There are many different things to consider when looking for Audio Books: formats, Media, Audio Players, Software, Services.
Digital Audio formats
Digital audio is usualy compressed (made smaller) in different ways, and to a different degree. Factors that can be changed (reduced) to achive this compression are sample rate (number of samples per second taken from the original audio, usualy in Hz); bit rate (amount of bits or disk space used to describe a sample, usualy in kb/sec), and number of chanels (e.g. mono, stereo, 5.1 surround, etc.). The higher these factors are, the better the quality, the smaller they are, the less space they take.

We group digital audio formats into two main categoties: Lossy compression formats, and Lossless compression formats. As the names suggest, lossy formats lose some data, and sometimes some quality. They also take up a lot less space, generaly TEN times less then lossless formats with no preceptible loss of quality. Lossy formats are the choise for listening on the go, sharing, or streaming over the internet.

Lossless audio formats do not lose any data. They take up much more space on your hard drive, and are recomended for long term archiving of audio where high fidelity of sound is important.

Probably the most popular digital audio format, and certainly the most famous is MP3. We will sometimes use MP3 as a kind of benchmark. We will say "better quality than MP3" (at the same bit rate), or "takes more space than MP3".

Compatible with nearly all software and hardware audio players.
256 Kb/s DVD quality 2mb/1min
128 Kb/s CD quality 1mb/1min
64 Kb/s FM quality 1mb/2min
32 Kb/s AM quality 1mb/4min
16 Kb/s Phone quality 1mb/8min
Open format. Offered by many online audio book services.
Sample rate 16 kHz to 48 kHz. Surround sound support.
Plays with no restrictions, unless specified by Digital Rights Managnment
Originaly developed in 1991 as part of MPEG video format. Widely popular as a non-streaming audio format.
Compatible with many (192) digital audio players including iPod.
format1 Phone quality 2mb/1hour
format2 AM quality 4mb/1hour
format3 FM quality 7mb/1hour
format4 Near CD quality 14mb/1hour
Supported by
Audible
only
Requires the Audible file manager or iTunes.
Developed and used exclusively by Audible as part of their online audio book service. Extremely compact format.
Compatible with Apple iPod, Microsoft Zune.
format1 Phone quality 2mb/1hour
format2 AM quality 4mb/1hour
format3 FM quality 7mb/1hour
format4 Near CD quality 14mb/1hour
Supported by
Apple
Requires iTunes or Quick Time.
Developed and used by Apple as part of their online music and audio book service. Used for Podcasts.
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