Tutorial CDR

Tutorial: CDR (China Digital Radio)

What it is. CDR is China's digital radio standard (GY/T 268.1-2013 and 268.2-2013). It is an OFDM system that sits in the FM band (about 87 to 108 MHz), either beside an analogue FM programme or on its own. It carries audio services, a text service and data. In OnAir you tune a CDR signal, the receiver finds its beacon, reads the system information and the multiplex, and lists the services; you pick one to hear.

What you need

• A radio that tunes the FM band. The mode asks for 2 Msps and works down to 768 ksps (the signal is about 0.5 MHz wide). An RTL-SDR is enough. See Supported radios.
• A normal FM antenna.
• A frequency. The mode's start frequency is 106.1 MHz and the tuning range is 87 to 108 MHz. The shipped data has no CDR station list, so you have to know where CDR is on the air (the standard is Chinese). If you have no signal, use the built-in test signal first.

Before you start

• CDR has two kinds of channel: all-digital modes, and modes beside an analogue FM programme. The receiver searches for the beacon in both positions: inside +-100 kHz of the centre for the all-digital modes, and 100 to 250 kHz from the centre for the modes beside an analogue FM programme.
• Keep the gain moderate; a strong analogue FM carrier next to the digital signal can overload a cheap radio.

Step by step

1. In the mode list open Broadcast radio and choose CDR (second line China digital radio).
2. Type the frequency and press Start.
3. The Radio tab shows "searching for a CDR signal", then "reading the multiplex". The network name appears when received.
4. In the STATIONS list on the left (or the Services section) pick a service.
5. Use the Re-sync button if the signal faded and the sound stays silent: it drops the lock and searches again without stopping the radio.
6. Save stream writes the raw sound of the service to a .dra file; Stop saving ends it.

What you should see

• A narrow OFDM block about 0.5 MHz wide, flat-topped.
• In the Radio tab: Text (the text service), Signal and Network; in the Receiver tab: Receiver (state, status), Physical layer (carrier offset, sub-frames), System information (frame and sub-frame, allocation, the modulation of the service description and the service data, the LDPC code rate and whether protection is unequal, hierarchy, nominal frequency, multi-frequency), Multiplex and Services.
• For the selected service: codec "DRA+", bit rate, sample rate and channels, language, the audio units received per logical frame.

Reading the screen

• The service list shows each service with its identifier and share of the multiplex in kbit/s.
• The Not supported section lists what is not decoded: the electronic service guide (service names and programme types), hierarchical modulation and unequal protection.

Options and settings

• Re-sync, Save stream, volume and mute.
• The test signal has options for an FM programme (the analogue programme beside the digital signal) and a text service.

Troubleshooting

• Never finds a signal. Wrong frequency or no CDR on the air. Try the test signal to check the chain.
• Locks, silent. The sound is DRA+, whose patent and licence status is unclear, so OnAir does not include a DRA+ decoder: the sound is silent until you install the optional DRA+ add-on (libonair_dra, built from the open-source decoder as docs/CODECS.md, section "DRA+ (CDR sound)", explains). Without it the panel shows an amber ! notice with a How to install button; the services, signal information and data work regardless. With the add-on loaded (a green "DRA+ decoder loaded" line) OnAir plays the selected service; if it then stays silent, press Re-sync.
• Service names show only numbers. Names belong to the electronic service guide, which is not decoded.
• Sound stutters or drops out. The signal is weak; improve the antenna.

Tips

• Checked with generated signals and impairments; recordings of real signals are welcome (see Signal samples).

Legal and privacy notes

Receiving broadcasts is legal in most countries. The patent and licence status of DRA+ is unclear, which is why the decoder is an add-on you install yourself; use of it is your own responsibility under local law.

Under the hood

GY/T 268.1-2013 physical layer: OFDM with a beacon (sync) signal and scattered pilots, per sub-frame system information (Viterbi, CRC-6), a service description channel (Viterbi) and the service data channel with LDPC and descrambling; GY/T 268.2-2013 multiplex: control and service multiplex frames carrying DRA+ audio units, text and data. The receiver resamples to 816 ksps.

Related

Mode page: CDR | FM | DRM | Signal samples | Supported radios