Question E2B11:
What is the function of the vertical interval signaling (VIS) code sent as part of an SSTV transmission?  

Choices:  
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 is  B: To identify the SSTV mode being used  

To identify the SSTV mode being used is correct.

Explanation:
- The VIS (Vertical Interval Signaling) code is a short digital code sent before the image to tell the receiver which SSTV mode to use (e.g., Martin 1, Scottie 1, Robot 36). This lets the decoder automatically set line count, scan rate, and color/mono parameters. Conceptually: $S = f(\text{VIS})$, where $S$ are the decoder settings.

Why the other choices are incorrect:
- A: Locking a color burst oscillator applies to analog TV (NTSC/PAL) color subcarriers, not SSTV. SSTV color is encoded via audio frequency mapping; VIS does not lock a color burst.
- C: Vertical sync in SSTV is provided by specific sync pulses (frame/line sync) embedded in the picture signal, not by the VIS code.
- D: Callsign identification is done by voice, text overlay, CW/MFSK ID, or separate digital ID—not by the VIS code.

Rules of thumb:
- VIS = “mode ID” sent before the picture so the receiver can auto-select the correct SSTV mode.
- Sync pulses = timing (line/frame sync), separate from VIS.
- Callsign ID ≠ VIS.