A MP3/RTP input plugin for Winamp

(Built from "LIVE555 Streaming Media")

AOL/Nullsoft's "Winamp" audio player (for Windows) can receive and play multicast (RTP) MP3 audio sessions, using a special input plugin: in_rtp.dll. This plugin allows Winamp to receive RTP-based MP3 streams directly, without the need for a separate multicast receiver 'helper application' (such as "playRTPMPEG").

Important Note: This plugin works for Winamp version 2 or 5. It will not work for the ill-fated "Winamp3".

Downloading and installing the "in_rtp.dll" plugin

Download the binary file in_rtp.dll, and copy it to your Winamp "Plugins" folder - usually "C:\Program Files\Winamp\Plugins".

Receiving MP3/RTP streams using the plugin

With the "in_rtp.dll" plugin installed, you can receive a MP3/RTP stream from Winamp by giving it the "Open Location" command (<control>-l ('ell')), and entering the 'pseudo-URL' rtp://<multicast-address>:<port> e.g.,

rtp://224.44.45.46:8888

You can also use this pseudo-URL if you start Winamp from a command shell, e.g.,

winamp rtp://224.44.45.46:8888

Notes

Please run Winamp version 2 (2.6 or later) or 5. The plugin will not work with "Winamp3".
Winamp sometimes gets confused (and appears to hang) if the input stream's MPEG parameters (e.g., its sampling frequency) changes while it's running. If this happens you can often recover by clicking Winamp's "play" button once again.

Additional options

To receive the more loss-tolerant 'ADU-based' RTP payload format "mpa-robust" (as defined in RFC 3119), add a 'directory path' of "/format=mpa-robust:<RTP-payload-format-number>" to the URL, e.g.,
```
winamp rtp://224.44.45.46:8888/format=mpa-robust:96
```
<RTP-payload-format-number> must be a dynamic RTP payload format number - i.e., in the range [96,127].
To receive a source-specific multicast session, append the desired source address to the multicast address, using a comma as separator, e.g.,
```
winamp rtp://232.33.34.35,10.11.12.13:8888
```
By default, the plugin sends multicast RTCP reports with a TTL of 255. To use a different TTL, add a 'directory path' of "/ttl=<RTCP-TTL>" to the URL, e.g.,
```
winamp rtp://224.44.45.46:8888/ttl=15
```
To output the plugin's version number (to 'stderr'), add a 'directory path' of "/version to the URL. (Note: The version number will be output as an error message, and Winamp will exit.)
This plugin can also receive special 'staggered, erasure-protected' MP3 streams - as pioneered by Multicast Technologies, Inc. Such a stream consist of several different multicast subsessions (i.e., multicast address,group pairs) each carrying the same audio, but encoded at different bitrates, and staggered over time. The highest-priority subsession is encoded at the highest bitrate; lower-priority subsessions are encoded at progressively lower bitrates. The goal of this scheme is to protect against packet loss - especially packet loss that happens in bursts. Whenever a high-priority audio packet is lost, then this lost audio can be recovered - a short time later - by waiting to receive the same audio encoded in a lower-priority subsession.
To specify such a stream in a 'rtp://' pseudo-URL, use the "|" (i.e., vertical bar) character to separate the different subsessions. (These subsessions should also be listed in priority order - highest-priority first.)
For example, the following pseudo-URL denotes a particular two-level, staggered, erasure-protected stream:
```
rtp://233.64.133.30:7000|233.64.133.31:7002/stagger=5
```
(The stagger parameter gives the total time range (in seconds) between the highest and lowest-priority subsessions. The default value is 5 (seconds).)

Reading ".sdp" files

With this plugin, Winamp can also read a SDP (Session Description Protocol) file that describes a MP3 session (as an alternative to entering the session's parameters in a "rtp://" 'pseudo-URL'). The name of this file must end in ".sdp". A ".sdp" file can either be a local file (which you can simply 'drop' on Winamp), or a file on a remote HTTP server (which Winamp can open using a "http://" URL).

Opening RTSP URLs

This plugin also has a built-in RTSP client. This allows you to open URLs that begin:

rtsp://

if this URL refers to a session that contains a MP3 stream.

If Winamp exits unexpectedly

If this plugin encounters an error (such as being given a malformed URL), it outputs an error message to 'stderr' and exits.

You can see this error message by running Winamp from the Windows 'Command Prompt' (shell), as follows:

winamp 2> err.txt

After Winamp exits, the file "err.txt" (in this case) should contain an error message.

About the plugin's implementation

The plugin was implemented using the (open source) "LIVE555 Streaming Media" source-code libraries, with high-level scripting in Tcl.

LIVE555.COM