DTMB is a major standard for digital broadcast. It is used in China, Hong Kong, and Cuba and may be adopted in other nations.
The GB20600-2006 standard described may just specify the transport stream format and not the digital layer, which is what we can process in software.
For information on DTMB hardware, see
Linux driver development
Linux Kernel developer Antti Palosaari <email@example.com> has written some basic DTMB support into the Linux kernel, see blog entry. He also wrote a HDIC HD29L2 DTMB (aka CMMB or DMB-T/H) demodulator driver (hd29l2) for kernel 3.3, but notes there is no longer any public hardware for that driver.
For discussion (in Chinese) of Linux in Hong Kong and China, see
CCextractor does not yet handle DTMB text extraction.
The HKCA1108.pdf regulatory document from the Hong Kong government states that "Receivers shall include provisions to decode and display subtitles conforming to ETSI EN 300 743." This is the specification that regulates European subtitles over DVB. It's possible that DTMB regulates only the physical signal but has nothing to do with subtitles or video encoding.
Similarly this Test Procedure document from the Cuban government says "The TV Set is intended for the reception of DTMB Digital Terrestrial Television in 6MHz bandwidth, according to the specifications GB 20600-2006 and DVB subtitles (ETSI EN 300 743). The DUT must support DVB subtitles (ETSI EN 300 743)."
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