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Topic: Mp3 and M4a file sizes (Read 13190 times) previous topic - next topic
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Mp3 and M4a file sizes

Ok, i have  a little question here, hope you can enlighten me:

i'm used to mp3 files but i have an album in m4a format that is about 30 minutes long but the total size of the files is around 186 MB. Is m4a lossless? If so then why is it that when i play the files on winamp, it always shows a bitrate of 128kb?! Does this bitrate refer to mp3 files only?

Mp3 and M4a file sizes

Reply #1 –
Quote from: navarre on
Is m4a lossless?

It can be - Apple doesn't use different extensions to differentiate between lossy and lossless .m4a files, and AFAIK, they're the only ones putting lossless audio in an .m4a container, i.e. I've never heard of Nero doing it (or doing lossless at all).

Lossless would make sense as you're looking at a bitrate in the neighborhood of 800 kbps.

Quote
If so then why is it that when i play the files on winamp, it always shows a bitrate of 128kb?! Does this bitrate refer to mp3 files only?

Dunno - I've never tried playing Apple Lossless files in Winamp and don't currently have it installed on this PC.

Do you have iTunes installed/can get to a machine that does and check the files there?

MediaInfo could also tell you.
"Not sure what the question is, but the answer is probably no."

Mp3 and M4a file sizes

Reply #2 –
Quote
it always shows a bitrate of 128kb?! Does this bitrate refer to mp3 files only?
Simple answer no...  Bitrate is the number of bits it takes to store one second of sound (and/or video), and it applies to any audio or video format. (A typical commercially-produced DVD might have a bitrate around 6000kbps.)

With a little math and (a little rounding):
File Size in MB = (Bitrate in kbps x Playing Time in minutes) / 140

(In order to calculate file size with variable bitrate files, you need to know the average bitrate, and for audio/video files you need the combined audio & video bitrates.)

With uncompressed files, you know the "exact" amount of data it takes, and you can calculate the bitrate from the sample rate (kHz), bit depth, and number of channels.  The bitrate for CD audio is 1411kbps, and uncompressed LPCM stereo on a DVD is slightly higher (1536 kbps) because it uses a 48kHz sample rate, vs 44.1kHz on a CD.


Longer answer...
With lossless compression, you can compress the file to about 60% of it's original  bitrate & size.  With lossy compression, you can compress to around 1/5th the bitrate/size and still retain excellent quality.  At 128kbps, you are about 1/10th the original size (assuming the original is a CD) and you may notice quality loss.

Some formats are more efficient than others, and most of these formats are lossy.  So, a 256kbps file usually sounds better than a 64kbps file.  (i.e. The 64kbps file is smaller, and more data has to be "thrown away".)    And, a 64kbps MP4/AAC might sound better than a 64kbps MP3.    So, you might be able to get-away with a lower bitrate if you use AAC.  (At 256kbps, they both might sound identical to the original.)

For best quality (at a given bitrate) variable bitrate (VBR) is used, so VBR is more common than CBR.  With VBR, silent and simple sounds are encoded at low bitrates, and more complex sounds are encoded at higher bitrates.  (This gives you higher quality at a lower average bitrate & file size.) 

Typically when you use variable bitrate, you don't tell the encoder what bitrate to use.  You tell the encoder what quality you want, and the encoder adjusts the bitrate moment-to-moment depending on the audio content.