It happened on the Golf channel last night. Here is a series of photos that shows the issue.
Picture 1 is channel 93, SD. I included the guide display so that the program can be identified. Note the letterbox at the top, left and right.
Picture 2 is again 93/SD but without the guide. Note the letterbox on all 4 sides. This is correct for 16:9 HD content being broadcast on a 4:3 SD channel with no loss of content.
Picture 3 is channel 593/HD. Note that there is no letterbox at the top of the picture. Compare this photo with the first photo. The guy's face is noticeably thinner in this photo.
Picture 4 is without the guide. Look at the word GOLF in the lower right and compare it with the second photo. The program is being incorrectly manipulated. The height looks to be HD, but the width is being compressed without data loss into SD dimentions.
