There’s more to audio codecs than file format. Equally important are things such as bitrate (directly affecting the file size), format of source audio files, and last but not least, compatibility with devices you’re going to listen the audio files on.
The bitrate is the single most important factor affecting the size of the resulting file. Other things equal, most formats will offer similar file sizes for similar bitrates – with AAC being a notable exception, as this format only supports variable bitrate. You may get slightly smaller files by choosing the OGG container, but this is by far not the deciding factor.
The deciding factor for selecting a certain bitrate is audio quality. There are endless comparisons on the net testing one format against another with certain bitrates. For your typical portable music applications, CD-quality audio, the following bitrates offer similar audio quality between different audio formats and, generally, are a good starting point. Note that the bitrates listed below are a ballpark; you may want to use higher or lower bitrates depending on your application, taste, or desired audio quality.
MP3: around 256 kbps (196-320 kbps is the optimal range).
OGG: optimal range around 196-256 kbps.
AAC: around 196 kbps.
The constant vs. variable bitrate debate is endless. Technically, constant bitrate (CBR) and variable bitrate (VBR) are two options you can choose when encoding audio files. Both choices will have their own set of strengths and weaknesses.
The main strength of constant bitrate (CBR) is universal compatibility. CBR appeared earlier, and is supported by a wider range of devices. VBR requires more computational power to decode using more sophisticated algorithms. Some (mostly older) audio players ‘choke’ when playing VBR-encoded MP3 files.
On the other hand, VBR produces files that are better optimized audio quality while maintaining similar file sizes (or produce smaller files while maintaining similar audio quality; your choice). Notably, the AAC format is always VBR, a choice consciously made by the developers of one of the best codecs currently available.
Choose CBR when maintaining compatibility with older hardware (such as many in-car MP3 players) is essential. You’ll probably want to up the bitrate a bit when using CBR.
Choose VBR when audio quality is what you’re after, or when (marginally) smaller file sizes are essential. Make sure your player copes well with VBR-encoded files. Remember, when using AAC, your files are always VBR.
The reasoning of using joint stereo is similar to the concept of VBR. However, the implementation and end result are completely different.
In order to cut down the size of the file and produce reasonable sound quality at lower bitrates, some audio formats (e.g. MP3) introduced so-called joint channel compression. Basically, this compression scheme assumes that differences between left and right audio channels are small, and encoding the different audio channels can be performed jointly, with differences between the channels derived and encoded. Joint stereo compression is a blessing at lower bitrates, as it squeezes a little bit extra quality out of the very low available bitrate, but should be avoided at higher bitrates. By default, MP3 will encode audio in true stereo (separate encoding for left and right audio channels) starting from 192 kbps. AAC was initially developed with true stereo in mind.
Resulting file size depends almost entirely on encoding bitrate. Granted, AAC files encoded into 192 kbps may be smaller than similarly encoded MP3s or OGG files, but that’s because AAC doesn’t offer you an option of using constant bitrate (CBR), applying greater compression to parts of audio carrying less complex range of audio signals.
To determine which single audio format produces the smallest file while maintaining acceptable audio quality, perform a simple test as described in Part I of this article.
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