Question E2B12:
What signals SSTV receiving software to begin a new picture line?  

Choices:  
A: Specific tone frequencies  
B: Elapsed time  
C: Specific tone amplitudes  
D: A two-tone signal    

Correct Answer is  A: Specific tone frequencies  

To begin a new SSTV picture line, the decoder looks for a short “line sync” tone at a specific frequency defined by the mode (commonly around 1200 Hz). Once that frequency is detected, the software starts the next scan line.

Why the correct answer is correct:
- SSTV encodes video information as audio frequency changes. Each picture line is preceded by a fixed “sync” tone at a specific frequency (e.g., about 1200 Hz in many popular modes like Martin/Scottie). The decoder detects this specific tone to trigger the start of a new line.

Why the other choices are incorrect:
- Elapsed time: Not used for line boundaries; timing would drift. SSTV relies on explicit sync tones, not free-running timing.
- Specific tone amplitudes: Amplitude is unreliable over HF paths due to fading; SSTV uses frequency, not amplitude, for syncing.
- A two-tone signal: While the VIS header uses two distinct frequencies (not simultaneous) to identify the mode, a new picture line is cued by a single specific sync tone, not a two-tone signal.

Useful rules of thumb:
- Brightness (or color component) is sent as a tone whose frequency maps to pixel value, typically:
  $$f = f_{\min} + (f_{\max} - f_{\min}) \cdot \frac{V}{255}$$
  where $V$ is the 8-bit pixel value, $f_{\min} \approx 1500\ \text{Hz}$ (black), and $f_{\max} \approx 2300\ \text{Hz}$ (white). Mid-gray is about $1900\ \text{Hz}$.
- Line sync tone: commonly around $1200\ \text{Hz}$ for a brief, mode-defined duration.
- VIS/mode ID: sent before the picture using specific frequencies (often 1100/1300 Hz), but line starts are still keyed by the line sync frequency.