Audio files compressed using the MP3Pro compressor, play back in a lower quality from an iPod.
Solution
Neither the iPod or iTunes works with the MP3Pro format, so audio is played back at the encoded resolution, which is compatible with standard MP3 devices.
MP3Pro Background
The MP3Pro compression format allows audio files to be encoded at a lower resolution, making the .mp3 file smaller in size. However, when played back on a device, or in an application that works with MP3Pro, the sound quality is perceived to be similar to the same file encoded at a higher resolution.
For example, an MP3 file encoded at 128 kilobits per second (kbit/s) using a standard MP3 compressor, such as iTunes, might be 1.6 MB in size. However, the same file encoded at 64 kbit/s using a standard MP3 compressor might only be 820 KB. The MP3Pro format allows a file to be compressed at 64 kbit/s, yet sound like it was encoded at 128 kbit/s when played back on a MP3Pro device or application. When the same file is played back without using MP3Pro, the file plays back in a "non-Pro" format at 64 kbit/s. This means the file sounds like a standard MP3 file encoded at 64 kbit/s file without any enhancements.