Question E2B10:
What aspect of an analog slow-scan television signal encodes the brightness of the picture?  

Choices:  
A: Tone frequency  
B: Tone amplitude  
C: Sync amplitude  
D: Sync frequency    

Correct Answer is  A: Tone frequency  

In analog SSTV, each pixel is sent as an audio tone where the instantaneous tone frequency represents the luminance (brightness). Higher frequency = brighter; lower frequency = darker.

Why the correct answer (Tone frequency) is correct:
- Brightness is encoded by the audio tone’s frequency during each pixel interval.
- Typical mapping in common modes: black ≈ 1500 Hz, white ≈ 2300 Hz, mid‑gray ≈ 1900 Hz.

Why the other choices are incorrect:
- Tone amplitude: Amplitude is not used for brightness; systems often amplitude‑limit to reduce fading effects.
- Sync amplitude: Sync identifies timing, not brightness; amplitude isn’t the cue for luminance.
- Sync frequency: Sync tones are fixed, reserved frequencies used to mark line/frame timing; they don’t carry brightness information.

Useful rule of thumb:
- If $L$ is normalized luminance ($0$ = black, $1$ = white), then the transmitted tone frequency is approximately
  $$f(L) = f_{\min} + L\,(f_{\max} - f_{\min}),$$
  with $f_{\min} \approx 1500\ \text{Hz}$ (black) and $f_{\max} \approx 2300\ \text{Hz}$ (white).