The reason why I recommend the test screen is that it has the following benefits: - The quality of external devices (or lack thereof) is not visible. - Some devices do tend to modify the output. If it doesn't appear on the test screen my recommendation is to swap out the other devices (TV Reciever with another device with video output that is known to work well on an other TV, and another cable). If this doesn't solve it, attach the cable to another port on the TV and see if this works.
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