Joint Stereo (mode 1) - To create the highest quality MP3 files, Xing recommends always choosing Joint Stereo (mode 1) over Stereo. Don't confuse Joint Stereo (mode 1) with the Joint Stereo coding used for MPEG layer 2 encoding - it is not the same.
Joint Stereo (mode 1) encoding for MPEG layer 3 allows the XingMP3 Encoder to use additional methods of encoding, specifically - MS Stereo (Middle/Side Stereo), and for lower bitrates only, Intensity Stereo, in addition to the Independent Channel coding used for Stereo. MS Stereo uses one channel to encode information that is identical on the left and right channels and the other channel to encode the differences between the two channels. Intensity Stereo encodes only bits that are perceived to be important to the stereophonic image. The XingMP3 Encoder uses Intensity Stereo only in low bitrate files (96kbps or less), where file size is critical to the user.
In Joint Stereo (mode 1), the encoder dynamically (frame by frame) chooses the method of encoding that produces the best quality for each individual frame. Dynamic encoding improves compression efficiency which results in a higher quality file using less bits.
Stereo (mode 0) encodes the left and right channels independently. The total bitrate remains constant, but the split between the channels can vary. The XingMP3 Encoder uses this flexibility to improve quality by allocating more bits to the channel with the more dynamic signal. For MPEG layer 3 encoding, Stereo (mode 0) limits the encoder to only one method of encoding - Independent Channels. Because Stereo (mode 0) is limited to one method of encoding, Joint Stereo (mode 1) in most cases produces higher quality. In the exceptions, the Stereo (mode 0) quality will be essentially equivalent to Joint Stereo (mode 1).
Dual Stereo includes two completely independent encoded channels (left/right), each with half the total bitrate. In effect, it is two Mono files packed into a single file. Dual Stereo is generally only used for multi-lingual audio programs.