DCT is a proprietary audio file format developed by NCH Software for their dictation software. As well as audio data, .dct files also contain dictation header information and can be encrypted. The format is aimed in particular at markets such as medical dictation, where confidentiality laws often require encryption.
DCT is a variable codec format, meaning it can contain audio data encoded with a number of codecs including MP3, CELP and GSM. It's apparently only coincidental that some of the codecs might use the compression technique known as DCT compression (Discrete cosine transform).
- NCH Software - File Formats. Note that while NCH Software list DCT as an "open" file format, the only software which appears to create or play it is written by them and there does not seem to be any published specification, which doesn't seem particularly open.
- NCH Software - Wave File Compression Codecs Compared