State the frequency limits generally accepted for telephone transmission of speech and typical signal levels encountered in the telephone network.
The correct answer and explanation is:
Correct Answer:
- Frequency limits for telephone transmission of speech: 300 Hz to 3400 Hz
- Typical signal levels in the telephone network: Ranges from -9 dBm to 0 dBm
Explanation:
The telephone network is designed to effectively transmit human speech while using limited bandwidth to optimize performance and resource use. The frequency range that is generally accepted for the transmission of speech in conventional telephone systems is from 300 hertz (Hz) to 3400 hertz (Hz). This range is often referred to as the voice band or telephone bandwidth.
This 300 Hz to 3400 Hz range is sufficient to capture the intelligible components of human speech, especially the fundamental frequencies and formants that are most crucial for understanding words. Although the human ear can detect sounds from around 20 Hz to 20,000 Hz, most of the energy and articulation required to understand speech lies within the narrower 300–3400 Hz range. This makes it practical to filter and transmit just this part of the spectrum in telephony.
The signal levels encountered in the telephone network typically range from -9 decibels referenced to one milliwatt (dBm) to 0 dBm. These levels are standardized to ensure consistency and quality in voice communication. The value of 0 dBm represents a power level of 1 milliwatt across a 600-ohm impedance, which is a standard reference in analog telephony.
For example, at the point where a voice signal is transmitted from a user’s handset into the network, the signal level may be around -9 dBm. As the signal passes through amplifiers and switching systems in the network, it is conditioned to maintain acceptable levels, usually close to 0 dBm at key transmission points. Keeping the signal within this range helps to avoid distortion and maintain clarity.
Together, these limits ensure that speech is transmitted efficiently, with acceptable quality and minimal interference, across local and global telephone systems.