How much space saving using AAC instead of AC3?

[vrpatilisl]

hi
I want to know how much file size is reduced of output mkv. When i encode ac3 to aac. I generaly passthru ac3. Is there any significant file size reduction.
Consider 384kbps ac3 to 160kbps AAC.
Thanxs

[MatLz]

Without speaking about overhead of different codecs in different containers, just use the ratio bitrate / bitrate.
Here, your aac audio size will be ac3 audio size * 160/384.

Or simply use the formula size = bitrate * duration

[hello_hello]

I use a quality setting (Q0.5) when converting to AAC. As a result I never convert AC3 audio as there's generally no significant reduction in the size of the audio stream. DTS is of course a different story. Generally you can save around 700MB per movie by converting it to AAC (using the quality setting I do at least), but because it's a quality setting no two encodes would have the same bitrate anyway.

I tried to have a look to see what sort of bitrates my DTS to AAC conversions use, but MediaInfo wasn't reporting the bitrate of AAC audio inside MKV files. I'm not sure why that is. If I had to guess, I'd say the average 5.1ch AAC encode comes out at around 300MB or less, which is fairly similar to AC3. I did just demux the audio stream from the last DTS soundtrack I converted to AAC to see what the file size was. 244MB. That's multi-channel audio, Q0.5 quality setting and a duration of 1 hour and 30 minutes.

Using your example.... 384kbps ac3 vs 160kbps AAC.... logically the AAC version would be less than half the size of the original given the bitrate is less than half while the duration would be the same. Around 40% of the original size by the look of it.
If you were to convert the AC3 audio would you mix it down to stereo? Most stereo AC3 audio is 192kbps. It's just that 160kbps seems a bit on the low side for multi-channel audio to me.

[vrpatilisl]

thanxs
which is better option ac3 passthru or aac

[kypec]

I use a quality setting (Q.05) when converting to AAC.
Didn't you mean Q0.5 by the way? .05 wound sound like total crap even through phone speaker IMO:D

thanxs
which is better option ac3 passthru or aac
Better for what?
Filesize reduction? AAC is better.
Sound quality? Original AC3 is better.
Hardware compatibility? Probably AC3 for stand-alone devices like DVD/USB players, set-top-boxes, multimedia players, TV sets whereas AAC should be also well playable by current handheld and portable devices, smartphones etc.

[hello_hello]

Didn't you mean Q0.5 by the way?

Yep, I must have been having a brain dead moment. I've edited my post to fix that.

[rcrh]

One thing to consider is that you normally go from a 5.1 track (surround sound) AC3 down to two tracks with AAC.

However, I have some OTA recordings of older shows and the broadcasts are in AC3 2.0. Does anyone have an opinion as to whether there is an advantage to staying with the stereo AC3 track? I'm not seeing it (or hearing it) if it's there.

Thanks.

[amtm]

If you can get transparent quality to your ears by encoding it at lower size in AAC, then no.

[hello_hello]

However, I have some OTA recordings of older shows and the broadcasts are in AC3 2.0. Does anyone have an opinion as to whether there is an advantage to staying with the stereo AC3 track? I'm not seeing it (or hearing it) if it's there.

For me it really comes down to original file size vs re-encoded file size, even if you're not hearing any difference between the two (and not knowing what quality setting/bitrate you use for AAC).

Assuming 192kbps for the AC3 and 128kbps for the AAC (or even MP3) you're only saving 64kbps. Which, if my math is okay, only saves you about 28MB per hour of video.
So assuming the 192kbps and 128kbps bitrates are fairly typical, maybe the question might be do you need convert an already lossy audio track to another lossy format in order to save around 28MB per hour? If you're really trying to stick to a budget with the file sizes then it's probably worth it to keep the video quality as high as possible.

[Keiyakusha]

Assuming 192kbps for the AC3 and 128kbps for the AAC (or even MP3) you're only saving 64kbps.
I understand that what you saying is an assumption, just want to post my own.
So I'd say for stereo track its like 96kbps aac using apple's encoder implementation, 128 for mp3 using lame and 256 for ac3. These 3 options have relatively similar transparency compared to the source