We start on the air, with the PlutoSDR and the lesson’s finder flowgraph:

1. Open L14-hd-find.grc. Select the File Source and the six blocks after it, through Float To Complex, and press D; then select the PlutoSDR Source and press E.

2. Run it and type a strong local station’s frequency into Center; the station list at hdradio.com shows which local stations carry HD Radio.

3. Watch the waterfall. The station is a rounded band whose width changes with the audio, and a station with HD Radio also shows two flat bands of constant width on either side of it, which an ordinary FM radio ignores.

4. Drag a box across one of those bands on the spectrum to zoom in on it, and right-click to zoom back out.

Those bands are HD Radio, a digital broadcast that carries the station’s program, and often extra programs, on the same channel. The course Plutos run in extended-frequency mode, 70 MHz to 6 GHz, so the FM band is in range.

The figure is the finder playing the lesson’s test file rather than an antenna, so it carries a “Simulated” label: the station at 100.3 MHz has the two bands, and the one at 99.9 MHz has none. The rest of the lesson builds that test signal in GNU Radio, explains the finder, and measures the bands, and nrsc5 decodes a recording of a real station.

HD Radio carries a station’s program digitally alongside, or in place of, the analog signal. A hybrid station, the layout in the figure, keeps the analog FM signal Lesson 7 received in the center, about 130 kHz wide on each side, where every FM radio still plays it, and puts the digital signal in two sidebands, one on each side, from 129.4 to 198.4 kHz from the channel center. An extended hybrid station limits its analog signal to about 100 kHz each side and adds digital carriers on the inside edge of each sideband, and an all-digital station drops the analog signal entirely. The standard that defines all three is NRSC-5.

The digital signal carries the main program, called HD1, and a station can add HD2, HD3, and more programs that have no analog version.

Each part of an HD Radio station’s spectrum uses a different modulation. The host is analog FM, and Lesson 7’s demodulator turned it into the multiplex: mono audio up to 15 kHz, the 19 kHz stereo pilot, the stereo difference signal as DSB-SC around 38 kHz, and RBDS, the US name for RDS, as differentially encoded BPSK at 1187.5 bits per second on 57 kHz, carrying the station name, the time, and text. To see it on the same station, open Lesson 7’s L07-rds-rx.grc, run it on its recording, and the panel fills with the station’s name; Lesson 7 shows how to switch it to the radio for a live station. PySDR’s RDS chapter, which the slides list as an activity, decodes the same signal step by step in Python.

The digital sidebands sit outside the host, and each one is OFDM, a block of many narrow carriers, each carrying QPSK. The next frames describe that block, and the rest of the lesson finds it, measures it, and decodes it.

Each sideband is not one carrier but 191 narrow ones, called subcarriers, spaced 363.4 Hz apart. NRSC-5 numbers them from the channel center, so subcarrier \(k\) sits at \(k \times 363.4\) Hz; the upper sideband runs from subcarrier 356, at 129.4 kHz, to 546, at 198.4 kHz, and the lower sideband is its mirror image. We use those numbers as block settings: the sideband meter later in the lesson takes its band from Variables named inner and outer.

The subcarriers come in frequency partitions of 19: 18 carry data, and one reference subcarrier, at 356, 375, 394, and every 19th after that up to 546, carries known symbols that the receiver uses to find the timing, frequency, and phase. An extended hybrid station adds partitions inside subcarrier 356. The sidebands are much weaker than the host, so that they do not disturb analog radios or the neighboring stations; the meters in this lesson read each sideband’s power relative to the host, in dBc.

Orthogonal frequency-division multiplexing (OFDM) divides a wide channel among many slow subcarriers instead of one fast carrier. The subcarriers are spaced so closely that their spectra overlap, but each one passes through zero at the center of every other one. That property is orthogonality, the O in OFDM, and it lets the receiver separate them with an FFT. The explorer two frames ahead shows it.

Each subcarrier carries one constellation point per OFDM symbol, so a symbol is a list of I/Q points, one per subcarrier: QPSK points on HD Radio’s data subcarriers. GNU Radio’s OFDM Carrier Allocator places the points on the subcarriers we list, a reverse FFT turns the whole list into one symbol of time samples, and the OFDM Cyclic Prefixer copies the end of each symbol in front of it as a guard interval, which separates each symbol from the one before it. These are the blocks of the GNU Radio wiki’s Basic OFDM Tutorial, which the slides point to.

For HD Radio at 1,488,375 samples per second, fft_len = 4096 makes each FFT bin exactly one 363.4 Hz subcarrier, and cp_len = 224 samples is NRSC-5’s guard. Symbols follow one another over time, each 4096 + 224 samples long. The receiver runs the same steps backward: drop the guard, take a forward FFT, and read each subcarrier’s point.

L14-hd-test-tx.grc recreates a hybrid station with the OFDM blocks of the previous frames. For each sideband, random QPSK points fill subcarriers 356 to 546 in the OFDM Carrier Allocator, a reverse FFT of 4096 points turns them into one symbol, and the OFDM Cyclic Prefixer adds the 224-sample guard. A Multiply Const sets each sideband’s level, here \(-23\) dBc. The FM host is filtered noise and a 19 kHz pilot, frequency modulated with the broadcast 75 kHz deviation setting, and an analog-only station sits 400 kHz below.

The sidebands carry random symbols, not NRSC-5 data, so nrsc5 cannot decode this file; it is a test signal for finding and measuring the sidebands. It writes L14-hd-test.cu8, unsigned 8-bit I and Q at 1,488,375 samples per second, nrsc5’s default input format. Nothing in it transmits: its last block is a File Sink.

