoding I'd lean towards higher bitrates. Not that I really know anything about the AC3 encoder, as if I re-encode it almost always as AAC, but I don't have to worry about DVD compliance.