Question E2B04:
How is color information sent in analog SSTV?  

Choices:  
A: Color lines are sent sequentially  
B: Color information is sent on a 2.8 kHz subcarrier  
C: Color is sent in a color burst at the end of each line  
D: Color is amplitude modulated on the frequency modulated intensity signal    

Correct Answer is  A: Color lines are sent sequentially  

To send color in analog SSTV, the system transmits the color components one after another (sequentially) for each scan line. The receiver buffers these components and recombines them to form the color image.

Why A is correct (Color lines are sent sequentially):
- In common analog SSTV color modes (e.g., Martin, Scottie), each line is sent as separate components in sequence, such as luminance (Y) followed by two chroma components (R−Y and B−Y), or as sequential R, G, B lines depending on the mode.
- This avoids the need for a color subcarrier and fits within the narrow SSB audio passband.

Why the other choices are incorrect:
- B: “Color information is sent on a 2.8 kHz subcarrier”
  - Analog SSTV does not use a dedicated color subcarrier. Pixel intensity is conveyed directly by an audio tone whose frequency varies with intensity. All information stays within the audio baseband.
- C: “Color is sent in a color burst at the end of each line”
  - That describes NTSC/PAL TV (e.g., NTSC’s 3.58 MHz color burst). SSTV uses line/vertical sync tones (e.g., around 1200 Hz for sync), not a color burst.
- D: “Color is amplitude modulated on the frequency modulated intensity signal”
  - SSTV encodes brightness/chroma solely by tone frequency. Amplitude is kept constant and is not used to carry color, which improves robustness against fading.

Useful rules of thumb:
- Pixel intensity is mapped to audio frequency over a defined range (typical for many modes):
  - Black ≈ 1500 Hz, White ≈ 2300 Hz; sync ≈ 1200 Hz.
- A simple model for intensity-to-tone mapping:
  $$f(I) = f_{\text{black}} + (f_{\text{white}} - f_{\text{black}})\,I$$
  where $I$ ranges from 0 (black) to 1 (white), e.g.:
  $$f(I) \approx 1500\ \text{Hz} + 800\ \text{Hz}\cdot I$$
- Color is sent as sequential components per line (e.g., $Y$, $R{-}Y$, $B{-}Y$), not via a subcarrier or amplitude modulation.