In many parts of the world, radio is slowly transitioning to a digital transmission platform—but so far, this new frontier has not been plumbed by pirates. Part of this is due to the relatively immature state of radio’s digital transition, but some of the systems have been around long enough that they’re ripe for experimentation.
In very simple terms, the primary thing to keep in mind is that the heart of a digital radio transmission system is the software that controls the transmitter. The more freely-available the software, the more possible to play with. In global contention, there are three contending platforms of note, though their DIY-potential varies:
Digital Radio Mondiale. The newest of the digital radio transmission systems, it works on the AM, FM and Shortwave bands. It wholly displaces the analog signal on the channel, which requires listeners to have a DRM-compatible receiver. That said, the system is open-source, which means its code is publicly available to tinker with. SourceForge has a wiki with software to both encode (transmit) and decode (receive) DRM signals; this project has been under development for ten years now.
Eureka 147 DAB. The oldest of the digital radio transmission systems, it functions on "new" (i.e., non-traditional broadcast) spectrum, thus requiring unique transmitters and receivers. DAB signals propagate in a manner very similar to FM signals. Some of the key patents expired on the DAB system just last year, which means it, too, is now in the realm of the homebrew. Last year, British engineer Rashid Mustapha did just that, broadcasting experimentally for four months atop a tower block in Brighton.
In his report to Ofcom, the UK’s broadcast regulator, Mustapha says available open-source software components to encode, multiplex (transmit) and decode DAB signals work well. More importantly, he highlighted the availability of spectrum for such small-scale operations: "it is likely that frequency blocks could be identified in many areas for transmitters radiating power levels in the order of 100 watts or so. In some locations it may be possible to re-use existing DAB frequencies, in coastal regions it may be possible to re-use the blocks of neighbouring countries and in others [sic] areas there could be [additional] options. . . ."
HD Radio. Designed for the AM and FM bands alone, the system involves adding digital sidebands adjacent to existing analog signals. Because of the need to coexist with analog transmissions, the digital signal is broadcast at a relatively weak power which means reception suffers. In addition, broadcasting both signals effectively fattens each radio channel, which raises the potential for interference between neighboring stations. Some pirates have enough problems keeping their analog signals clean—doubling or tripling a station’s footprint on the dial is just inviting attention.
Although the software runs on the Linux operating system, HD’s proprietor, iBiquity Digital Corporation, has smothered it in patents and other trade-secret chicanery that you must pay to play with it. David Maxson, who wrote the definitive engineering textbook on the HD Radio system, explains it this way: "In theory, a manufacturer could develop a product that is [HD] compliant but that has not earned the right to use the HD Radio name and logo. In practice, the manufacturer would find it necessary to license iBiquity patents to manufacture and sell its . . . product. While doing so, the manufacturer may be enticed . . . to take the next step and join the HD Radio family." Not many manufacturers have entered these waters, especially on the receiver-side.
If somebody were to decompile or reverse-engineer the HD system, or perhaps acquire an illicit copy of the transmission software from a licensed station (if that is even possible), it’s a good bet that iBiquity would be all over them like a fat kid on cake. Cracking the "black box" that is the HD system would break iBiquity’s business model, and in the HD universe that’s unfortunately the paramount concern.
On the television side, my knowledge is much more scant. Outside of one unconfirmed report of a pirate digital television broadcast in California some years ago, there’s been no real activity in this realm in the United States. Much of the rest of the world has adopted a different digital transmission standard, however, and there’s a relatively sophisticated level of homebrewing: here’s a guide for building a DTV transmission system capable of broadcasting up to four channel-streams. A long-time designer of quality low-power FM transmitters has also branched out into the DTV realm, but it’ll cost you.