Question E2B01:
In digital television, what does a coding rate of 3/4 mean?  

Choices:  
A: 25% of the data sent is forward error correction data  
B: Data compression reduces data rate by 3/4  
C: 1/4 of the time interval is used as a guard interval  
D: Three, four-bit words are used to transmit each pixel    

Correct Answer is  A: 25% of the data sent is forward error correction data  

In digital TV, “coding rate” refers to forward error correction (FEC). A rate of $3/4$ means 3 information bits are carried in every 4 transmitted bits; the extra 1 bit is FEC.

Why A is correct:
- Coding rate $R = k/n$, where $k$ = information bits and $n$ = transmitted bits.
- For $R = 3/4$: $k=3$, $n=4$. So 1 out of 4 bits is FEC.
- Fraction of transmitted bits that are FEC: $1 - R = 1 - \tfrac{3}{4} = \tfrac{1}{4} = 25\%$.

Why the others are incorrect:
- B: Coding rate is about FEC, not source compression. Compression ratio is unrelated to the FEC coding rate.
- C: Guard interval (GI) is a separate OFDM parameter (e.g., GI = 1/4, 1/8, 1/16, 1/32). It is not what “coding rate 3/4” describes.
- D: Pixel word sizes/bit depth concern video format, not the channel’s FEC coding rate.

Useful formulas and tips:
- Coding rate: $R = \dfrac{k}{n}$.
- FEC overhead (fraction of transmitted bits that are parity): $1 - R$.
- Parity-to-payload ratio: $\dfrac{n-k}{k} = \dfrac{1-R}{R}$.
- Example for $R=3/4$: overhead $= 25\%$; parity-to-payload $= \tfrac{1}{3}$.
- Rule of thumb: Lower $R$ ⇒ more redundancy (stronger error protection, lower net throughput). Higher $R$ ⇒ less redundancy (weaker protection, higher net throughput).