Humax NA-3712 - Manual (Page 26)

A/V Output Setting
Set the Audio mode which is output from RCA/Cinch Jack.
(STEREO/MONO)
Select the aspect ratio of TV which is connected to STB(4:3 or 16:9).
Select the format which the video is to be displayed on screen.
When your TV set is 4:3 and video source is 16:9,
STB will display in Letter-box or Center mode as selected.
When your TV set is 16:9 and video source is 4:3,
STB will display in the Pillar-box or Full mode as selected.
Adjust this value between 3.58 and 4.43 if your TV displays the video
only in black and white color. Normally NTSC 3.58 mode is used
worldwide, but some TV sets can support only NTSC 4.43 mode.
When you watch TV after connecting a cable between
the RF OUT jack on STB and the Input jack (RF Input) on TV.
If you select Channel 3 or Channel 4 and then set VHF Tuning to
Channel 3 or Channel 4, all services from STB will be viewed
through TV Channel 3 or Channel 4.
Audio Output
Screen Mode
Display Format
NTSC Mode
VHF Tuning
GB25