What do I need to know about MP3 and video bitrates?

If you download audiobooks or spoken MP3 files, or deliver them by e-mail, you may want to know how you can download them faster and store more of them on your portable MP3 player or computer hard drive.

MP3 files that contain music need a lot of information to be processed. You probably won’t want to alter them because decreasing the file size will reduce the sound quality.

However, if you’re ripping or downloading audiobooks or spoken audio, you can make some file changes that will allow your to speed up downloads and store more files on your MP3 player.

A bitrate is the number of computer bits that are processed in a unit of time. The higher the bitrate of a file, the better the quality of sound or video.

The bitrate is often expressed in thousands of bits per second, or kbit/s. For example, 128 kbit/s is 128,000 bits processed per second.

Hertz is the frequency of electrical vibrations per second. The abbreviation “kHz” represents one thousand cycles per second. MP3 music files typically use a 44.1 kHz rate, which is the same rate used for compact discs.
Most MP3 music files are encoded at 128 kbit/s or 192 kbit/s.  That rate is needed because there is a lot of  frequency and variation in the sound of music.

Recorded voice doesn’t have nearly the range or complexity of music.  Because of this, you’ll find that audiobooks and voice recordings have surprisingly good quality when recorded with a bitrate of 48 kbit/s and a frequency of 32 khz.   Furthermore, the spoken word typically can be recorded in mono, instead of stereo, making the resulting file even smaller.

As a result, you can “resample” your spoken audiofiles to a lower bitrate and frequency to store more than twice as many files on your MP3 player and download them more quickly.

Typical MP3 Audio bitrates

  • 4 kbit/s, minimum required for speech
  • 8 kbit/s, telephone sound quality
  • 32 kbit/s, AM radio quality – this is the bitrate that Audible.com uses
  • 48 kbit/s, FM radio quality, speech
  • 96 kbit/s, FM radio quality, music
  • 128 kbit/s, Typical “acceptable” music quality
  • 256 – 320 kbit/s, Near audio CD quality

Typical Video bitrates

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