3GA is derived from the 3GPP multimedia file and contains just the audio stream that was stored in the 3GP file. The 3GA audio container can use a number of audio codecs including AMR-NB, AMR-WB, AMR-WB+, AAC-LC, HE-AAC v1 or HE-AAC v2. It is used by a large number of mobile devices.
3GA is the audio element of the 3GP format which itself is based on the structure of MPEG-4 Part 14, otherwise known as MP4. 3GA contains the audio portion of the 3GP file which was esigned to reduce the overhead in terms of storage and bandwidth to allow audio files that could be handled by smartphones.
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MP3 is a digital music format which allows CD tracks to be reduced to around a tenth of their normal size without a significant loss of quality. MP3 gets rid of a lot of the information recorded in a song that our ears are not able to hear and then uses complex algorithms to reduce the file size. This then enables you to get hundreds of songs on to a CD and it also has opened up a new market over the internet - the download market as download times have been significantly reduced.
The MP3 format is a lossy format. That means that an MP3 file does not contain 100% of the original audio information. Instead, MP3 files use perceptual coding. In other words, that means it removes the information that your ear doesn't notice thereby making the file smaller. The reason lossy formats are used over raw is that raw audio files are too large to travel over the internet at any great speed. By using lossy formats it enables even dial up users to download mp3 files at a reasonable speed. Raw file formats generally require 176,000 bytes per second compared to a lossy format which requires 17,600. The difference is massive and so are the download times.