Signal processing in a telephone system means doing three things to a voice signal on its way along a line: amplifying it to restore strength lost on the wire, filtering it to remove frequencies that do not belong, and shifting it in frequency so several can share one line. Gains and losses are added in decibels, where a voltage gain of 10 is 20 dB and gains of 10, 10 and 10 in a row make 1,000, or 60 dB.
Part of Telecommunications
How a voice signal is boosted, cleaned and repositioned in frequency between a microphone and an earpiece, with the decibel arithmetic used to track it.
A Telephone Already Processes The Signal Before It Reaches The Wire
The telephone band of 300 to 3,400 Hz is itself a processing decision, as Sound Wave Basics explains. The lower cutoff rejects power-system hum and the upper rejects hiss, while the band keeps enough vowel and consonant energy for word recognition. Sidetone control, which feeds back a small fraction of your own voice so the phone does not feel dead (Anti-Sidetone Network), is another. A signal is also converted between impedances by transformers (Impedance Matching). None of this needs a transistor.
Three Problems On A Long Line, Three Tools
The signal gets smaller. Resistance, leakage and capacitance cost level along the wire. The remedy is a repeater: Amplification.
Unwanted energy gets in. Hum from power lines, hiss, and the neighbouring conversations on a shared pair. The remedy is to keep only the band you want: Filtering.
One conversation uses a pair for a small slice of what it can pass. The remedy is to move each conversation to its own frequency: Carrier Systems.
Decibels Turn Multiplication Into Addition
Stages that multiply gains become simple sums when expressed in decibels. For voltage ratios, dB = 20 × log₁₀(ratio):
- Voltage gain 10: 20 × log₁₀(10) = 20 dB
- Voltage gain 100: 20 × log₁₀(100) = 40 dB
- Three stages each gaining 10: 10 × 10 × 10 = 1,000, which is 60 dB; also 20 + 20 + 20 = 60 dB
- A power ratio of 10 is 10 dB, and 10 × log₁₀(8) ≈ 9 dB, the “about 9 dB (8×)” that Modulation Concepts gives for single sideband over AM
Sound pressure level uses the same scale against a 20 µPa reference: 60 dB SPL is a pressure of 20 µPa × 1,000 = 0.02 Pa. Cascading Stages shows the same multiplication for tube stages, and a line loss is the same sum in reverse: a section that loses 20 dB followed by a repeater that gains 20 dB leaves the level where it began.
Feedback Is Processing Too
Negative feedback trades gain for stability, as Feedback Principle works out: an amplifier with a gain of 100 and a feedback fraction of 0.09 has closed-loop gain 100 / (1 + 100 × 0.09) = 100 / 10 = 10, and distortion and drift fall by the same factor of 10. A repeater chain made of stages with stable, known gain is much easier to line up than one made of stages whose gain wanders.
What You Need
- A way to measure level. An audio oscillator (see Oscillator Circuits) and a voltmeter or earpiece to compare
- A written record of every stage’s gain or loss in dB, so a line can be lined up and fixed
- A reference test tone at a single agreed frequency in the voice band
Common Mistakes
- Multiplying decibels. They add; gains multiply.
- Mixing voltage and power decibels. A voltage ratio uses 20 log, a power ratio 10 log.
- Amplifying before filtering. The amplifier boosts the noise along with the voice.
- Judging by ear without a number. Two stages that “sound fine” can hide a 10 dB error.
Safety
Supply voltages
Tube amplifiers and rectifiers run from DC commonly 150 to 600 V (see Rectification). Discharge capacitors and keep one hand clear when probing. Transistor stages on 9 V are not a shock hazard but a battery short can still overheat wire.
What’s Next
- Learn next: Amplification, restoring level on a line
- Combine with: Filtering, deciding which frequencies stay