Need to convert 24-Bit 48k PCM WAV to something smaller for BluRay [SOLVED]

[FulciLives]

Hello!

I have a Blu-Ray that I want to back up and it has 24-Bit 48k PCM WAV audio. When I extract the movie and this audio track and try to make a Blu-Ray file structure with tsMuxeR I get a final size that is like a teeny tiny bit just over (and I mean just barely) the limit of a BD25

So I need to make the audio smaller but I'm not sure what is the best way to do that. I was originally just going to convert it to a 16-Bit 48k PCM WAV audio file but now I'm thinking maybe I can make it smaller but retain quality if I can compress it to Dolby TrueHD or DTS-HD MA but I don't know if that's possible.

Thoughts?

Oh and the audio track is one channel.

[poisondeathray]

I don't know off hand if 1ch configuration is supported by TrueHD or DTS-HD MA , but there are no free encoders for lossless HD audio. You need either the DTS-HD Master Audio Suite or Dolby Media Encoder (both are quite expensive)

[jman98]

Well, PCM is just PCM. If it's only one channel it's trivial to encode it yourself to AC3 despite being 24 bit. It's not free, but CoolEdit can do it if you use the AC3 plugin that's freely available. I'm sure that BeSweet can do this too. If it was 5.1 channel PCM it would be a lot more complicated, but you're more or less just dealing with a standard WAV to AC3 issue here.

If you use BDRB on your source file, it has an option to re-encode HD audio to AC3 and hopefully it could easily do the job for you. I think eac3to, aften and ffmpeg could also likely do the job. I would give up trying to get this in HD audio. AC3 at 640 Kbps is really good and I use this on some home made BluRays I've made from old laserdiscs.

Dolby TrueHD probably supports 1 channel output as Dolby has always been good to support multiple channel options from 1 and up, but I am doubtful (but I truly do not know) that any DTS version except DTS-HD EX would support 1 channel. I've only seen DTS-HD EX used at lowish bit rates for audio commentaries and I don't know what bit rates it supports, but I've only seen 192 Kbps used.

[Island_Dweller]

I don't know off hand if 1ch configuration is supported by TrueHD or DTS-HD MA , but there are no free encoders for lossless HD audio. You need either the DTS-HD Master Audio Suite or Dolby Media Encoder (both are quite expensive)

DTS-HD MA Suite does mono just fine.

But as jman98 says, ac3 should be easiest. 192kbit/s ac3 should be more than enough for a mono track.

[FulciLives]

My concern in making it AC-3 or DTS is that I want to keep it 24-Bit but I think only the HD versions support 24-Bit and by HD versions I mean Dolby TrueHD and DTS-HD MA

I'm not sure about that though.

I guess my line of thought is: damn this thing is just a bit too big to fit a BD25, how can I least destroy the audio to make it smaller?

Anyway I went ahead and converted the 24-Bit 48k PCM WAV to a 16-Bit 48k PCM WAV and that took the audio file size from 732MB down to 488MB and it still doesn't fit on a BD25 so I guess I'll just convert it further to 640kbps AC-3 and forget about the whole 24-Bit thing since it obviously isn't gonna fit otherwise.

Thanks for the replies.

[jman98]

Wow. If you only have 732MB to begin with and that's still too big, you can try 640 Kbps AC3 but if that doesn't work, I'd advise running it through BDRB and just let it shrink the video, leaving the audio alone.

By the way, did you produce a MONO 16 bit PCM WAV file as output? A lot of programs default to stereo for WAV files and if you explicitly make it mono, you could save even more space if you got a stereo 16 bit output file.

[FulciLives]

When converting the 24-Bit 48k PCM WAV to 16-Bit I did in fact keep it 1 channel (I double checked). It was still ever-so-slightly too big so I took the 16-Bit PCM WAV and converted it to a 640 AC-3 audio file (again keeping it 1 channel) and now it finally fits a BD25 (I'm burning it now actually).

Thanks to all who read and participated in this thread.

- John "FulciLives" Coleman

[El Heggunte]

...... converted it to a 640 AC-3 audio file (again keeping it 1 channel) and now it finally fits a BD25

Maybe it's too late now , but IMNSHO 640kbps is superoverkill for a mono AC3 stream. 128kbps is more-than-adequate for a 1.0 AC3 file.

[FulciLives]

Well it fits at 640kbps so I'd rather overkill than underkill and making it fit is what it was all about and if it fits at 640kbps (which it does) then I'd rather use that than go lower (saving space means nothing since I'm burning it to a Blu-Ray disc, I mean either it fits or it doesn't).

[Island_Dweller]

... so I took the 16-Bit PCM WAV and converted it to a 640 AC-3 audio file ...

Why not use the original 24-bit PCM to encode to 640 ac3?

[jman98]

... so I took the 16-Bit PCM WAV and converted it to a 640 AC-3 audio file ...

Why not use the original 24-bit PCM to encode to 640 ac3?

