Also known as: HD Radio, IBOC, in-band on-channel, NRSC-5
Overview
HD Radio is the trademark name for the North American in-band on-channel (IBOC) digital radio system, standardised as NRSC-5 and licensed by Xperi (formerly iBiquity).1 Rather than moving digital radio to a new band as DAB does, IBOC squeezes OFDM digital sidebands into the spectrum immediately around an existing analog FM or AM station, so one transmitter radiates both the legacy analog signal and a digital version on the same assigned channel. Receivers blend seamlessly from analog to digital as the digital signal locks, and can offer additional program streams (HD2, HD3) tucked into the same channel.
Technical characteristics
| Property | Value |
|---|---|
| Arrangement | In-band on-channel; hybrid (analog + digital) or all-digital |
| Waveform | OFDM sidebands, QPSK-mapped subcarriers |
| FM sidebands | Digital energy outside the analog MPX, within the 200 kHz mask |
| AM mode | Narrow OFDM sidebands within the ~20 kHz AM channel |
| Audio codec | HDC, a proprietary HE-AAC derivative |
| Extra services | Secondary programs (HD2/HD3), text, and album art |
| Blend | Automatic analog↔digital transition on the FM host |
History
iBiquity Digital developed IBOC from earlier competing proposals, and the FCC authorised hybrid daytime operation in 2002; the NRSC formalised it as the NRSC-5 standard. All-digital AM (MA3) and denser FM configurations were added over the following years. iBiquity was acquired by DTS and then Xperi, which continues to license the HD Radio brand and the proprietary HDC codec that keeps the system closed despite the public NRSC-5 layer descriptions.
Deployment
HD Radio is deployed primarily in the United States and, to a lesser degree, Mexico and Canada, where a large fraction of FM stations and many AM stations run hybrid IBOC. It is the North American answer to digital radio, chosen over DAB precisely because it needs no new spectrum and preserves the analog service. AM HD Radio has seen limited uptake because its digital sidebands can raise adjacent-channel interference on the crowded medium-wave band.
Decoding it with GopherTrunk
GopherTrunk does not decode HD Radio; it is a trunked land-mobile scanner (P25, DMR, NXDN, TETRA and similar) and IBOC is a broadcast system outside its scope. The proprietary HDC codec also means even general-purpose SDR tools can demodulate the OFDM layer (e.g. the open-source nrsc5 project) but rely on reverse-engineered audio decoding. For GT readers, HD Radio is a useful example of layering an OFDM digital signal alongside an existing analog carrier without a new channel assignment.