On Digital Terrestrial Radio Broadcast

I have been so absorbed in digital radio recently that I bought a clock radio that support HD Radio last week. I also read Wikipedia articles and technical docs on NRSC-5 (the offcial name for HD Radio) and DAB.

NRSC-5 is the digital radio broadcast standard in the US. NRSC-5 was developed by a private company, iBiquity, which gave it a trademark of HD Radio. It was selected by FCC in 2002 for digital radio broadcast. NRSC-5 supports both AM and FM bands. This article will focus on FM.

NRSC-5 has two modes, hybrid and all-digital. In hybrid mode, a station broadcasts analog audio signal plus two rectangular sub-carriers that contain digital audio and data. In all digital mode the full bandwidth is used for digital audio.

Waterfall plot of NRSC-5 hybrid mode

NRSC-5 has many advantages over analog FM,

• NRSC-5 is more robust against multi-path and fading thanks to OFDM. NRSC-5 also supports single frequency network.
  
• NRSC-5 can multiplex several audio streams encoded in modified HE-AAC on the same frequency. The first stream, HD-1, is the main program. While HD-2/3/4 are supplemental programs. NRSC-5 supports up to 4 audio streams in hybrid mode and 7 streams in all digital mode. All NRSC-5 signals I receive are in hybrid mode.
  
• NRSC-5 supports text, images, and emergency alerts in additional to audio. Several stations around broadcast traffic and weather maps to be displayed on vehicle head units.

I think analog FM broadcast will still exist in the next ten years for these reasons,

• Lack of NRSC-5 receivers. HD Radio is a trademark of a private company. A manufacture needs to pay license fees to produce HD Radio branded devices. So there are very few NRSC-5 capable tabletop/portable radios on the market. While many car radio supports NRSC-5, my car does not. In contrast, Europe's DAB is an ETSI open standard and many manufactures produce DAB capable radios.
  
• There is no federal push for NRSC-5 (This is US). About half of FM stations in my city does not use NRSC-5. I read somewhere that radio stations have to pay for NRSC-5. In comparison, EU requires new cars to support DAB starting in 2021. Norway have switched off analog FM.
• Mediocre audio quality. In hybrid mode, the digital data rate is about 100 kbps. A station needs to allocate it among all audio streams. Most stations I receive allocate 48 kbps to HD-1 and 32 kbps to HD-2/3 each. HE-AAC at 48 kbps does not outperform analog audio. In comparison, a DAB station have about 1 Mbps data rate. Each audio stream gets ~100 kbps.
• NRSC-5 signal uses more bandwidth than analog FM. Adjacent stations could interfere. So FCC initially limited the power of digital signal to 1% of the analog signal. Later in 2010 FCC relaxed the limit to 10%. Still the coverage of digital broadcast is less than that of analog.
• The rise of 4G and Internet streaming services. Many people (including me) listen to music/podcasts on their phone during commute.

References

• NRSC-5 receiver for RTL-SDR: https://github.com/theori-io/nrsc5
• HD Radio homepage: https://hdradio.com/
• Wikipedia: https://en.wikipedia.org/wiki/HD_Radio