i recently running low of disc space. i'm thinking to convert my new cd album to mp4. but the problem is a lot of mp4 format out there, aac, m4a, mp4... some do video and some don't and so on. Which one should i use. Does anyone has any better format recommendation, why? and if i am to encode to mp4 what bitrate should i use? i heard 128kbit is already cd quality. My thinking now is to conver to 160kbit .mp4 format, but i do consider the tools available for mp4 such as mp3gain for mp3, tag editor and so on. how do you collect your music? Any suggestion and tip would be much appreciated. Thanks in advance.

you are a bit confused ....
1st of all
MP4 is NOT the same as MP3
a MP3 is a MPEG 1 Layer 3 encoded stream
the MP4 in audio is just a container (a hull) for an audio stream
the AAC (advanced audio encoding) is the format the audio is saved in an MP4 scenario
this AAC stream contains nothing more than the pure audio stream ... if you load an AAC stream into winamp for example it cannot even tell how long the track is
now this and more informations like title bit rate and many more information are saved within the MP4 container (maybe together with a video)

the filetypes are ... AAC - pure audio stream
MP4 a MPEG4 video with audio
m4a an AAC stream inside a MP4 container (often mislabled as mp4)
m4v just MP4 video

long time ago we made some subjective test on audio quality of aac encoders and I found that a 96kbit aac stream sound better than a 128kbit mp3 stream (which means on 128k mp3 you can clearly hear the differenc while I was unable to hear any difference with 96k AAC compared to the original audioCD

I personally do not want to convert my MP3's to M4a since it always involves a quality loss ... but any new CD I do convert is now converted into an m4a file (containing AAC VBR ~150kbit/s)
for this I do use EAC http://www.exactaudiocopy.de/
with FAAC http://rarewares.org/aac.html
using these CLI options

as for reencoding I would recommend http://www.dbpoweramp.com/ with very easy to use MP4 plugins

for tags 'n' renaming you can use http://www.mp3tag.de/en/index.html
which is fully mp4 compatible

sorry for any mistake

Yes. It's just like AVI, which is also a container format that can hold many different formats. Only by knowing the container type you can't tell anything about the quality or the codecs (MP3, DivX, etc.) involved.

Actually, only MP4 is the official extension. M4A and M4V are not official but often used to differentiate between audio-only files and files with video.

Always remember that. Never convert a lossy format to another lossy format, at least not for archiving purposes.

I don't recommend FAAC. It's quite old and there are other encoders with much better quality. Nero is offering a very good (if not the best) command line encoder for free, which you can download here (no nag screens, nothing).

I "still" use MP3, by the way. File sizes are okay (~192 kbps) and quality is great if you use the LAME encoder with recommended settings, which you can look up here.

For encoding and tagging I use foobar2000. It's not just an encoding or tagging tool. It's an audio player with very sophisticated conversion and tagging capabilities. I switched from Winamp some time ago as it is far superior.

opss, i get that right now. i'm aware of the file size for mp4 is a bit larger then mp3 at same bitrate. however mp4 achieve better sound quality in lower bitrate. so what bitrate should i convert my cd audio to? i do concern the quality of re-encode because i don't really know i might use it in a multimedia application (presentation and so on). i'm too lazy to go get my cd and redo the rip-ing. thanks

an mp4 shouldn't be any bigger at the same bit rate

as said ... 96k does give you the best bit vs quality rate
with 128+ you can't do anything wrong

now that is new ... I will check that out when I get a new CD
I used nero when gaining my first experiences ... but I don't like NERO and FAAC did the job just fine