Tutorial HD Radio

Tutorial: HD Radio (NRSC-5)

What it is. HD Radio (in-band on-channel digital radio, IBOC, standard NRSC-5) adds digital sidebands to FM and AM stations, mainly in the United States. The analogue programme stays on the air; next to it the station sends up to eight digital programmes (HD1 to HD8), programme data (title, artist, album), station logos and pictures. In OnAir you tune the station, the receiver locks the sidebands and shows the programmes and data.

Sound needs an add-on that you install yourself. HD Radio audio is coded with HDC, a patented codec owned by Xperi, so OnAir does not ship it. Until you provide the HDC library (a patched FAAD2, libfaad_hdc, that you build yourself: see docs/CODECS.md and Voice codecs) the digital audio is silent. Everything else works without it: the station and programme list, the title/artist/album text, the station information, the pictures and the signal measurements. The ? button next to the sound shows whether the library was found, the exact file names OnAir looks for, and has Open codecs folder and Look again (no restart needed). The analogue FM or AM programme of the station can be heard in the FM or General receiver modes.

What you need

• For FM: any radio that tunes 88 to 108 MHz with at least 2 Msps (the mode asks for 2 Msps). The channel is about 400 kHz wide because the digital sidebands lie either side of the analogue carrier.
• For AM (medium wave, 530 to 1710 kHz): a radio that reaches medium wave (an HF radio, an RTL-SDR in direct sampling mode, or an upconverter). 1 Msps is plenty.
• Antenna: a wire or telescopic antenna for FM, a loop or long wire for AM.
• A frequency. The default is 98.5 MHz (the mode's start frequency, in the frequency database as the HD Radio default). HD Radio stations are on the ordinary FM channels (88.1 to 107.9 MHz, 200 kHz raster). The Scan tab can help find them. There is no station list in the shipped data.

Before you start

• A strong analogue carrier makes the sidebands harder to see on a weak receiver; avoid overload.
• The digital sidebands are much lower in power than the analogue carrier. They need a clean signal: a local, strong station is best.
• AM hybrid (MA1) and all-digital (MA3) stations are on the air mostly in the Americas.

Step by step

1. In the mode list open Broadcast radio and choose HD Radio (second line USA, FM and AM).
2. In the tuner choose Band FM or AM. For AM choose the raster: 10 kHz raster (the Americas) or 9 kHz raster (elsewhere). Use the arrows to step to the next channel (200 kHz in FM).
3. Press Start.
4. Open the Radio tab. It has three views: Station, Data and Signal.
5. In Station: pick a programme (HD1, HD2, ...) in the Programs list. The Now playing section shows title, artist, album, genre and comment. With the HDC library installed, sound plays ("sound: HD1 (HDC)"); without it the line says that HD Radio sound needs the HDC codec, which you install yourself.

What you should see

• A central analogue carrier (FM) or a narrow AM carrier with digital sidebands left and right, at lower level.
• The state moves to locked; the MER of the sidebands appears (lower and upper for FM).
• Programme cards fill in after a few seconds once the station information arrives.

Reading the screen

• Station: Programs (HD1 to HD8 with name and type), Now playing (title, artist, album, genre, comment), the station logo and alerts.
• Data: Programs again, Files received (LOT) (pictures such as album art and station logos, with a preview), and Data services (a table with Service, Belongs to, Port, Transport, Content, Received). Pictures are shown; traffic, weather and other services are listed with the amount received but their content is not decoded.
• Signal: Receiver (state, waveform, service mode, input rate and level, receiver load), Measurements (MER lower and upper sideband, channel bit error rate, carrier offset, block count), Counters (P1 and P3 frames, PIDS frames, audio PDUs, AAS packets) and Audio (HDC decoded, errors, lost frames).

Options and settings

• Band FM / AM, the raster, and the channel arrows.
• The test signal can synthesise a station for trying the screens without a radio.

Troubleshooting

• No lock. The channel may have no HD Radio. Not every FM station is hybrid. Try a strong local station; check the sample rate.
• Locks, silent. The HDC library is not installed or was refused. Click ? for the exact reason and steps.
• Text but no sound, or sound drops out. Marginal MER. Improve the signal; the Audio counters show decoded versus lost.
• AM does not tune. The radio does not reach medium wave. Use direct sampling or an upconverter.

Tips

• Checked with generated signals and impairments; recordings of real signals are welcome (see Signal samples).
• Build the HDC library yourself following docs/CODECS.md and put it in the codecs folder; the ? popup lists the exact folders and file names for your system.

Legal and privacy notes

Receiving broadcast radio is legal in most countries, but HDC is patented and its use is your own responsibility; check local patent and law. OnAir ships no HDC code.

Under the hood

NRSC-5 (iBiquity/Xperi): OFDM digital sidebands either side of the analogue FM carrier (hybrid) or in an all-digital arrangement; for AM, MA1 (hybrid) and MA3 (all-digital). Layer 1 has the physical channels P1, P3 and others with convolutional coding and interleaving; Layer 2 carries the programmes and Advanced Application Services (AAS). Audio is HDC, a codec related to AAC. The receiver resamples to 744187.5 Hz.

Related

Mode page: HD Radio | Voice codecs | FM | General receiver | Supported radios