Implementing my radio player took more experimentation and research than actual coding. As with most software, things turn out more complicated than first anticipated!
To tune in into an Internet radio station, normally you would click on a link on the station's website. From this link your browser will download a play list file, which in turn contains the URL that your favourite media player can use to access the station's live stream. My goal was to make this accessible in a more convenient manner, by allowing access to several radio stations in one place.
So I decided to create my own play list file that contained all the relevant URLs for my favourite stations. I wrote a PHP script that would generate this this play list by downloading each station's play list and extracting the relevant stream URL.
Play list file formats
There are three main formats used for these play list files, which are ASX (Windows Media Player), M3U and PLS (WinAmp).
Easiest to parse was M3U, this is just an ordinary text file that has the URL as a line of text. As PLS is an INI file, this seemed easy using PHP's built in functions to parse INI files. I quickly found that the PHP functions were unreliable so I wrote a kludge to get the URL myself, using a simple regular expression.
In a previous article I discussed problems with ASX files not being XML, so I avoided using this format unless the station did not also offer M3U or PLS. By sheer luck the few stations I am accessing via ASX are well formed XML. Unfortunately the structure of these ASX files vary, so it took a few iterations of the XSLT code to extract the stream URL realiably.
The file format I ultimately create my play list is is XSPF, as this is a well defined format based upon XML. From the master XSPF play list, I can readily generate ASX, M3U and PLS by using XSLT. Having the playlists available for means good support for tuning in with various different media players. I would personally recommend using the XSPF play list in VLC Media Player.
Streaming from a web page
I felt the stations should ideally be available directly a browser, without requiring an external application to be installed. After trying a few different options I opted to use the "XSPF Web Music Player" written in Flash. Getting Flash working is an exercise in itself made easy using the UFO JavaScript library. This setup offers good compatibility with typical browser setups.
Fixing the stream URL
The biggest problem with this was presented by a strange behaviour of Shoutcast servers. When you access the stream URL for a Shoutcast server using a web browser, you are served up a web page about the stream. This HTML content is provided instead of the MP3 stream, on the basis of the User-Agent header of the HTTP request.
Generally browser plugins have their HTTP requests made for them by the browser, so HTML content is delivered to a Flash movie or a Quicktime regardless. The exception to this is the Windows Media Player plugin, which bypasses the browser completely; disregarding proxy settings etc.
Shoutcast servers do provide a special URL that allows you to ensure the MP3 stream is returned regardless of the User-Agent. You just append ;stream.nsv for example http://example.com:1234/ becomes http://example.com:1234/;stream.nsv.
Appending this string confuses any servers that aren't using Shoutcast though. So it became necessary to probe each server, to adjust the URL where required. This works by impersonating Firefox and when HTML comes back, that's Shoutcast.
Memory usage
One significant problem appears to be that of memory consumption while listening to a stream. If you're listening for a few hours or so, expect your memory usage to be in the 100s of megabytes. This is enough to cause problems in a typical PC setup. A workaround for this is to stop and restart the stream occasionally.
The reason behind this is that the MP3 stream is treated as if a very long MP3 file is being downloaded. What you've heard so far remains in memory even though, with the exception of Real Player, you can't seek backwards. This is a well known problem that affects nearly all media players and browser/plugin combinations except VLC Media Player.
Recommended use
If you use the M3U or PLS play lists, you may well just see IP addresses and port numbers rather than meaningful station names. This is a limitation of the file formats used, and isn't fixable. Using the XSPF play list is recommended as it has the station name and website link. Also you can use the XSPF play list in VLC Media Player, without having to worry about your memory usage.