A carrier system puts several telephone conversations on one pair of wires by shifting each voice band to its own frequency slot, then separating them at the far end with band-pass filters. Each voice channel needs at least the 3,100 Hz between 300 and 3,400 Hz, so the number of conversations is limited by the bandwidth the line can pass, not by the copper. The risks are neighbouring channels leaking into each other and mixing products falling on the wrong slot.
Part of Telecommunications
Frequency-division multiplexing of voice channels on a shared line: the principle, why bandwidth is the limit, and where it fails.
One Pair Can Carry Many Conversations Because A Conversation Occupies Only A Slice Of Frequency
A wire pair does not care what frequencies cross it, up to a limit. A voice uses only 300 to 3,400 Hz. If the first conversation is left where it is and the second is moved up by modulating it onto a higher carrier tone, the two occupy different parts of the spectrum on the same wires. At the far end a filter tuned to each slot picks out one conversation and rejects the other, and the second is moved back down to the voice band.
The corpus already contains this idea at telegraph scale. Relay Circuits describes superimposing audio-frequency signals of different frequencies on the DC telegraph current: a 1,000 Hz channel, a 2,000 Hz channel and a 3,000 Hz channel could coexist on one wire, each separated by a bandpass filter at the receiver, and calls the principle “the foundation of modern broadband communication”.
A Voice Channel Costs At Least 3,100 Hz, Or 6,800 Hz With Both Sidebands
Modulating a voice onto a carrier creates sidebands on both sides, as Modulation Concepts derives for AM: the carrier plus components at the carrier frequency plus and minus each audio frequency. The two sidebands each carry the same information, and the carrier carries none.
- Voice band: 3,400 − 300 = 3,100 Hz wide
- AM with both sidebands: carrier ± 3,400 Hz, so the signal spans 2 × 3,400 = 6,800 Hz
- Single sideband, carrier and one sideband suppressed: 3,100 Hz
So single sideband fits about twice as many channels in the same space, and saves power too: Modulation Concepts gives about 9 dB (8×) more effective signal than AM. The cost is complexity, since the carrier must be restored precisely at the receiver.
The real slot per channel is wider than these minima, because filters do not cut off sharply and a guard space is needed. This article does not state a standard spacing, because none is in the corpus and none is invented here.
Why Bandwidth Is The Scarce Thing
- A line only passes so much. Capacitance between the conductors acts as a low-pass filter (see Wire Line Systems), and loading coils make it worse above the voice band. The usable frequency range of a pair is therefore limited and shrinks with length.
- Wire is expensive to build. A pole line lasts 30 to 50 years if built carefully (Pole Line Building). Every conversation moved onto an existing pair is a pole line not built.
- The radio spectrum is shared. The same trade, one slot per user, governs radio, where the carrier article advises writing down every frequency assignment to avoid interference (see Telecommunications).
Whatever the pair’s usable bandwidth is, the number of channels is at most that bandwidth divided by the slot width, before guard space. Measure the first, do not guess it.
What A Carrier Channel Needs
- A carrier oscillator at each end, stable enough that the slot does not drift into the neighbour. Quartz crystals have Q of 10,000 to 1,000,000 against 100 to 500 for a good LC circuit (Oscillator Circuits)
- A modulator that shifts voice up, and a demodulator that brings it back
- Band-pass filters at both ends of each channel, as in Filtering
- Amplifiers, because each stage has loss (Amplification)
- A line that has not been loaded
Worked Example: Mixing Products Land On Neighbouring Channels
Take the relay-circuit example of carriers at 1,000, 2,000 and 3,000 Hz. Any non-linear stage (a saturating amplifier, a worn contact, a dirty joint) mixes two tones into their sum and difference.
- 2,000 − 1,000 = 1,000 Hz, the first channel’s carrier
- 2,000 + 1,000 = 3,000 Hz, the third channel’s carrier
- The second harmonic of 1,000 Hz is 2 × 1,000 = 2,000 Hz, the second channel’s carrier
So even a modest non-linearity feeds each channel’s carrier into the others. Remedies are to keep levels low enough that amplifiers stay in their linear range, to space carriers so that low-order products fall into guard spaces, and to put a filter ahead of each demodulator. Frequency stability matters equally: a carrier that drifts slides its sidebands into the neighbour.
Common Mistakes
- Loading a line that will carry carrier. The loaded line cuts off above the voice band.
- Over-driving an amplifier. Level that is too high makes mixing products, heard as crosstalk.
- Crowding channels. Filters need a skirt; leave guard space.
- Treating the carrier tone as free. In AM about two-thirds of the power sits in the carrier, which carries no information.
Safety
Tube power supplies
A tube-based carrier amplifier runs from rectified DC, commonly 150 to 600 V (see Rectification). Discharge the filter capacitors before touching the chassis, keep one hand behind your back when probing, and do not work alone.
What’s Next
- Learn next: Filtering, the part that keeps channels apart
- Combine with: Superheterodyne for the same select-by-frequency idea in a receiver