*Question G2E14:
What could be wrong if you cannot decode an RTTY or other FSK signal even though it is apparently tuned in properly?*  

*Correct Answer is  D: All these choices are correct*  

### AI Response: (may contain errors)  

To understand why the correct answer is "All these choices are correct," let's break down each option:

1. **Mark and Space Frequencies Reversed**:
   - In RTTY (Radio Teletype) and other FSK (Frequency Shift Keying) signals, two frequencies are used: the mark frequency (representing a binary '1') and the space frequency (representing a binary '0').
   - If these frequencies are reversed, the decoder will interpret the signals incorrectly, leading to garbled or no output.

2. **Wrong Baud Rate**:
   - Baud rate refers to the speed of the signal transmission, measured in symbols per second. If the baud rate set on your decoder does not match the baud rate of the incoming signal, the timing will be off, resulting in an inability to decode the message correctly.
   - Common baud rates for RTTY include 45.45, 50, and 75 baud. Always ensure your settings match the transmission.

3. **Listening on the Wrong Sideband**:
   - RTTY signals are typically transmitted in either upper sideband (USB) or lower sideband (LSB). If you are tuned to the wrong sideband, the signal will not be decoded properly.
   - For example, if the signal is transmitted in USB and you are listening in LSB, the frequencies will be inverted, leading to a failure in decoding.

### Summary:
- All three options can independently cause issues with decoding RTTY or FSK signals. Therefore, if you cannot decode a signal that appears to be tuned in properly, you should check:
  - The mark and space frequencies
  - The baud rate
  - The sideband setting

### Useful Rule of Thumb:
- Always verify your settings against the specifications of the signal you are trying to decode. This includes checking the frequency, baud rate, and modulation type (USB or LSB). 

By understanding these factors, you can troubleshoot and resolve issues with decoding RTTY and FSK signals effectively.