L14-hd-find.grc plays the test file, any unsigned 8-bit recording made at 1,488,375 samples per second, or the PlutoSDR live. Such a file stores each sample as two unsigned bytes from 0 to 255, so the first five blocks subtract 127.5, scale by 1/128, and pair the bytes into complex samples. The PlutoSDR Source delivers complex samples directly, so it skips those blocks. To switch to the radio, select the File Source and the six blocks after it, through Float To Complex, press D to disable all seven, and enable the PlutoSDR Source; GRC will not run with both paths enabled.

The QT GUI Frequency Sink and Waterfall Sink use an FFT of 4096 points, so each bin is one subcarrier wide. The three meters measure the power in a band, which the next frames use.

On the frequency sink, the shelves begin 129.4 kHz from the station, at 100.1706 and 100.4294 MHz, and end at 100.1016 and 100.4984 MHz. Their tops are flat because every subcarrier carries the same power, and their edges are sharp because the subcarriers stop at 356 and 546. The FM host is rounded, and on the waterfall its width changes with the audio, while the shelves stay the same width all the time. That difference is how we recognize HD Radio on a live band: look for two straight, flat bands about 130 to 200 kHz either side of a station.

The shelves sit about 25 dB under the top of the host on the display, but the meters, not the display heights, give the power ratio between the bands.

A band-power meter is four blocks we have used before. A Frequency Xlating FFT Filter moves the band’s center to 0 Hz and keeps only the band, \(\pm 100\) kHz for the host and \(\pm 34.7\) kHz around \(\pm 163.9\) kHz for each sideband, and decimates by 6. Complex to Mag^2 gives each sample’s power, a Moving Average over 0.2 s averages it, Keep 1 in N slows the result to 50 readings a second, and Log10 with a factor of 10 converts each reading to dB. Subtracting the host’s meter from a sideband’s meter gives that sideband in dBc, and a third Subtract shows the upper sideband minus the lower.

On the test file the meters read \(-23.04\) and \(-23.06\) dBc, the level the test transmitter sets, and the sides differ by 0.02 dB. The same flowgraph on the real station recorded in the nrsc5 project’s sample file reads about \(-24.3\) and \(-23.8\) dBc, close to the test file. On a live station the two meters need not agree, and the lab compares them. The Inner subcarrier slider steps the meters’ inner edge down one partition at a time, from 356 to 280, so the same meters work on an extended hybrid station: slide it down until the readings stop rising.

To watch noise at work, open L14-hd-test-tx.grc, change the Variable noise_amp from 0.03 to 0.15, run it to write a new test file, and run the finder on it. The figure is that run. The floor rises by about 14 dB, the host still stands far above it, and the shelves, which are much weaker, now stand only a few dB above the floor. The meters read about 0.8 dB higher than before, because each one measures all the power in its band and the noise in the band adds to the sideband’s. On a live station the same thing happens as the signal weakens, so a meter reading is trustworthy only while the shelves stand well above the floor.

Finding and measuring the sidebands needs only the spectrum. Decoding them needs the rest of NRSC-5: the receiver locks onto the reference subcarriers, reads each subcarrier’s QPSK point, corrects errors, and decompresses the audio. GNU Radio has no HD Radio receiver module, so the open-source nrsc5 project is the decoder, and this step is optional. It plays the chosen program and prints the station’s name, the song title and artist, and measures of signal quality as they arrive.

nrsc5 has no PlutoSDR driver, but it reads recordings, and its README lists complex float at 744,188 samples per second, half our rate, as one input format. The finder writes exactly that: enable its Rational Resampler, which halves the rate, and its SigMF Sink along with the PlutoSDR Source, run about 10 s on the station, and stop. GRC draws the SigMF Sink with an orange border, its mark for a block it lists as deprecated; it still runs in GNU Radio 3.10. It writes hd.sigmf-data, about 60 MB, and hd.sigmf-meta, which records the rate and the center frequency at the start. Then run the commands above.

nrsc5 is a C program, not part of radioconda: build it from source on Linux, install it with the project’s Homebrew formula on macOS, or use the project’s MSYS2 build on Windows, and use a recent version, because --iq-input-format is newer than some packages. We have not run it in this course yet. With an optional RTL-SDR, nrsc5 100.3 0 tunes a station live. The related project gr-nrsc5 is an HD Radio transmitter for GNU Radio, not a receiver, and this course does not transmit on the FM band; gr-rds, from Lesson 7, still decodes the RBDS inside the host.

To look for HD Radio live, disable the File Source and the six blocks after it in L14-hd-find.grc, enable the PlutoSDR Source, and type the station’s frequency into Center. About 1.5 MS/s streams continuously over the Pluto’s USB 2.0 link. Centering on the station puts the Pluto’s DC correction on the host, where it does not touch the sidebands or the meters.

The finder sets the Pluto’s gain by hand, from the Pluto RX gain slider, rather than with its automatic gain, so a reading at a given gain means the same thing every time. Start near 30 dB, and lower it if the floor rises or copies of strong stations appear, because the sidebands are far weaker than the host and an overloaded receiver buries them. Not every station carries HD Radio, so check the station list at hdradio.com, and then type its frequency into the finder.

A weak station reads high, as the noise frame showed. As in Lesson 7, receiving is unrestricted and transmitting is not; nothing in this lesson transmits.

Today we found the HD Radio sidebands beside an FM station, measured their power with meters built from familiar blocks, and built an OFDM signal from GNU Radio’s own blocks. The L14 Lab is the bench version: build the sideband meter yourself, use it on a capture whose configuration you do not know, and measure a live station on the PlutoSDR, with an optional nrsc5 decode.

Lesson 15 turns to receiver work that nrsc5 performed for us today. Every QPSK subcarrier here needed its phase and timing recovered before the receiver could read its symbols, and Lesson 15 builds that recovery for a single mPSK carrier in GNU Radio.