# ECE 448 Software Defined Radios ## Lesson 14 — HD Radio Spring 2027 · Dr. Neil Rogers

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--- ## Where We Were - Lesson 7 received an FM station and its RDS data. - Lesson 13 decoded tones in bins, one FFT frequency at a time. - Today we find a digital signal that sits beside an FM station. Note: Before class, run L14-hd-test-tx.grc once so L14-hd-test.cu8 sits next to L14-hd-find.grc. If the room has a PlutoSDR and an antenna, switch the finder to its PlutoSDR Source and find a local HD station on the waterfall ahead of time. --- ## Today's Plan 1. HD Radio modes and what is in the spectrum 2. The digital sidebands and OFDM 3. Recreating and finding the sidebands in GNU Radio 4. Measuring them 5. Decoding with nrsc5 (optional) Note: This follows the slides: overview, spectrum, RBDS, sidebands, the OFDM concept and implementation, then tools. --- ## Find the Shelves on the Air Simulated: the finder's spectrum and waterfall on the L14 test file at 100.3 MHz, with a flat shelf on each side of the station on the spectrum and two straight bands on the waterfall; the station at 99.9 MHz has none

Simulated: the finder's spectrum and waterfall on the L14 test file at 100.3 MHz, with a flat shelf on each side of the station on the spectrum and two straight bands on the waterfall; the station at 99.9 MHz has none

Run L14-hd-find.grc on the PlutoSDR at an FM station: flat bands beside it are HD Radio. Note: Live demo: in L14-hd-find.grc, disable the File Source and the six blocks after it, enable the PlutoSDR Source (1,488,375 samples per second, extended-frequency mode), and type a strong local station into Center. Drag a box across one shelf on the spectrum to zoom in. hdradio.com/stations lists the local HD stations. The figure is the finder playing the simulated test file. --- ## HD Radio Modes

- Hybrid keeps the analog host and adds a sideband on each side. - Extended hybrid widens the sidebands inward; all digital drops the host. Note: NRSC-5 defines all three. Hybrid keeps about 130 kHz of analog signal on each side; extended hybrid limits it to about 100 kHz. --- ## What Is in the Spectrum | Part | Where | Modulation | | :-- | :-- | :-- | | Analog host | within about ±130 kHz | FM | | Inside the host | 0 to 57 kHz after demodulation | audio, pilot, stereo DSB-SC, RBDS BPSK | | Digital sidebands | ±129.4 to ±198.4 kHz | OFDM, QPSK on each subcarrier | Note: RBDS is the US name for RDS: differentially encoded BPSK at 1187.5 bits per second on 57 kHz, carrying the station name, time, and text. Demo: run Lesson 7's L07-rds-rx.grc on its recording to show the station name; PySDR's RDS chapter, the slides' activity, decodes it in Python. --- ## The Digital Sidebands - Subcarriers are 363.4 Hz apart: one FFT bin at `fft_len` 4096. - Each sideband is many narrow subcarriers, numbered 356 to 546. - Partitions hold 18 data subcarriers and 1 reference subcarrier. - The sidebands are much weaker than the host. Note: Subcarrier k sits at k times 363.4 Hz from the center. The reference subcarriers are 356, 375, 394, and every 19th up to 546. The meter's inner and outer Variables use these numbers. --- ## The OFDM Concept - OFDM sends data on many closely spaced subcarriers at once. - Their spectra overlap, but each is zero at its neighbors' centers. - An FFT at the receiver separates them again. Note: OFDM divides a wide channel among many slow subcarriers. Orthogonality is the O in OFDM. --- ## OFDM in GNU Radio - Each subcarrier carries one I/Q point per symbol: BPSK, QPSK, or QAM. - An inverse FFT of `fft_len` = 4096 builds a symbol; each bin is one subcarrier. - The guard, `cp_len` = 224 samples, goes in front of each symbol. Note: The blocks are the OFDM Carrier Allocator, a reverse FFT, and the OFDM Cyclic Prefixer, as in the GNU Radio wiki's Basic OFDM Tutorial, which the slides link. The receiver drops the guard, takes a forward FFT, and reads each subcarrier's point. --- ## OFDM Explorer Open the OFDM explorer (it is also on the lesson page). - At spacing 1.00 every neighbor crosses zero at the center subcarrier. - An echo longer than the guard reaches into the next symbol's FFT window. Note: Start at HD Radio: the red dots sit on zero. Move the spacing off 1.00 and the neighbors leak in. Drag the echo past the guard and the red sliver appears inside symbol 2's FFT window. --- ## The Test Signal L14-hd-test-tx.grc: two OFDM chains of Random Uniform Source, Chunks to Symbols, Stream to Tagged Stream, OFDM Carrier Allocator, reverse FFT, and OFDM Cyclic Prefixer; an FM host and neighbor; noise; and a File Sink of unsigned bytes

L14-hd-test-tx.grc: two OFDM chains of Random Uniform Source, Chunks to Symbols, Stream to Tagged Stream, OFDM Carrier Allocator, reverse FFT, and OFDM Cyclic Prefixer; an FM host and neighbor; noise; and a File Sink of unsigned bytes

