This is the first entry out of a series of posts, where I compare various aspects of popular podcasts. Part 1 is about file formats, bitrates, samplerates, channels, size, length and filenames.

The selection is based on Flattr podcast charts (mostly german), iTunes Store Top 10 Podcasts (english, german), podcasts by public radios (english, german) and some random ones to get more diversity.

List of Analyzed Podcasts

Here is the list of podcasts with links to the analyzed episodes, the available file formats and filenames (only for one format):

Podcast Filename of Episode File Formats
CRE 188 cre188-telecomix.mp3 MP3
techzing 162 techzing-162.mp3 MP3
Hoaxilla 70 hoaxcast070_wahrsager.mp3 MP3
Planet Money Podcast 20111230_blog_pmoney.mp3 MP3
Startups For the Rest of Us 61 startups-for-the-rest-of-us-061.mp3 MP3
Alternativlos 21 alternativlos-21.mp3 MP3, OGG
Bits und so 272 bits-2011-12-23.mp3 MP3, M4A
Elementarfragen 9 ef09mp.mp3 MP3, M4A
Ubuntu UK Podcast uupc_s04e22_high.mp3 2xMP3, 2xOGG
Fanboys Podcast 49 FAN049.m4a MP3, M4A
This American Life 420 420.mp3 MP3
ARD Radio Tatort 2011_podcast_unter_verdacht.mp3 MP3
Deutschland Radio Wissen drw_201112301745_die_wissensnachrichten_ des_tages_79e879d8.mp3 MP3
Freakonomics freakonomics_podcast122111.mp3 MP3
OE1 Geschichte Podcast OE1_geschichte_111230.MP3 MP3
Wait Wait ... don't tell me 20111231_waitwait_04.mp3 MP3
Radio New Zealand Afternoons aft-20111223-1606-the_panel_christmas_special-00.ogg MP3, OGG
Inside Home Recording 97 IHR_2011_12_05.m4a MP3, M4A
Radio Tux 2011-12-23.RadioTux.WeihnachtsSpecial.2011.ogg MP3, OGG

It's easy to see that the by far most common file format is MP3, for an overview of file formats see Audio File Formats for Podcasts. In addition to the MP3 version, some podcasts offer Ogg Vorbis (file extension OGG, strong in open source community) and AAC (file extension M4A, strong in apple community).

Some filenames are also very unintuitive and long. That's especially annoying on MP3 players with small displays, because it can be very hard to find a specific podcast episode.

Audio File Format Information

The next table lists more detailed information about the used audio codec, samplerate, bitrate, channels, size and length:
Podcast Audio Codec Samplerate Bitrate Channels Size Length
CRE 188 MP3 44100 96k CBR Joint Stereo 72.4M 1:40:15
techzing 162 MP3 44100 128k CBR Joint Stereo 108M 1:51:48
Hoaxilla 70 MP3 22050 64k CBR Joint Stereo 25.4M 52:36
Planet Money Podcast MP3 44100 64k CBR Mono 9.98M 20:47
Startups For the Rest of Us 61 MP3 44100 160k CBR Joint Stereo 43.58M 36:14
Alternativlos 21 MP3 44100 115k VBR Joint Stereo 82.2M 1:35:39
Alternativlos 21 Ogg Vorbis 44100 74.9k VBR Joint Stereo 53.7M 1:35:39
Bits und so 272 MP3 44100 64k CBR Mono 63.2M 2:11:40
Elementarfragen 9 MP3 44100 128k CBR Simple Stereo 119M 2:04:23
Elementarfragen 9 LC AAC 44100 128k CBR Joint Stereo 121.7M 2:04:23
Ubuntu UK Podcast MP3 high 44100 126k VBR Simple Stereo 63.5M 1:07:17
Ubuntu UK Podcast MP3 low 44100 32k CBR Mono 16.1M 1:07:17
Ubuntu UK Podcast Ogg Vorbis high 44100 59.5k VBR Joint Stereo 30.1M 1:07:17
Ubuntu UK Podcast Ogg Vorbis low 44100 36.2k VBR Mono 18.2M 1:07:17
Fanboys Podcast 49 MP3 44100 64k CBR Mono 32.8M 1:08:17
Fanboys Podcast 49 HE AAC 44100 56k CBR Joint Stereo 29M 1:08:17
This American Life 420 MP3 44100 64k CBR Mono 28.2M 58:40
ARD Radio Tatort MP3 44100 128k CBR Simple Stereo 51.8M 53:57
Deutschland Radio Wissen MP3 44100 64k CBR Mono 2.92M 06:06
Freakonomics MP3 44100 96k CBR Simple Stereo 22.1M 30:36
OE1 Geschichte Podcast MP3 44100 64k CBR Mono 2.50M 04:52
Wait Wait ... don't tell me MP3 44100 64k CBR Mono 5.33M 11:05
Radio New Zealand Afternoons MP3 22050 48k CBR Mono 18.7M 51:53
Radio New Zealand Afternoons Ogg Vorbis 48000 49.7k VBR Mono 19.3M 51:53
Inside Home Recording 97 MP3 44100 128k CBR Joint Stereo 59.7M 01:02:08
Inside Home Recording 97 LC AAC 44100 128k CBR Joint Stereo 62.4M 01:02:08
Radio Tux MP3 44100 128k CBR Joint Stereo 163M 02:49:26
Radio Tux Ogg Vorbis 44100 96.5k VBR Joint Stereo 123M 02:49:26

Suprisingly, nearly all MP3 and AAC podcasts use constant bitrate coding (CBR), Ogg Vorbis podcasts variable bitrate coding (VBR). The bitrates range from 32kbps up to 160kbps.
In CBR encoding, the bitrate is kept constant across the entire file, whereas VBR coding tries to maintain the quality during the whole stream by choosing the optimal amout of data to represent each audio frame. VBR will almost always produce equal or better perceived quality results than CBR for files of the same size or average bitrate (from hydrogen), however, some hardware based MP3 decoders don't reliably support VBR.

Furthermore there are two ways to represent stereo audio in lossy codecs: in simple stereo each channel is treated as a completely separate audio signal, whereas joint stereo supports more complex methods of stereo coding for the sake of efficiency.
Joint stereo is very widespread in MP3 podcasts and basically all Ogg Vorbis/AAC files use it.

Podcast Comparison Series

If you are interested in metadata and encodings of popular podcasts, read through the posts of the podcast comparison series:

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