K3b is a multi-purpose burning/ripping/transcoding tool for Linux under the KDE graphical environment (it can be run under GNOME as well so long as the correct libraries are installed). It is generally pretty stable and does a good job of keeping the meta-data (e.g. title, album, etc) for tracks. The MP4 file format is therefore well matched to be outputted from K3b.

Requirements

Linux (I'm using Ubuntu Dapper Drake 6.06)
Wine (0.9.9-0ubuntu2 .deb package from Synaptic/apt-get). Wine must be set up and installed - you at least need a 'Windows' and a 'Windows\System32' directory. The encoder itself doesn't require any extra DLLs.
Sox (12.17.9-1 .deb package from Synaptic/apt-get)
K3b (0.12.17 compiled from source at http://k3b.plainblack.com/download )
Nero Digital Audio Reference MPEG-4 & 3GPP Audio Encoder (1.0.0.2 downloaded from http://www.nero.com/nerodigital/eng/Nero_Digital_Audio.html )
/dev/shm filesystem (this is often installed by default - you can get by without it, but your hard disk will end up doing more work)

Instructions

These instructions assume you have installed all the required items above.

Configure Wine

I won't go into too much detail about this here, but it's important that Wine can access all the files on your drive. I usually do this by keeping the default of Z:\ to be the 'root' (i.e. '/') filesystem. The program 'winecfg' is very useful in setting up this.

Install Nero Digital Audio executables

This bit is pretty easy. You just unzip the files to a directory somewhere.
You might want to test out that the executables can run by doing something like:
$ wine neroAacEnc.exe
in the 'win32' directory of the extracted files.
You should see some output like this:
err:font:ReadFontDir Can't open directory "/home/chris/.wine/dosdevices/c:/windows/Fonts" ************************************************************* * * * Nero Digital Audio Reference MPEG-4 & 3GPP Audio Encoder * * Copyright (C) 2006 Nero AG * * All Rights Reserved Worldwide * * * * Package build date: May 26 2006 * * * * See -help for a complete list of available parameters. * * * ************************************************************* ERROR: no input file specified

My Wine isn't set up for GUI stuff for Wine too well yet, hence the error message at the top. We don't have to worry about that though, because this is just a command line application.

Create 'neroaac' wrapper script

So I made a wrapper script around neroAacEnc.exe and neroAacTag.exe. The code for it is below. Copy the code into a file '/usr/local/bin/neroaac'. Make sure you chmod it to something like 755.

#!/bin/bash TEMPFILE=/dev/shm/neroaac-$RANDOM cd /home/chris/winec/nero/win32 sox -x -s -w -t raw -r 44100 -c 2 "$1" -w $TEMPFILE.wav wine neroAacEnc.exe -if $TEMPFILE.wav -of $TEMPFILE.mp4 wine neroAacTag.exe $TEMPFILE.mp4 -meta:title="$3" -meta:artist="$4" -meta:comment="$5" -meta:track="$6" -meta:album="$7" -meta:year="$8" mv $TEMPFILE.mp4 "$2" rm $TEMPFILE.wav

IMPORTANT: Change the line that reads "cd /home/chris/winec/nero/win32" to be the directory where you have neroAacEnc.exe and neroAacTag.exe located.

Just in case you are interested, this program will take in the following parameters:

Source filename (or '-' for stdin). The data this script takes in will be RAW 16 bit, 44.1kHz, big endian, stereo to match the format that k3b outputs.
Output filename. e.g. "myfile.mp4"
Track Title
Track Artist
Track Comment
Track Number
Album
Year

It's a bit hard to test this script on its own because of the format of the input file. K3b itself doesn't even export RAW audio in this format when you have RAW selected as a format!

Configure K3b to use 'neroaac' as an External Plugin

I'll do this with the aid of some screenshots.

Load up K3b and start a new Audio CD project:
http://img223.imageshack.us/img223/6554/k3b2bs0.png
Drag an audio file into the project and hit the arrow button in the project with the tooltip 'Convert audio tracks to other audio formats':
http://img80.imageshack.us/img80/6590/k3b3ox5.png
In the Audio Project Conversion box that appears hit the icon that looks like a mechanical gear. You might have to change the Filetype to something like 'Flac' in order to do this:
http://img90.imageshack.us/img90/9279/k3b4hh0.png
In the next dialog box add a new program to the list and enter the following values in the 'Configure Plugin' box:
http://img243.imageshack.us/img243/6195/k3b6dm1.png
The command line here is:
neroaac - %f %t %a %c %n %m %y
Close the 'Configure Plugin' box. Also close the 'Audio Project Conversion' box.

Usage

You can now use the 'Convert Audio Project' to convert any files that K3b can read into .MP4 files. Note that if you want all the meta tags (e.g. "Album") set, you will have to open the 'Burn' dialog for the Audio project and enter the CD Text information and then hit the "Save" button.

You will also be able to rip to .MP4 by selecting your CD reader/recorder in the tree and using the left-most button:
http://img213.imageshack.us/img213/5307/rippingxj3.png

Known flaws/limitations:

The progress bar goes to 100% quickly when converting a file and then sits there for a while. This is because K3b judges the completeness of the operation by how much of the file it has fed to the program. The program needs to take the *whole* file straight up because it needs to make a .WAV file out of it before sending it to neroAacEnc.exe. SOX will not support outputting a WAV file to a data stream.