Each side carries random QPSK on subcarriers 356 to 546, written as unsigned bytes. Note: The FFT has 4096 points at 1,488,375 samples per second, so each bin is one subcarrier, and the guard is 224 samples. The symbols are random, not NRSC-5 data, so nrsc5 cannot decode this file. The 4 s file is 11.9 MB, in nrsc5's default format. Nothing transmits. --- ## The Sideband Finder L14-hd-find.grc: a byte File Source through UChar To Float, Add Const, Multiply Const, Deinterleave, and Float To Complex, or a PlutoSDR, into a frequency sink, a waterfall, and three band-power meters

L14-hd-find.grc: a byte File Source through UChar To Float, Add Const, Multiply Const, Deinterleave, and Float To Complex, or a PlutoSDR, into a frequency sink, a waterfall, and three band-power meters

Five blocks turn unsigned bytes into complex samples. Note: Subtract 127.5, scale by 1/128, pair the bytes. GRC shows the PlutoSDR Source grayed out, with a Rational Resampler and SigMF Sink that record for nrsc5. To go live, select the File Source and the six blocks after it, press D, and enable the PlutoSDR Source. --- ## Finding the Sidebands Simulated: the sideband finder with meters at -23.04 and -23.06 dBc and 0.02 dB between them, the spectrum with two flat shelves, and the waterfall with two straight bands

Simulated: the sideband finder with meters at -23.04 and -23.06 dBc and 0.02 dB between them, the spectrum with two flat shelves, and the waterfall with two straight bands

The shelves have flat tops and sharp edges, and their width never changes on the waterfall. Note: The figure comes from the simulated test file. The shelves start at 100.1706 and 100.4294 MHz. The display shows power per bin, so the shelves' height is not the power ratio; the meters measure that. --- ## Measuring Sideband Power Simulated: Upper sideband -23.04 dBc, Lower sideband -23.06 dBc, Upper minus lower 0.02 dB

Simulated: Upper sideband -23.04 dBc, Lower sideband -23.06 dBc, Upper minus lower 0.02 dB

- Each meter shifts its band to 0 Hz, filters it, and averages $\vert x\vert^2$ for 0.2 s. - The sideband meter minus the host meter gives dBc; a third reading compares the sides. Note: The host band is plus or minus 100 kHz, each sideband is plus or minus 34.7 kHz around plus or minus 163.9 kHz, and every filter decimates by 6. The nrsc5 project's real sample file reads about −24.3 and −23.8 dBc in the same flowgraph, close to the test file. On a live station the two meters need not agree. --- ## What Noise Does Simulated: the finder with noise_amp 0.15: the floor near -66 dB, the shelves about 8 dB above it, and the meters reading -22.24 and -22.27 dBc

Simulated: the finder with noise_amp 0.15: the floor near -66 dB, the shelves about 8 dB above it, and the meters reading -22.24 and -22.27 dBc

With `noise_amp` raised to 0.15, the shelves sink toward the floor and the meters read high. Note: Demo: change noise_amp from 0.03 to 0.15 in L14-hd-test-tx.grc, rerun it, rerun the finder. The floor rises about 14 dB and the meters read about 0.8 dB high, because the noise in each band adds to the sideband's power. --- ## Decoding with nrsc5 (optional) ``` nrsc5 --iq-input-format cf32 -r hd.sigmf-data 0 # play HD1 nrsc5 --iq-input-format cf32 -r hd.sigmf-data 1 # play HD2 ``` nrsc5 runs the rest of NRSC-5: sync, QPSK, deinterleaving, decoding, and audio. The finder records the Pluto as SigMF at 744,188 S/s, half rate, which nrsc5 reads. Note: GNU Radio has no HD Radio receiver module, and nrsc5 has no PlutoSDR driver, so it decodes a recording. In the finder, enable the PlutoSDR Source, the Rational Resampler, and the SigMF Sink, run about 10 s on the station (about 60 MB), stop, and run the command. nrsc5 is not part of radioconda; build a recent version from source on Linux, with the project's Homebrew formula on macOS, or with MSYS2 on Windows. Not yet run in this course. Program 0 is HD1. With an optional RTL-SDR, nrsc5 tunes live. gr-nrsc5 is a transmitter, not a receiver. --- ## Key Point

HD Radio adds two OFDM sidebands, 129.4 to 198.4 kHz from the station's center and much weaker than the analog host. They appear as flat shelves on a spectrum, band-power meters measure them, and nrsc5 decodes them.

--- ## On the SDR - Run the PlutoSDR at 1,488,375 samples per second, centered on the station. - Look for shelves 129 to 198 kHz either side, with manual gain near 30 dB. - The shelves should stand 10 dB above the floor. Note: Centering puts the Pluto's DC correction on the host, away from the meters. Manual gain, not the Pluto's AGC, so a reading at a given gain means the same thing every time. Overload buries the sidebands, so back off the gain if the floor rises. The finder only receives, and nothing transmits. --- ## Looking Ahead - The L14 Lab builds the meter, identifies a station's mode, and measures a live station. - Lesson 15 recovers carrier phase and symbol timing for mPSK. Note: nrsc5 performed the carrier and timing recovery for us today; Lesson 15 builds it for a single carrier.