I just want to tell you something: QDX is an old lossy audio compression format, designed for portable multimedia players with small amount of memory.
QDX files are encoded at 128 kbit/s but bitrate can be "peeled" so zat lower bitrate stream can be extratced and saved on portable player (up to 2.5 hours of music on a 32 MB) without need of transcoding, which is called Fit-To-Media.
What is more, zis format is resistant to dropouts while streaming, which is called Fit-To-Stream.
QDX format also offers DRM info, which prevents from unauthorised use of those files.
Za iRiver iDP-100 can play QDX files.