AC3 5.1 to AAC 5.1 Suitable Mode/Profile/Bitrate? Options

[Makaveli7184]

I have a large number of 5.1 AC3 audio file encoded at CBR 384 kbps that i want to transcode to AAC. Channel mapping is not a problem. Bytes are scarce, so my goal is to minimize bitrate while preserving audio transparency.
Now i've read in various online posts that AAC standards recommend around 260 kbps for 5.1 audio. I'm using a certain application (i'm not interested in suggestions of using another one or another encoder for that matter) that uses up-to-date AAC codecs. This application gives me the option of encoding in CBR AAC (bitrate ranging from 4 to 448 kbps in increments of 4, and profiles HE2/HE/LC are completely user selected) and VBR AAC (where i can choose a quality preset from 1 to 5, 5 being highest quality and largest filesize and where profile is strictly automatic, HE-AAC 2 kicks in when using Q1, HE-AAC when using Q2, and from Q3 to Q5, format profile is LC).

Giving all the above, and knowing that audio transparency and quality is usually subjective, what would be a good option?
My initial preference is using VBR Q3/LC which gives me 5.1 audio files encoded at around 250 ~ 300 kbps. Do i need to go higher?? Can i go lower without losing perceptive quality?? Should i consider lower CBR with HE profile??

Waiting for some suggestions. Thank you.

[onkl]

MP3directcut which can also do fading, so something like that might even be possible for AC3.

If size is not crucial, you can raise the bitrate to make sure quality doesn't degrade too much.

[Makaveli7184]

MP3directcut which can also do fading, so something like that might even be possible for AC3.

DelayCut doesn't do fading. I'll check for similar tools. But how can a tool do fading without re-encoding?

If size is not crucial, you can raise the bitrate to make sure quality doesn't degrade too much.

Well, size is important, and i was pretty happy with re-encoding to AAC vbr 3 (at around 280 kbps), so to me, going to vbr 4 at around 380 kbps is the max i'm willing to raise the bitrate to. Plus i'm encoding in VBR so i'm not too hung up on the actual bitrates (something that i learned long ago on these forums ).

[Dynamic]

MP3directcut which can also do fading, so something like that might even be possible for AC3.

DelayCut doesn't do fading. I'll check for similar tools. But how can a tool do fading without re-encoding?

In MP3 and AAC-LC, there's a global gain value in each chunk of audio which is in steps of approx 1.5 dB. This allows mp3gain and aacgain to modify the volume of the whole song (by walking through and raising or lowering every chunk's gain value by the same amount). mp3directCut and similar tools can boost or quieten only a selection within a song by applying the increment or decrement to only the chunks within the selection, and they can apply a "logarithmic fade" that goes from, say, 0 dB to -48 dB in steps of -1.5 dB throughout the selection. The overlapping of the chunks in the MP3 decoder smooths out any edge effects you might expect from the sudden step-wise jumps, so they tend to sound smooth without any audible popping noises.

I don't play around with AC3 much or know how AC3 is put together, but there are other codecs of a similar vintage (such as MP2) that don't feature global gain fields, so the fading might not be possible without transcoding (though you could possibly transcode only that section of audio and stitch it on to the original AC3 of the rest of it - you'd need to experiment to check for glitches at the join points)