YMMV but at least here in the southeast USA, the default GSTV signal can be turned off by pressing a combination on the 4 soft touch keys on the outside of the screen. If your buttons are like so,

1 | CONTENT | 5

2 | CONTENT | 6

3 | CONTENT | 7

4 | CONTENT | 8

try the sequence 5 6 5 . Here where I am at, that mutes the sound. Sometimes 7 8 7 also turns the screen off.

But! You need to put it back to screen on, sound on, or the staff will notice and you risk them changing the combination on you.