mpeg2 PS, transcoding just audio

[smok3]

so there is an mpeg2 camcorder

http://somestuff.org/blog/images/sony_sr72e.jpg

that generates this type of stream:
Video #0
Codec : MPEG-2 Video
Codec profile : Main@Main
Codec settings/Matri : Custom
PlayTime : 1mn 15s
Bit rate : 9100 Kbps
Bit rate mode : CBR
Width : 720 pixels
Height : 576 pixels
Aspect ratio : 16/9
Frame rate : 25.000 fps
Standard : PAL
Chroma : 4:2:0
Interlacement : Top Field First
Bits/(Pixel*Frame) : 0.878

Audio #0
Codec : AC3
PlayTime : 3s 616ms
Bit rate : 256 Kbps
Bit rate mode : CBR
Channel(s) : 2 channels
Sampling rate : 48 KHz
Video0 delay : 1mn 26s
ChannelPositions : L R

for certain silly purposes i need the audio to be mp2..., so iam trying:
mencoder -ovc copy M2U00004.MPG -o M2U00004_.MPG -oac twolame -of mpeg

which throws bunch of errors, but the NEW file is ok and useful.

The question is, is it possible to kinda rewrite the audio in the SAME file, so i dont waste disk space/time for copiing video over...?

[wolf82]

I think You can use AviDemux to convert audio quickly (without touching video stream). It's something like VirtualDub, but works with MPEG too.
I doubt there is a tool for in-place audio conversion. Anyway it wouldnt be much faster, as streams have to be re-multiplexed.

[smok3]

yeah, turns out it is not to bad on production machine (which has some raid disks), speed is like 25x real time.