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.