Also known as: double sideband, DSB, DSB-SC, DSB-AM
Double sideband (DSB) is any form of amplitude modulation that transmits both of the two mirror-image sidebands produced when a message multiplies a carrier.1 Because the upper and lower sidebands each carry a complete copy of the baseband signal, DSB occupies twice the bandwidth of the message and — in its plain form — spends power on redundant information.
How it works
Multiplying a message m(t) by a carrier cos(2πf꜀t) shifts the message spectrum up to ±f꜀, producing a lower sideband (the frequency-inverted copy below the carrier) and an upper sideband (the erect copy above it). Ordinary broadcast AM is DSB with a large carrier (DSB-AM or DSB-LC): the carrier is left in place so a cheap envelope detector can recover the audio without regenerating a reference. If instead the message is multiplied directly with no DC offset, the carrier cancels and the result is double-sideband suppressed-carrier (DSB-SC) — the same two sidebands but with the power-hungry carrier gone.
Suppressing the carrier saves a great deal of transmit power (in broadcast AM the carrier carries at least two-thirds of the total), but it breaks envelope detection. DSB-SC must be demodulated with a product detector: the receiver multiplies the incoming signal by a locally generated carrier of the exact same frequency and phase, then low-pass filters. Getting that local carrier phase-coherent usually needs a phase-locked loop or a Costas loop, because a small phase error attenuates the recovered audio by cos φ (and a 90° error nulls it entirely).
The redundancy is easy to see in the spectrum. Both sidebands are exact mirror images of one another about the carrier, so either one alone already contains every bit of the message. DSB therefore spends twice the bandwidth of the baseband and, in the carrier form, most of its power on things the receiver does not strictly need — which is precisely the inefficiency that single sideband and vestigial sideband were invented to remove. DSB survives where that inefficiency buys real simplicity: an envelope detector for DSB-AM is a diode and a capacitor, cheap enough to put in every pocket radio ever made.
Variants
- DSB-AM (DSB-LC) — full carrier retained; simple envelope detection; the everyday AM broadcast signal. Its two sidebands are the reason a shaped single-sideband transmitter can throw one away.
- DSB-SC — carrier suppressed; roughly 6 dB more efficient use of transmit power for the same sideband strength, at the cost of coherent detection. It is the building block of analog color TV chroma, FM stereo difference channels, and many subcarrier schemes.
- Quadrature multiplexing — because a DSB-SC signal uses the full ±f꜀ band, two independent DSB-SC messages can share it in phase quadrature (sine and cosine carriers), the analog ancestor of I/Q modulation.
Relevance to SDR
DSB is more a conceptual anchor than a mode you tune every day. Standard AM broadcast is DSB-AM, and a software receiver can demodulate it either by envelope tracking or, better, by phase-locking to the carrier and doing synchronous DSB detection for lower distortion. DSB-SC itself appears inside composite signals: the 38 kHz stereo difference channel of broadcast FM is DSB-SC around a suppressed subcarrier that the receiver regenerates from the 19 kHz pilot. Understanding DSB is also the natural stepping stone to single sideband and vestigial sideband, which start from the two-sideband picture and remove or trim what DSB keeps. GopherTrunk targets digital trunking modes, so it does not demodulate analog DSB voice, but the same product-detector and carrier-recovery ideas underlie the coherent I/Q processing it uses on digital carriers.
In practice
The efficiency gap is stark in numbers. A DSB-AM broadcast transmitter at 100% modulation puts two-thirds of its power in the carrier and only one-sixth in each sideband, so a 1 kW station radiates barely 167 W of useful sideband energy per sideband; the rest heats the antenna as an unmodulated reference tone. DSB-SC recovers that carrier power for the sidebands, and SSB recovers the redundant-sideband power on top of it — which is why a 100 W SSB signal reaches as far as a kilowatt-class DSB-AM one. The trade the designer is buying with DSB is not efficiency but receiver simplicity and tuning tolerance: an envelope detector needs no frequency reference and does not care about a modest tuning error, whereas a DSB-SC product detector must reconstruct the carrier’s frequency and phase or the audio fades and distorts.
Sources
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Double-sideband suppressed-carrier transmission — Wikipedia, for the DSB/DSB-SC definitions and coherent detection requirement. ↩