Hi! I just had a few quick questions. I want to convert AC3/DTS to AAC. I was looking at about 224-256 kbps for 5.1 files and about 128 - 192 kbps for stereo files. Firstly, is the above bitrates good in terms of sound quality. I am sure there are a lot of people here who must've done a lot of listening tests so you guys can help me out here. Secondly, which version of AAC (LC, HE, HE2) would be good for the above 2 cases while retaining good quality. I was looking at LC but someone mentioned in a forum that HE is good for multichannel files. Hence I had to ask this question. Thanks for your time.

You would want to go with LC-AAC. HE-AAC should only be used if you plan on encoding at around the 64kbps bitrate range. LC-AAC should be used for 96kbps encoding and above. That is with stereo CD files though. You will be transcoding from lossy sources (DD/DTS are lossy) so that right there limits what kind of help we can give you. Generally speaking, when ripping from an audio CD, AAC performs well at 128kbps and 160kbps. From my experience, once you hit 192kbps, Nero/iTunes AAC and Lame mp3 all begin to sound alike. Actually, I can't properly ABX the differences between them so they sound the same to me.

Only you will be able to tell what sounds good and what doesn't. I suggest starting off with some stereo 128kbps AAC/AC3 files and conduct a blind ABX test. If you can't hear the difference then great, 128kbps will suite you. Just remember that our ears, music, and equipment is different from yours so we can only tell you what we find acceptable. This can be different from what you fine acceptable though.

Thanks for the reply. My equipment surely wouldn't be as good as the ones you guys use. Plus if someone wants to come over to hear the music, atleast I would be confidant that the settings and hence the quality of the audio would be good since its been tested out by the pros. Btw your answer was for stereo files. What about 5.1 files?

I'd go with 80kb/s/channel.

80kps per channel would be equal to 400 or 480 kbps (assuming subwoofer also gets 80k). That way it would seem better to keep Dolby. But I wanted to choose AAC since it supports multichannel and I was told I can get almost the same quality with a comparatively lesser bitrate (thats why I chose 224 - 256 range).

This advice is probably the best you're going to get. Multiply the bitrate you need for stereo by three for 5.1 channels. Nobody can give you a definite answer as we don't have your equipment, your listening conditions, or your ears.

Cheers, Slipstreem.

No idea about the 5.1, but I've got an iPod and previously used 128 kbps AAC for it (I've switched to VBR MP3s now anyway ;P). The 128 kbps files sounded great, but I never tried them on any other equipment apart from some cheap computer speakers and my iPod. You should try doing a few tests yourself, as we all have different opinions on what is "acceptable". I'd definitely go for 128 kbps if space is an issue due to a large audio collection or something.

Sorry about the long delay. I haven't had much experience with 5.1 files. I did some ABX experiments quite a bit of time ago and found out that 256kbps AAC was suitable for my needs. I was transcoding from an AC3 Dolby Digital lossy source as well. Slipstreems advice of multiplying the bitrate by 3 seems to be the standard. However, I did not like having videos with bitrates of about 384kbps given that the source audio was encoded at around 400kbps VBR. Since I was transcoding from a lossy source, I figured I could get away with using a smaller bitrate.

What will be your primary ways of playing these videos? Are you going to be using dedicated hardware such as a PS3 or will you be using a computer? If you are going to use a computer then I suggest looking into HE-AAC for 5.1 audio. From my experience, I could get away with using a lower bitrate while still retaining the quality. It limited playback of the files to my computer (in order to get the true quality of HE-AAC, I don't have any hardware aside from my cellphone that supports HE-AAC files), that is why I stopped using it and went back to LC-AAC for my movie encoding needs.

If I go based on what Slipstreem suggested, that would a bitrate of around 384 - 576 kbps. Given Dolby is 448 kbps, I am now reconsidering if it is even worth transcoding.



I wanted to make "video library" on my computer. My target was to use about 10 MB per min (about 1393 kbps). I already figured out how to incorporate chapters and subtitles (vobsub) in the final file (with matroska). I would be using H.264 for video. I had to choose between matroska and mp4. Matroska could fit all the different types without any glitches. The cost was I would never be able to play it in any hardware player. Atleast till there is support for matroska. MP4 doesnt support Dolby so thats another issue to consider. I was very strongly in favour of 256 LC-AAC (I chose LC because I heard the support for HE was minimal). If I choose Dolby, I have to go for matroska and if I choose AAC, I have option between matroska and MP4. I will test the chapters and subtitles with MP4 first. If that goes good, then I think I'll go for 256 kbps LC-AAC and MP4 for the sake of compatibility.

Well, bear in mind that advise is from somebody who positively despises 320kb/s AC3. And yes, I am on record as making a clean distinction in an ABC/hr test at MPEG.