Reducing bit rate of AC3 file of DVDs

[rock_king_my]

Hi

I just ripped a AC3 audio file from a DVD with PGCDEMUX.

The bitrate is about 192kbps, now I want to reduce/shrink it to 128kbps then remux it back to the video.

This is to reduce the size of the file. I think that if you shrink a DVD, all is shrunk is the video, not the audio.

What's the easiest way of doing this, if you could do a guide with pics that would be great!

PEACE FOR XMAS!

[jman98]

This can be done, but it's VERY complicated. Trying to save space on a DVD by re-encoding the audio is like trying to save cents on the cost of buying a new HDTV when you should be worrying about saving dollars. Reducing the audio bit rate will BARELY save you any disc space. It's much much easier to shrink the video with something like DVDShrink.

If you must do this, the process is:
1) Demux the audio (you've done that)
2) Convert the AC3 file into 6 separate mono files, one file for each channel of the 5.1 audio stream (I have no idea at all how to do this)
3) Re-encode the 6 separate audio files into a new 5.1 AC3 file with the lower bit rate
4) Use something like MuxMan to re-mux the audio and video and subtitles (if any) together and then use VOBBlanker to put the new VOB in place of the old one.

I have no guide, but that's what you do. Please note that even if you succeed in shrinking the audio, your DVD will probably still be too big. This is why you should not worry about shrinking audio - it doesn't save enough space to be worth the trouble of doing it.

[ntscuser]

If the existing bitrate is 192kbs then it's 2.0 channel not 5.1
The minimum "legal" bitrate for 5.1 is 384 kbs
Reducing the bitrate of 2.0 channels to below 192kbs will destroy any existing surround effect.

[RabidDog]

Not to mention converting from 192 to 128 will save you fractions of a penny in Bitrate. You can do it quite easily using something like besweet but you really need to be down converting from 448kb to 192kb to make any sort of meaningful saving. You dont need to decode all separate channels just plug in the input file and output params to your conversion software. I doubt 2.0 channels would have much surround effect? (2.0=stereo?)

[ntscuser]

I doubt 2.0 channels would have much surround effect? (2.0=stereo?)

They have a great deal of surround effect, it is how Dolby Surround works. The surround information is encoded out of phase with the front stereo using the same two channels and can be extracted using a Dolby Surround or other type surround decoder.

[rock_king_my]

Hi.


I got the original bitrate wrong. It's actually:
Audio: PCM (AC3) 48000Hz 6ch 448Kbps [Output] 261MB

This was 5.1 DD and I just want to shrink it to 2.0 stereo. INTENTIONALLY. It's to make compilation DVDs for relatives who don't have home theatre system.

I managed to shrunk it with BESWEETGUI to:
Audio: PCM (AC3) 48000Hz 6ch 128Kbps [Output] 75MB

So this works for me but I think the audio of the 75MB is not as as loud as the 261MB (volumewise). Qualitywise sounds ok thou.

Is there a way to increase volume in the output by 25 or 50% in BESWEETGUI? I can't see the settings for this.

[aedipuss]

don't bother. use the 5.1, as it will still play even through a mono TV.

[guns1inger]

You are taking a perfectly good 6 channel audio track, and butchering it to a non-compliant, crappy sounding 6 channel audio track to save less than 200 MB, and all for the wrong reasons. The DVD player will happily down mix it to stereo for anyone who doesn't have a full surround system. This is built into the DVD spec and all players.

[politiken]

Hi - The audio level issue is a known problem - Look to this: https://forum.videohelp.com/topic317891.html

So far you gained additional 4% space for the video, but if the video (using DVDShrink) already was above 80% of the original, you really wont be able to notice the difference. Personally I would not go below 192 bit, and I would only do a conversion from LPCM to AC3 - (LPCM is often 3 times the size of AC3). Your original DVD could already hold a 2-channel (stereo) version in AC3, which you can select using DVDShrink.

Pol

[guns1inger]

Your original DVD could already hold a 2-channel (stereo) version in AC3, which you can select using DVDShrink.

Pol

Less and less discs have a stereo mix unless that was how it was originally released. There is simply no reason to have one for modern films because the player does all the work for you.