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This comes from the early days of radio communications and is the traditional amplitude modulation by an analog signal like human voices. It is also short for Double Sideband Suppressed Carrier or DSBSC. The latter is just multiplying the audio signal by the carrier sinewave. This is not used much any more now that filtering and phasing methods of generating SSBSC are inexpensive, but it was mostly on HF for voice communications. It has about 6 dB SNR improvement over AM.
You see, there is a note in that picture: "One channel gap must be specified." It is a result of DSB using. It is a cheap modulator - good TV modulator will have filter to suppres second sideband. There is a need to suppress second sideband - some TV sets have AFC error on DSB signals, aslo you will produce additional interference in multichannel systems. It is practical observation - I made TV modulators for TV cable systems ten years ago.
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