A technical explanation of audio files
Compression is a way to store data using less storage space. The music data on audio CDs is not compressed because back in the 1980s when the standard was introduced, the computing power necessary to decompress data on the fly just didn’t exist.
The data is truncated, however, because the music is sampled at a certain rate and limited to a range of frequencies. However, the people who created the standard had the goal of leaving enough data so that no human would be able to tell the difference between an audio CD and digital audio recorded with a higher sample rate and greater frequency range.
MP3s take up a lot less storage space because (1) they use data compression; and (2) they use “lossy compression” which means they remove some of the data. I think it’s better to think of it as smart truncation. The MP3 encoding algorithm is removing data that the algorithm determines is unimportant for humans.
MP3s vary in quality based on (1) the bit rate (lower being worse); and (2) the quality of the encoder. There is also such a thing as variable bit rate (VBR) encoding which maintains a target listening quality and varies the bitrate based on what’s needed for a particular moment in the audio stream to have the targeted quality level.
The maximum size of an MP3 file is 320 kbps CBR (continue bit rate), and the vast majority of humans cannot tell the difference between CD-quality audio and a 320 kpbs MP3 encoded with a quality encoder, even if they are listening on the highest-end audio equipment in a completely quiet room.
Amazon currently uses “variable bit rates aiming at an average of 256 kilobits per second (kbps)” which seems similar to the “Extreme” setting on the Lame encoder (and maybe that’s exactly what Amazon is using). Even though this is a little bit lower in quality than 320 kbps CBR, this is still a quality level where the vast majority of humans can’t tell the difference between that and a less truncated file even under optimal listening conditions.
In the past, when bandwidth and storage were at more of a premium, purchased MP3 files often had lower quality than that. But based on current Amazon quality standards, it seems to me that you can confidently buy Amazon MP3s.
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My first MP3 player was a Sony with only 2GB of memory, and it stored the music in a proprietary Sony format that was only 64 kbps. Sony claimed it was superior quality to MP3, and I am sure they were right because it’s well known that MP3 doesn’t have the best quality compared to some other formats, especially at lower bit rates, but it was still a pretty heavily compressed file and probably had quality somewhat below 128 kbps MP3s which are considered low quality by audiophiles. Yet I was immensely pleased by how great the music sounded on that device.
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I also notice that standalone MP3 players have become much less popular items because everyone is listening to music on their smartphones. But Apple charges a lot of money for memory on their iPhones, so if you buy a dedicated MP3 player that accepts micro-SD cards, you could carry around a much larger collection of music at a much lower price.