 An upgraded version of the system was released in February 2007, called DAB+, which uses the modified discrete cosine transform (MDCT) based HE-AAC v2 (AAC+) audio codec.DAB is not forward compatible with DAB+, which means that DAB-only receivers are not able to receive DAB+ broadcasts.However, broadcasters can mix DAB and DAB+ programs inside the same transmission and so make a progressive transition to DAB+.