This is a series of blog posts on the all digital HD radio tests performed by iBiquity. It is also a good study on how to revitalize the AM broadcast band with technical suggestions and programming ideas.
Part I: Discusses the all digital HD radio test on WBCN. One test on a favorable subject does not success make
Part II: Discusses the history of AM broadcasting, regulations and technical developments/setbacks
Part III: Discusses some solutions to the above problems and notes that good programming and hard work is also necessary for success.
Part IV: An example of a successful, stand alone AM station in the Northeast.
All digital AM IBOC is more trouble that it is worth, it excludes small operators that cannot afford the upgrades or licensing, it creates interference and degrades audio quality.
There are better ideas including improved receiver technology, carrier frequency synchronization, wider daytime audio bandwidth and cutting away some dead wood. Other AM improvement posts: