How to Compress 45 Minutes of Audio... on a Floppy Disk!

Codecs have forever changed the way we distribute audio. Gigantic files shrink to far more reasonable sizes without sacrificing too much quality, and lossless technologies allow us to be more efficient in preservation and storage. However, today we have the privilege of high-capacity drives. Imagine what it would be like to store an entire podcast on a single floppy disk. Sean Haas from the Advent of Computing portal decided to accept that mission, and this is the result…

1.44 megabytes. Fresh out of the box, a 3.5-inch floppy disk barely crosses the two-million-byte mark, but once you give it a format, 30 percent of its capacity is left behind. An unpleasant but necessary rule we lived with for many years. A floppy with bad sectors was a headache, each one was worth a fortune, and we did everything possible to bring it back from the dead. That same obsession also extended to data storage. A floppy had to be used to its fullest, and many programs applied a special format to gain extra space.

Despite those efforts, the floppy disk became obsolete, and there are some things that simply cannot be stored on them... or can they? It's all a matter of trade-offs, including quality parameters. For example: a 45-minute podcast. In MP3 format, a podcast of that duration would exceed 72 megabytes... that's 50 floppies. However, Sean Haas of Advent of Computing took it as a challenge. What does it take to store a podcast of almost an hour on a floppy disk?

The Solution: AMR

The key behind all this hides in something we know very well: mobile communications. Sean explains that if you’ve ever made a call with a mobile phone, then you’ve already “listened” to the AMR codec, Adaptive Multi-Rate audio codec. With a fixed frequency of 8 kHz and a maximum bitrate of 12.2 kilobits per second, AMR rules the GSM world, and it’s the reason our calls often sound terrible. Its main advantage is that it has been specially optimized for voice, and a podcast fits perfectly.

Sean took his podcast and converted it to AMR, but even with very aggressive bitrates, it was still too large. The solution was to process the MP3 with FFmpeg to speed up its playback by 20 percent (command 'atempo=1.2'), and then apply AMR. Although FFmpeg doesn't encode AMR, the program SoX has the necessary support. A couple of additional commands, and the 72 megabytes in MP3 shrank to 1.2 megabytes under AMR. Now, the million-dollar question: How does it sound?

How Does It Sound?

Sean discovered several problems when turning this project into a full-fledged podcast. A user can load whatever they want into an RSS feed, but it must receive validation on the other side. Apple Podcasts rejected the file immediately. In contrast, Spotify not only accepted the feed but also enables playback in a browser, an interesting detail because there is no official AMR support in the main browsers on the market. Want to give it a try? The links appear below.

Official site: Click here

Download SoX: Click here