A signal synthesis circuit synthesizes luma signals and chroma signals output by a video signal source, to output composite video broadcast signals (CVBS). The signal synthesis circuit includes a first gain adjusting unit, a delay unit, and a second gain adjusting unit. The first gain adjusting unit receives and adjusts the gain of the luma signals from the video signal source, and the delay unit delays phases of the luma signals output by the first gain adjusting unit. The second gain adjusting unit receives and adjusts the gain of the chroma signals from the video signal source so that the luma signals output by the delay unit and the chroma signals output by the second gain adjusting unit can be synthesized into the CVBS.