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Topic: Convert MP2 stereo to mono losslessly? (Read 5053 times) previous topic - next topic
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Convert MP2 stereo to mono losslessly?

I'm wondering if the specs allows such manipulation and if there's any software for this task

VCD often features Dual-Mono sound tracks and I'd want to only get a single channel out (224kpbs Stereo to 112kpbs Mono)

Convert MP2 stereo to mono losslessly?

Reply #1 –
I don't know for the specs but for the software you can try BeSweet.

[Edit]arfff no...sorry. It's not losslessly.

Convert MP2 stereo to mono losslessly?

Reply #2 –
Technically (looking at the file format), it should be possible to extract one of the channels from a dual-mono MP2 file to create a single-channel MP2 file, without decoding and re-encoding the audio data.

Unfortunately I don't know any software that can do this.

Convert MP2 stereo to mono losslessly?

Reply #3 –
Is it douable without too much hassle or next to impossible?
Any skilled coders up for this task?

Convert MP2 stereo to mono losslessly?

Reply #4 –
There's one problem:

224 kbps dual channel does not mean 112 kbps for each channel.

The amount of bits used to represent the channels usually differ. If you want to extract only the first (or only the second) channel of a 224kbps-MP2 frame, it might not fit into a 112kbps MP2 frame - You'll have to use a higher bitrate for those frames.

The opposite is also true. You can save space by reducing the bitrate if the channel of interest uses fewer bits than available in a 112 kbps frame.

So you'll end in a VBR stream.

Next resulting problem:

The allocation coding and maximum possible amount of used subbands depends on the ratio frame_size/channels. It may be impossible to transcode without requantizing the subband samples.

Fazit:
It may be possible for some streams. The resulting streams will be VBR. Some of the bits (at the end of each frame) will be unused and thus wasted.

IMHO:
It's not worth a try and there's no real demand for such a tool - unless you pay someone for implementing it.


bye,
Sebastian

Convert MP2 stereo to mono losslessly?

Reply #6 –
SebastianG: Are you sure this also apply to dual-channel and not only to stereo and joint-stereo?

Convert MP2 stereo to mono losslessly?

Reply #7 –
Quote
SebastianG: Are you sure this also apply to dual-channel and not only to stereo and joint-stereo?
[a href="index.php?act=findpost&pid=230232"][{POST_SNAPBACK}][/a]


Pretty sure, yes. The specification does not force an encoder to do a 50/50 splitting of the bandwidth. The only difference between Dual Channel and Stereo is the mode-flag in the header. Call it "meta data" if you want. This is also true for MP3.

bye,
Sebi

Convert MP2 stereo to mono losslessly?

Reply #8 –
...though it's easy to see why an encoder might choose to split the bandwidth 50/50 for dual channel - rather like an encoder might choose to use only long blocks, or m/s frames, or whatever: It could choose this strategy to put speed over quality. For dual mono, it's no worse a strategy than using CBR for a regular signal.

If it's not 50/50, then this implies the result of splitting to mono will be a VBR output (if it's possible at all) - that's almost useless, as very few mp2 decoders will accept VBR.

Cheers,
David.
 

Convert MP2 stereo to mono losslessly?

Reply #9 –
Apparently, BeSpit 0.9b6 can acheive this via this command line (found on doom9)
Code: [Select]
BeSplit -core( -input stereo.mp2 -prefix e:\channel -type mp2 -demux )

http://forum.doom9.org/showthread.php?threadid=68300

Haven't tried though...