Tutorial FM

Tutorial: FM broadcast radio with stereo and RDS

What it is. FM broadcast radio (87.5 to 108 MHz in most of the world) is the simplest signal to start with and a good test of your antenna. An FM station carries stereo sound (a 19 kHz pilot tone and a 38 kHz difference signal) and RDS, a slow data stream with the station name, the radio text, the programme type, traffic flags and more. OnAir tunes a station, plays it, and shows all of that.

What you need

• Any radio that tunes 87.5 to 108 MHz. An RTL-SDR is plenty. See Supported radios. The mode asks for 2 Msps.
• A wire or a telescopic antenna. For weak stations, an FM-band antenna outdoors helps more than gain.
• A frequency. The frequency database gives the band edges: 87.5 MHz international (up to 108 MHz), 88.1 MHz as the first channel in the Americas (200 kHz spacing, to 107.9 MHz), 76.0 MHz in Japan (76 to 95 MHz) and the OIRT band from 65.8 MHz in Eastern Europe (mostly retired). The Scan tab lists the real stations near you; no station list is shipped.

Before you start

• A very strong local station can overload a cheap radio and ghost across the band. Reduce the gain if you see "ADC level low" or "AGC is lowering the gain" messages in the status area, or if stations appear in two places.
• Stereo and RDS both need a clean signal. Mono is robust at lower levels than stereo, and RDS needs a good signal-to-noise ratio too.

Step by step

1. In the mode list open Broadcast radio and choose FM (second line Stereo and RDS).
2. Open the Scan tab. Set from MHz and to MHz (default 87.0 to 108.0), tick full RDS if you want the scan to wait for the whole station name, radio text and programme identification of each station, and press Scan FM band. A full scan takes about two minutes. The result table lists MHz, station, type, SNR and sound (stereo or mono).
3. Press Start, then choose a station from the Stations list or type a frequency.
4. Open the Radio tab. The station, the programme type, the radio text and the stereo and RDS lamps appear as they are decoded.

What you should see

• A narrow peak 150 to 200 kHz wide in the spectrum, with the stereo pilot (19 kHz) and the RDS sub-carrier (57 kHz) visible as small bumps in the multiplex plot.
• In the Radio tab a table: Frequency, Programme (the station name, and the programme type), Call letters (US stations only), Sound (stereo or mono), RDS (shown as "N % blocks ok" once RDS has synchronised), Traffic, plus meters for Carrier, Deviation, Pilot, Offset and Level. A good RDS signal shows well over 90 % blocks ok.
• Radio text scrolls as the station sends it. Radio text can be music titles, news or advertising.

Reading the screen

• Carrier / Deviation / Pilot / Offset: signal strength, how wide the modulation is, whether the 19 kHz pilot is locked (stereo possible) and how far the station is from the dial.
• Traffic: shows "station (TP)" for a station that carries traffic programmes and ANNOUNCEMENT while a traffic announcement is running.
• RDS standard: Auto (RBDS on odd tenths), RDS (Europe) or RBDS (US). RDS and the US RBDS share the data but name the programme types differently, and RBDS stations send call letters. Auto uses RBDS on the US channel raster (87.9, 88.1 ... 107.9 MHz). Europe also has stations on those channels, so pick RDS (Europe) there.
• Traffic (TMC) list: RDS-TMC traffic messages for stations that carry TMC, with locations shown as codes because the country's location table is not included.
• Saved stations: press Save station to keep the frequency and the live RDS name in your own list; click one later to tune to it. The list is separate from the band scan.

Options and settings

• Stereo: Auto (stereo when the pilot is locked) or Mono. Mono gives cleaner sound on a weak station.
• De-emphasis: 50 us (Europe, Middle East) or 75 us (Americas, South Korea). If treble sounds dull or harsh, you have the wrong one.
• Volume, Mute / Unmute.
• The dial slider in the Radio tab tunes across the stations of your scan.

Troubleshooting

• Hiss and no RDS. The station is too weak. Improve the antenna; add gain only until the noise floor rises clearly.
• Stations repeat across the dial. Overload. Reduce the gain.
• Station name never completes. RDS needs time and good SNR; the name arrives in pieces. The "full RDS" scan option waits for it.
• Station names are garbled. Wrong RDS standard in the US/Europe overlap.
• Wrong tone. Wrong De-emphasis.
• Sound stutters. Reduce the sample rate or close other programs.

Tips

• An FM scan is an excellent way to check a new radio and antenna.
• The scan covers the band from 87.0 MHz so no station at the edge is missed.
• The same FM receiver is inside the Second mode feature: add FM next to another mode from the same capture if the sample rate allows.

Legal and privacy notes

Listening to broadcast radio is legal almost everywhere. RDS carries only what the broadcaster chooses to send.

Under the hood

Wideband FM (about 75 kHz peak deviation), stereo multiplex (ITU-R BS.450: 19 kHz pilot, 38 kHz suppressed-carrier L-R), and RDS (IEC 62106; US RBDS NRSC-4-B): a 57 kHz sub-carrier (the third harmonic of the pilot) with 1187.5 bit/s differential biphase data in 26-bit blocks with a 10-bit CRC and offset words. RDS-TMC follows ISO 14819.

Related

Mode page: FM | FM radio with RDS (short version) | DAB and DAB+ | General receiver | Supported radios | Troubleshooting