An Example of URL Reverse Engineering
Slowly going up Rrose’s back catalog since her last appearance in Munich, I end up on her 2012 set for the Resident Advisor Podcast.
The version on Mixcloud is a shy 69k mp4, while SoundCloud only streams the dreaded 128k mp3.
I know Resident Advisor brodcasts 320k versions on their feed. I look for the <enclosure .../> tag of episode 318:
~$ curl "https://ra.co/xml/podcast.xml" | xq | less -R
Unfortunately, I notice that the feed only keeps the last 400 episodes, and definitely does not include what I am looking for. I also notice that they use chrt.fm to resolve mp3 files. For example, last episode’s file is referenced via:
<enclosure type="audio/mpeg" url="https://chrt.fm/track/C54868/http://feeds.soundcloud.com/stream/1455221263-resident-advisor-ra873-solid-blake.mp3" length="171982635"/>
The URL structure is consistent across entries:
https://chrt.fm/track/C54868/http://feeds.soundcloud.com/stream/<id>-resident-advisor-ra<number>-<artist>.mp3
I seem to have all the information except the SoundCloud track id. One could dig in the page source and find the api uri. I use yt-dlp1 instead:
~$ yt-dlp -F "https://soundcloud.com/resident-advisor/ra318-rrose"
[soundcloud] Extracting URL: https://soundcloud.com/resident-advisor/ra318-rrose
[soundcloud] resident-advisor/ra318-rrose: Downloading info JSON
[soundcloud] 236925033: Downloading JSON metadata
The id is 236925033. We’ll need to follow a couple of redirects and save the output to a file with curl:
~$ curl -L "https://chrt.fm/track/C54868/http://feeds.soundcloud.com/stream/236925033-resident-advisor-ra318-rrose.mp3" --output RA.318-Rrose.mp3
There you are:
exiftool RA.318-Rrose.mp3
ExifTool Version Number : 12.50
File Name : RA.318-Rrose.mp3
Directory : .
File Size : 138 MB
File Modification Date/Time : 2023:03:06 19:33:31+01:00
File Access Date/Time : 2023:03:06 19:33:09+01:00
File Inode Change Date/Time : 2023:03:06 19:33:31+01:00
File Permissions : -rw-r--r--
File Type : MP3
File Type Extension : mp3
MIME Type : audio/mpeg
MPEG Audio Version : 1
Audio Layer : 3
Audio Bitrate : 320 kbps
Sample Rate : 44100
Channel Mode : Joint Stereo
MS Stereo : Off
Intensity Stereo : Off
Copyright Flag : False
Original Media : False
Emphasis : None
ID3 Size : 387427
Encoder Settings : Logic Pro 9.1.7
Date : 2607
Time : 1825
Year : 2012
Title : RA.318 Rrose - 2012.07.02 [320k]
Album : Resident Advisor podcast
Artist : Rrose
Initial Key : C
User Defined URL : (URL) http://rrose.ro http://eaux.ro
Comment : Find out more about RA.318 with Rrose on Resident Advisor: www.residentadvisor.net/podcast-episode.aspx?id=318
Picture Format : JPG
Picture Type : Other
Picture Description :
Picture : (Binary data 376602 bytes, use -b option to extract)
Genre : Techno
Date/Time Original : 2012:26:07 18:25
Duration : 0:57:19 (approx)