How Should I Compress My Files?
What is File Compression?
File compression refers to reducing the size of a recording file – usually by applying a compression algorithm, like the MP3 algorithm, to produce a smaller file that still sounds great. While it's similarly named to dynamic compression, they are not related.
Compression is important on your end, because your files need to be hosted somewhere for people to download (either with Cast or with another host), and also for your listeners because they need to download and store your files while they're listening to them. Larger files are going to take longer to download, use up more of your listener's data plan, and take up more local storage space.
In short: you want the smallest possible file that still sounds good.
All computer files can be compressed in two broad ways:
lossless compression: this reduces the file size in a way that lets you reconstruct the original file exactly; think of this as taking notes using a shorthand. Lossless compression doesn't provide huge reductions in filesize, and is as a result not commonly used for podcasts.
lossy compression: this reduces the file size by actually throwing out some data; think of this as editing down an essay to say the same thing in a more succinct way. Lossy compression results in a huge filesize savings, and, when applied well, the files still sounds fantastic.
Compression algorithms have all been designed to capitalize on the things that human beings are good and bad at hearing; they aim to alter your recording files in ways that sound nearly the same to the human ear, but result in big file-size savings. In particular, many algorithms have been further optimized to compress the human voice very well without altering its sound. (This is, obviously, what you want.)
If You're Mixing With Cast
Cast's mixer chooses compression values that we've found work well for podcasters, and result in podcasts that sound great to your listners, but don't waste the free space on their devices. If you're mixing with Cast, you're all done here. Take a break – you've earned it!
If You're Mixing On Your Own
If you're compressing your audio on your own, your first step might be to take a look at the compression used on some of the podcasts you listen to by opening the files in an audio tool.
We have a few suggestions, though:
- create mono files: most podcasts don't need to separate out the left and right audio channel; by creating a mono file, you'll cut down on the overall audio by half
- create constant-bitrate (CBR) MP3 files: CBR files play back more consistently across different apps and platforms. While variable-bitrate (VBR) often promises better compression, VBR playback is extremely fraught and often results in files that play right in one app and don't play right in a second app.
- choose a bitrate between 64 and 92kbps: while these bitrates seem low coming from a music background, they sound great for the human voice, and strike a good balance between quality and filesize.