Question

For commercial telephonic communication, the frequency range adequate for speech signals is

• A. 20 Hz - 20 kHz
• B. 300 Hz - 3100 Hz
• C. 200 MHz - 600 MHz
• D. 300 kHz - 8000 kHz

Hint:

\textbf{Exam Tip:}

• \textbf{Audible Range:} \(20 \text{ Hz} - 20 \text{ kHz}\) (Music/Hi-Fi).
• \textbf{Voice Band (Telephony):} \(300 \text{ Hz} - 3400 \text{ Hz}\) (approx \(300 \text{ Hz} - 3.1 \text{ kHz}\)).
• Telephone lines act as a \textbf{Band Pass Filter} to remove noise outside this range.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:

The question asks for the standard bandwidth allocated for voice transmission in commercial telephone systems. It is important to distinguish between the full range of human hearing and the specific range required for intelligible speech.

Step 2: Analyzing Frequency Ranges:

• Human Audible Range:
  The human ear can detect frequencies from approximately \(20 \text{ Hz}\) to \(20 \text{ kHz}\). This range is necessary for high-fidelity audio (like music) to sound natural and full.
  
• Speech Spectrum:
  Although human speech contains frequencies extending up to \(10 \text{ kHz}\) or more, the majority of the energy and the components necessary for intelligibility (understanding words and recognizing the speaker) are concentrated in the lower frequency range.
  
• Telephony Standards:
  Commercial telephone systems limit the bandwidth to save transmission capacity. The standard "voice band" defined for telephony allows frequencies roughly between \(300 \text{ Hz}\) and \(3400 \text{ Hz}\) to pass. This range captures the fundamental frequency of the voice and the significant formants required for speech recognition.
  

Step 3: Evaluating the Options:

• Option (1) 20 Hz - 20 kHz:
  This is the entire audible range. Transmitting this wide band is unnecessary for voice communication and inefficient for commercial telephony.
  
• Option (2) 300 Hz - 3100 Hz:
  This range falls directly within the standard voice band (\(300 \text{ Hz}\) to \(3400 \text{ Hz}\)). It is the accepted range for commercial telephonic communication to ensure speech is intelligible while conserving bandwidth.
  
• Option (3) 200 MHz - 600 MHz:
  This is in the Ultra High Frequency (UHF) range, typically used for television broadcasting and cellular carrier frequencies, not baseband voice signals.
  
• Option (4) 300 kHz - 8000 kHz:
  This falls into the RF spectrum (Medium and High Frequency) used for radio broadcasting (AM/Shortwave), not for the audio signal itself.
  

Step 4: Final Conclusion:

The adequate frequency range for speech signals in commercial telephony is the voice band, which corresponds to option (2).