I don't know what FulciLives used, but I've seen licensed Dolby encoders refuse to touch anything but 16 bit WAV input files. That could be the reason. Also, I think it's worth considering that most humans probably can't tell a difference between the 24 bit and 16 bit WAV files any way.

[Island_Dweller]

I don't know what FulciLives used, but I've seen licensed Dolby encoders refuse to touch anything but 16 bit WAV input files. That could be the reason. Also, I think it's worth considering that most humans probably can't tell a difference between the 24 bit and 16 bit WAV files any way.

You're probably right on both counts, but still, it seems like a waste of a good resource.

[FulciLives]

I was under the impression that a standard AC-3 audio file had to be 16-Bit and that only TrueHD could be 24-Bit

Anyway, in this case, I wanted the movie to fit a BD25 without re-encoding the video so this meant redoing the audio. It the end it fit by going from 24-Bit PCM to 16-Bit PCM and then to 640kbps AC-3 and for a film soundtrack (bear in mind an older film with a mono soundtrack) I think I did OK.

Anyway I am curious about if a standard AC-3 audio file can be 24-Bit and is this Blu-Ray Disc compliant? Although in this case had I done it in 24-Bit then I don't think it would have fit.

[Island_Dweller]

Anyway I am curious about if a standard AC-3 audio file can be 24-Bit and is this Blu-Ray Disc compliant? Although in this case had I done it in 24-Bit then I don't think it would have fit.

Lossy audio codecs such as AC3 do not have a bit depth like say, a wav file or a flac files does. Depending on what is used to decode it and irrespective of source bit depth, ac3 can be decoded to 16 bit, 24 bit, 32bit float and so on. So, it really does not hurt to encode ac3 with the highest quality source you have, if your encoder can accept it. It doesn't matter if your source is 16 bit or 24 bit, the file size is solely determined by the bitrate you choose for encoding, since ac3 is CBR.

To be bluray compliant, AC3 must have at most 6 channels, a bitrate not exceeding 640kbit/s and a sampling rate of 48 KHz.

[FulciLives]

So you are saying a 24-Bit 48k PCM file converted to 640kbps AC-3 will be the same size as a 16-Bit 48k PCM file converted to 640kbps AC-3

However the 24-Bit file has more "info" so technically it is being compressed more than the 16-Bit source, right?

Yet it seems that you are saying it is always best to go with 24-Bit if you have it ???

[Cornucopia]

I think what Island_Dweller was saying was that 16 vs. 24 bit is not a determiner of the bitrate of the end file. It IS however a determiner of the QUALITY of that end file (if your encoder can make use of a higher-than-16bit source).

Let me clarify that with an analogy: If I had 2 16bit, 48kHz, stereo LPCM WAV files, they both have the same "structural quality". But only by listening (or very complicated spectrographic & statistical analysis) would one be able to tell if the inherent "content quality" was better or worse.

So too, the BITRATE of an AC3 determines its structural quality. The quality of the source and capabilities of the encoder determine its content quality.

(These are my arbitrary terms, I'm sure we could find better/more appropriate ones)

A lot would depend upon what the encoder could do: can it retain the inherent dynamic range of 24bit (<=144dB) vs. 16bit (<=96dB), and in the case of a lossy codec, does this help? Even with my background, I'm not sure. I'm just hopeful that it would, and so if I had an encoder that accepted 24bit in addition to 16, I would try to start from the best possible (less ERROR, right?).

Scott

[hello_hello]

So you are saying a 24-Bit 48k PCM file converted to 640kbps AC-3 will be the same size as a 16-Bit 48k PCM file converted to 640kbps AC-3

By changing the bitdepth through converting the 24 bit PCM file to 16 bit PCM, you're effectively altering it before converting to AC3. If the converter accepts 24 bit input, then you'd go directly to AC3. Alternatively a decent conversion GUI would know (or allow you to specify) the maximum bitdepth an encoder will accept, so you should be able to encode anything, which will automatically be down-sampled for you before it's sent to the encoder if necessary.

Because when sampling audio, the bitdepth determines the number of discreet values which can be assigned to each sample, the greater the bitdepth the greater the accuracy. By downsampling to 16 bit first, you're effectively reducing that accuracy. Mind you chances are you'll never hear a difference, but theoretically it's better to compress the original source.

[Island_Dweller]

Scott and hello_hello have already explained very clearly, so I will only add the following.

So you are saying a 24-Bit 48k PCM file converted to 640kbps AC-3 will be the same size as a 16-Bit 48k PCM file converted to 640kbps AC-3

If both PCM files have the same number of samples, then yes, the ac3 files will be the same size after encoding.

However the 24-Bit file has more "info" so technically it is being compressed more than the 16-Bit source, right?

Yet it seems that you are saying it is always best to go with 24-Bit if you have it ???

Yes, the 24 bit has more information, but AC3 samples are in frequency domain instead of time domain. See this article, for instance, http://www.mp3-tech.org/ac3.html.

[FulciLives]

Thanks for all the info guys!