Question
What is the function of the vertical interval signaling (VIS) code sent as part of an SSTV transmission?
Answer Options
- A) To lock the color burst oscillator in color SSTV images
- B) To identify the SSTV mode being used
- C) To provide vertical synchronization
- D) To identify the call sign of the station transmitting
Correct Answer: B
Explanation
Before an SSTV image transmission begins, a special encoded signal known as the Vertical Interval Signaling (VIS) code is sent. This is a very short transmission, typically a sequence of two specific audio tones, which serves a vital handshake function between the transmitting and receiving stations.
The primary function of the VIS code is to identify the SSTV mode being used. Since there are many different SSTV modes (e.g., Robot 36, Scottie 1), each with unique timings, resolutions, and color sequences, the VIS code automatically tells the receiving software which mode to select. This allows the receiver to correctly synchronize its internal timing and accurately decode the incoming image data.
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