Question E2B08:
What technique allows commercial analog TV receivers to be used for fast-scan TV operations on the 70-centimeter band?  

Choices:  
A: Transmitting on channels shared with cable TV  
B: Using converted satellite TV dishes  
C: Transmitting on the abandoned TV channel 2  
D: Using USB and demodulating the signal with a computer sound card    

Correct Answer is  A: Transmitting on channels shared with cable TV  

To use a standard analog TV set for 70‑cm fast‑scan ATV, we put our AM/vestigial‑sideband NTSC signal on the same RF frequencies used by cable TV “cable-ready” channels that lie within 420–450 MHz. In CATV mode, many analog TVs tune those channels directly, so no special receiver is needed.

Why A is correct (Transmitting on channels shared with cable TV):
- Many analog TVs are “cable-ready” and can tune mid/superband CATV channels whose RF carriers fall in the 70‑cm band.
- If you transmit NTSC AM/VSB ATV on one of those CATV channel frequencies, a TV set in CATV mode can receive it directly.
- Common 70‑cm ATV video carriers that match CATV channels:
  - Ch 57: 421.25 MHz
  - Ch 58: 427.25 MHz
  - Ch 59: 433.25 MHz
  - Ch 60: 439.25 MHz
- These alignments are the practical “trick” that makes a consumer analog TV usable for ham fast‑scan ATV on 70 cm.

Why the others are incorrect:
- B: Using converted satellite TV dishes
  - Satellite TV uses very different frequencies and modulation (e.g., 12 GHz down to 950–1450 MHz IF, digital/QPSK). A small DBS dish is ineffective at 70 cm, and its receiver cannot demod NTSC AM/VSB ATV.
- C: Transmitting on the abandoned TV channel 2
  - Channel 2 is 54–60 MHz (VHF), not in the 70‑cm band (420–450 MHz). Also, “abandoned” or not, it doesn’t help us use a UHF amateur band or match CATV tuners there.
- D: Using USB and demodulating the signal with a computer sound card
  - Fast‑scan ATV is a wide NTSC AM/VSB signal (~6 MHz channel). A USB voice signal is narrow (~2.7 kHz), and a sound card cannot handle the multi‑MHz RF/video bandwidth.

Useful formulas and rules of thumb:
- NTSC channel width: $$B \approx 6\ \text{MHz}$$
- Video carrier placement within the 6‑MHz channel: $$f_{video} = f_{lower} + 1.25\ \text{MHz}$$
- Audio subcarrier (if used): $$f_{audio} = f_{video} + 4.5\ \text{MHz}$$
- Vestigial lower sideband extends about 0.75 MHz below the video carrier.
- Stay inside 420–450 MHz: choose video carriers at or below about $$f_{video} \le 439.25\ \text{MHz}$$ so the 6‑MHz channel fits within the band (e.g., CATV Ch 57–60).