AC-3 to AAC Conversion - BitRate

[oriz]

TL;DR Is AC-3 to AAC 96kbps fine, or does it make sense to set a higher Bitrate?

Hi all, it is about time, that I have finally created an account. I am reader here for some time now, but today I have a very specific question about video/audio conversion.

I would like to convert some videos into MP4 and I have done some research about Handbrake and its settings. For the audio settings it sets the first track to AAC and the second track to AC-3. As I have read, this is for compatibility.

I was now wondering what a good bitrate would be to set? I know for example that Subler uses 96 kbps as default setting and Handbrake is currently set to 160 kbps. Since the AC-3 source is already compressed, I was wondering if it's worth to spend more then 96 kbps (Dolby Pro Logic II) on the AAC track.

Since I have not hooked my Apple TV 3 to a receiver or anything like that, I know that my ATV3 is currently playing the AAC Version to me.

 

[oriz]

Does no one have an opinion about the bitrate to choose for an AC-3, 6 Ch to AAC, 2 Ch conversion?

 

[MacUser2525]

I go with 128k for stereo 2 channel and 384k for 5.1 surround sound and do not bother with the compatibility second audio track, works for me YMMV.

 

[Unami]

I go with 128k for stereo 2 channel and 384k for 5.1 surround sound and do not bother with the compatibility second audio track, works for me YMMV.

i usually go with 160-256k when converting a 256k ac3, to have some headroom. it's a pretty small difference in file-size (160 is 32kb/s more than 128, so that's 4kB/s, multiplied with 5400 seconds - 90 minutes - so that's about 20MB more on an average movie, 40 when you compare 96 to 60)

 

[MacUser2525]

i usually go with 160-256k when converting a 256k ac3, to have some headroom. it's a pretty small difference in file-size (160 is 32kb/s more than 128, so that's 4kB/s, multiplied with 5400 seconds - 90 minutes - so that's about 20MB more on an average movie, 40 when you compare 96 to 60)

True enough but when you get into them insane bitrate 5.1 files sometimes 1/4 to 1/3 of the file size is the sound once converted if you just pass it thru. I have done the blind test on them leave intact sound and converted sound have my brother click on play then sat there and tried to hear a difference never found any I could tell so went with what I had tested at.

 

[oriz]

Hey Thanks for your answers @MacUser2525 and @Unami

@MacUser2525 I guess the same would / could be true with 96 kbps, isn't it? Since AC-3 is already lossy, you introduce more loss anyways, so an AC-3 384 kbps to an AAC 384 kbps will not be 1:1 the same (in terms of sound quality).

I currently think about if it is even worth it (in terms of sound quality, file size), to spend more than 96 kbps for the AAC track, as this track will have a bad quality anyway. Of course, if you have a lossless source then this would be different.

Currently I can't hear a difference, but I have also not a very good setup.

 

[MacUser2525]

Be the same with the lossless as well unless you are a bat most people cannot tell the difference between them. It is all an illusion the it is bigger so has to be better mentality that is foisted upon people all the time by the hype and advertising machinery to convince yet more people they need something new and more expensive. Now when I was younger and you had albums you could actually hear the differnce between that hundreds of dollar turntable and the cheepie in the sound reproduction. Digital files unless you have total garbage for speakers not going to happen except for the very tiny minority who have excellent hearing. In short go with what you can live with for a bitrate.

 

[oriz]

So, I did some more encodes of the same material to test my settings. It seems that Handbrakes 160 kbps is equivalent to Subler's 80 kbps (per channel) setting. I was not able to find if Handbrakes setting is per channel or not, but from my tests (e.g. file size comparison) it seems to be the case, that Handbrake shows the bitrate for both channels.

 

[tjwilliams25]

I agree with @MacUser2525, really it's up to what your ears like and what you want file size-wise. Personally I do as little compression as possible, not because I can tell the difference, but because I have a very large drive that I put my media on so I'm not too worried about it. Go with what you think may be best for you.

 

[Sedulous]

Why re-encode the AC3 track? Just set it to passthrough to maintain the AC3 as-is and create a second AAC track.

Edit: ah, slightly misread OP. Yeah, 128 is fine for AAC audio.

 

[gnasher729]

Every time you encode audio, you add faults to it according to the bitrate. This adds up. Even if you had a 96KBit mp3 file, converting it to 96KBit AAC makes it a lot worse (you get all the faults from the MP3 file, plus all the faults from the AAC conversion at low bit rate). I'd go at least 128 or perhaps 160 Kbit.

 

[oriz]

@tjwilliams25 Yeah, that's what I do now. However, I would have liked a more reasoned, technical approach about why to choose which bitrate.

@Sedulous No problem for misreading. It is still a very good tip, though if someone is new to this

Every time you encode audio, you add faults to it according to the bitrate. This adds up. Even if you had a 96KBit mp3 file, converting it to 96KBit AAC makes it a lot worse (you get all the faults from the MP3 file, plus all the faults from the AAC conversion at low bit rate).

Yup, I know that, hence the question if a high bitrate makes any sense in terms of quality. AC-3 is already compressed, so if you encode with 128 or 160 Kbit, does it really make sense? See my goal is optimization in terms of quality and file size. So of course I can just set Handbrake to 320 kbps and call it a day, however I might end up with a large file, that does sound exact the same as a 80 kbps encode but takes four time the space on my hard drive.

However, I am currently fine with a 96 kbps setting per channel, which is the equivalent of 192 kbps setting in handbrake and I – of course – pass through the ac-3.