Understanding Telephony: Bandwidth, Transmission Lines, and Sound

Bandwidth Transmitted by the Voice Channel

Band transmitted by the voice channel. The telephone system does not transmit all frequencies between the audible range of 20Hz and 20kHz, as this would require excessive bandwidth. To determine the bandwidth used, the parameter of intelligibility, the logatoms, is used. 80% intelligibility is sufficient, which corresponds to transmitting frequencies below 2,100Hz or above 1,500Hz.

Transmission Line Conductors

The transmitted signal is alternating and consists of the sum of signals with different frequencies within the band of 300Hz to 3,400Hz. A pure 800Hz tone is used for testing.

Primary Parameters of a Transmission Line

These parameters can be measured directly in a section of a transmission line one mile long:

• Kilometric Resistance: Resistance corresponding to one kilometer of line.
• Kilometric Conductance: The inverse of resistance, corresponding to one mile of line, and related to insulation capacity.
• Self-Induction (LF): Corresponding to one kilometer of line, measured directly in the open with special equipment.

Resistance and conductance are detrimental to the line, as they are responsible for power losses.

Secondary Parameters of a Transmission Line

These are divided into two:

• Characteristic Impedance (Z0): The impedance that a line presents at one end.
• Propagation Constant: This has two components:
  • Phase Constant (β):
  • Attenuation Constant (α):

These two parameters cause two types of distortions in the transmitted signal.

Impedance Matching

The signals involved are:

• Incident Wave: Generated by the generator.
• Transmitted Wave: Applied to the load from the junction of the two circuits.
• Reflected Wave: Reflected back to the generator at the junction of these circuits.

Ideally, we want the power transmitted to the load to be maximized. However, part of the power is reflected, leading to a loss of power. Consequently, the transmitted signal suffers attenuation with respect to the original signal.

Qualities of Sounds

The telephone system is designed primarily for the transmission of vocal sounds. The main qualities of sound are:

• Intensity
• Pitch
• Timbre

The human voice can convey frequencies from 100Hz to 9,500Hz, although our hearing range is much greater, extending from 20Hz to 20,000Hz.