DRM is unlikely to replace analog AM broadcast on HF band

I learned a bit about DRM broadcasting on HF band recently. I don't think it will succeed in replacing AM shortwave broadcast.

DRM is a digital broadcast scheme. It features 16/64QAM, OFDM, xHE-AAC. A DRM station usually takes up 10kHz bandwidth in HF band. It may carrier one audio program and some auxiliary data (text, program guide, emergency warning, etc).

Analog AM is good enough for voice. DRM can provide FM radio like audio quality on shortwave band. However, most shortwave stations are operated by governments (e.g. BBC, VOA, CRI) or religious groups (e.g. FEBC). They usually have voice programs. AM's ~4kHz audio bandwidth is good enough for this use case. Another use case for shortwave radio is emergency. Analog modulation also works well for this purpose.

Complexity and cost. An analog AM receiver simply need LO, mixer, filters, envelop detector, audio amplifier. A DRM receiver must have a DSP that can demodulate OFDM, a xHE-AAC decoder, and a micro-controller. All these adds to the cost and power consumption of DRM receivers. I could not find any DRM receiver for less than 1000 CNY in China. I tried listening to DRM on SDR, the effort is not trivial. I also experimented with the CML DRM1000 module, which I do not recommend.

Lastly, mass media broadcasting itself is going downhill IMO. The mobile Internet is capable of carrying high quality audio and video streams from independent "creators". I have not subscribed to cable TV for years. 

  

Tecsun M-801 media player review

I acquired a used Tecsun M-801 music player.

The good,

• Built-in FM antenna with decent reception.
• User replaceable 18650 battery.
• USB-C port supports changing and acting as a USB speakerphone.
• It can record audio into a TF card.
• The built-in mic can be used for answering phone calls via bluetooth.
• It exposes its battery level to phones through bluetooth.
• The audio player can resume from previous location within a file. 

The bad,

• The recorded audio is in MP2 format at 160 kbps instead of MP3.
• Does not play AAC audio files. 
• The only knob has two functions. I prefer a dedicated volume knob.
• The USB-C port is not implemented properly to work with USB-C adapters. 
• No AM reception. 
• The speaker has a noticeable noise floor.
• Volume is not synced to phone in bluetooth mode. 

 

Qodosen DX-286 portable radio review

I bought a Qodosen DX-286 portable radio at https://www.amazon.com/dp/B0D3V3H356 for 80 USD + tax.

I compared it with a tecsun PL330 last evening.

Good,

• The tiny speaker is surprisingly loud and deep. Better than tecsun PL330.
  
• Stellar performance in FM band. It clearly received 38 stations without pulling out the antenna. While tecsun got only 19.
• Good MW performance. It picked up 15 stations. Tecsun got 12 stations.
  
• It decodes FM RDS/RBDS.
• It uses a widely available 18650 battery and can charge the battery through USB.
• The build quality feels nice.
• The tuning knob has good tactile feedback.
  
• It is extremely compact.
• It has many powerful settings.
• The LCD display has good viewing angle and even backlight.
  

Meh

• It does not receive SSB or air band. But I have other radios for these.
• It is power hungry thanks to the TEF6686 chip. The quiescent current in FM band is 200 mA.
  
• The 18650 battery is not included in the package.
• I wish the RDS info could be displayed on a dot-matrix with more characters.
• Buttons serve multiple functions. I must not lose the user manual.
• The English user manual has a few translation errors.
• The noise floor in FM is higher than PL330's.
• The radio mutes for ~100ms between tuning, probably due to DSP.
• The buttons do not have a bump on 5, making it hard to use in the dark.
• It takes ~1 sec to boot.
  

Bad

• The "world" clock setting is counter-intuitive. IMO, the correct order is to set UTC time and then choose the local timezone.
• The kickstand feels flimsy.
• Charging port is micro USB, not type C. 
• The volume control is weird. The volume knob is a potentiometer. The MCU reads its output to set the actual volume.
• The carrying case is a waste of plastic.
  

A teardown of the radio is at https://www.shouyinji.com/thread-2518-1-1.html

Jay Allen's review at https://radiojayallen.com/qodosen-sr-286-am-fm-sw-high-performance-portable/

Note that DX-286 is an improved version of SR-286.

Xiegu X6100 HF transceiver review

The X6100 is a HF and 50MHz transceiver. It is a budget alternative to IC-705 if you do not need VHF/UHF bands.

I tried FT8 on it for a few days.

Good

• Relatively low price
• Bright LCD screen
• The firmware is not locked down.
  
• Internal battery
• Built-in USB interface
• Built-in antenna tuner
• A large community, many hams have the transceiver.
• alternative R1CBU firmware.
  
• The manufacture is still adding new features like wfserver to the firmware.

Bad

• Changing receiver options is tedious due to the untouchable screen.
  
• hamlib's CAT control can be buggy at times.
• Built-in USB interface lacks isolation. Noise from computers would get into the receiver.
• Only 5W output on internal battery.
• It supports WiFi but I never succeeded in connecting to one.
• The CW decoder never worked for me.
• Some birdies exist.
  

Update in Nov 2023,

I sold the transceiver because I lost interest in QRP HF.

Reduce the audio volume on Tecsun PL-330

I found a method that does not require soldering at Baidu tieba. In FM mode, long press the number 7 key, then the display will show a blinking two digit number. Turning the volume knob will change the number and also the speaker loudness. Short press the 7 key to save the value.

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I have been using my new PL-330 radio everyday. I got annoyed by two things related to its digital volume control,

• It sets the volume to at least 10/30 for alarms, which is too loud for me.
  
• The lowest volume, 1/30, is too loud when I use earphones of low impedance.
  

So I opened the case to see if I can fix it. Immediately I saw a 16 pin chip. It is a CD1622CB, a clone of SONY's CXA1622 power amplifier.

According to CXA1622 datasheet (shown below), pin 14 controls the gain, i.e. the volume. The gain is greatest if pin 14 is connect to GND, and 0 if pin 14 is connected to pin 15.

On the PCB, a capacitor (labeled 1C25) sits between pin 14 and GND. I measured the voltage on pin 14 while I rotated the volume knob. The results matched the datasheet. I also shorted pin 14 and pin 15, the speaker got muted.

So to reduce the speaker volume, I need to raise the voltage on pin 14. This can be achieved by adding a suitable resistor between pin 14 and pin 15. I experimented with 4.7k, 10k, and 22k. I found 10k most suitable. So I soldered it on.

And now I can use a volume of 6/30 with earphones. And alarms are no longer annoying.

UPDATE

I found the volume levels were not granular enough after the resistor mod. So I removed the resistor and soldered an 1 Watt 20 Ohm resistor in series with the speaker.

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