I use a comcast HD DVR box. I set the comcast box the way so that it does not touch the 480i content. But 720p and 1080i will be transmitted as 1080i. I want my 860 TV to 'upconvert' the signal to 1080p. Here is when I hear the 'pop' sound from TV (I triple-checked that the sound is coming from the TV):
1. Watching SD (480i) channel. Switched to HD channel (1080i).
2. Watching HD (1080i) channel. Switched to SD channel (480i).
The picture mode for HD is 'Just Scan' and for 4:3 (SD) is '16:9'.
This 'pop' sound is loud enough to notice. It happens intermittently.

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