Understanding A/V terms and formats

Common digital audio formats:

  • AIFF (MAC)
  • ACC (MAC)
  • MP3
  • Windows Media – Audio
  • Quicktime
  • RealAudio
  • WAV

Different file formats:

  • Allow for different file sizes and qualities. all audio files have the following attributes
  • File formats
  • Date rate (I.e compression)
  • Channels (mono, stereo or multi-channel)
  • Bit depth quality
  • Compressed or uncompressed format

compressed and uncompressed formats

It is possible to divide digital audio file formats into the following:

  1. Uncompressed. Examples include… CD audio, DV audio, Flac, . aif and .wav files.
  2. Compressed. Examples include… MP3, Real Audio, Dolby AC3 (DVD video)

The two “categories” of audio compression

Lossless. This is a process whereby clever algorithms (such as FLAC ) are employed to “compact” the data in a file without losing any information. Lossless compression can reduce the file size but no-where near as much as lossy.

Lossy. These codecs find ways to throw away information in the file which it thinks the majority of listeners ears won’t miss. File size reduction can be dramatic (10 times smaller) but the effect on audio quality is usually noticeable.

• This section will focus on lossy compressions because it is by far the most used form of audio file reduction technology (iPod, Mobile phones, DVD, Freeview, Cable TV etc).

Why compress audio files?

• Audio compression is employed for 3 primary reasons …

• to reduce file size so that more audio may be stored on a given media format (MP3 players, DVD- video discs, MiniDiscs etc)

• to reduce file size so that files will download from a web site faster

• to reduce Data Rate so that files will stream (broadcast) over a network such as the internet

Compressed audio file settings

• Compressed audio files can vary in size and quality depending on these primary settings …

• Sample Rate

• Number of channels – stereo, mono, surround sound or multi-channel

• Choice of audio compression codec • Audio compression codec settings

Compressed audio file settings

• By altering these settings, either before or after recording, you can achieve differing …

• Date Rates
• Files sizes
• Audio quality

Typical Bit Rates (Podcast)

  • Unfortunately, there is no such thing as a typical bit rate for a podcast, as they vary greatly from as low as
    32 kbps up to around 128 kbps. As a rough guide to the use of different bit rates however, the following may be helpful:
  • 320 kbps – Virtually indistinguishable from original CDs
  • 128 kbps – Typical for musical MP3s and quality podcasts
  • 64 kbps – Common bit rate for speech podcasts
  • 48 kbps – Reasonably common for longer speech podcasts
  • 32 kbps – Poor, usually used to reduce download times

BBC Guidelines

  • British Broadcasting Corporation
  • The BBC have been broadcasting since 1922 and are generally acknowledged as setting standards in the world of broadcasting. As a rule of thumb, if it’s good enough for the Beeb, the chances are it’s good enough for the likes of you and me!
  • The BBC have adopted podcasting as a new broadcasting medium and are now heavily into podcasts. As such they have published a range of standards and guidelines for their new media formats.
  • In summary, the BBC’s recommendations for podcast bit rates are:
  • MP3 Mono Speech: 64 kbps, 44.1 kHz, constant bit rate
  • MP3 Stereo Music: 128 kbps, 44.1 kHz, constant bit rate

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