My laptop has S-video (7 pins), but my TV has S-video (4 pins), can i transfer movies on my PC onto TV, with what sort of cable? Does the picture quality get worse because of the change of screen resolution?

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My laptop has S-video (7 pins), but my TV has S-video (4 pins), can i transfer movies on my PC onto TV, with what sort of cable?

That would be 7-pins to 4-pins S-Video converter cable (and not the other way around). You should refer this when asking for the cable at a video or electronic store.

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Does the picture quality get worse because of the change of screen resolution?

Most of the case, yes. For standard TV, the maximum resolution is 704x480 (NTSC) or 704x576 (PAL). The NTSC system are mostly used in USA and Japan. Most PCs use at least 800x600 resolution, so there will be a loss of image quality. Using a higher (eg. 1024x768) PC resolution would help, but very little and almost unnoticeable. Please do aware that on most TV screen, about 2-5% of the outer region may not visible. Especially the left and right outer regions.