Also known as: vestigial sideband, VSB, 8VSB
Vestigial sideband (VSB) is an amplitude-modulation variant that transmits one complete sideband plus a shaped remnant — a “vestige” — of the other, together with a reduced carrier.1 It is a compromise between wasteful double sideband and hard-to-filter single sideband: VSB gets most of SSB’s bandwidth saving while keeping the near-DC content that SSB filtering would destroy.
How it works
Pure SSB removes an entire sideband with a sharp filter, but a real filter cannot have an infinitely steep edge, and any attempt to cut right at the carrier also strips out the signal’s lowest frequencies — fatal for a video signal that carries DC brightness and large flat areas. VSB solves this by shaping the transmit filter so its response falls off gradually through the carrier along a symmetric Nyquist slope: the small amount of lower-sideband energy that survives near the carrier exactly complements the upper-sideband energy that is attenuated there. When the receiver’s own filter has the matching odd-symmetric roll-off, the two partial contributions add back to a flat response, so low-frequency content is recovered without distortion. The vestige only needs to span the first megahertz or so, so the total occupied bandwidth is far closer to SSB than to DSB.
The odd symmetry is the crucial trick. Around the carrier the transmit filter’s response is not a cliff but a straight-ish ramp from full transmission on the wanted-sideband side down to zero on the vestige side, and it is antisymmetric about the carrier point: whatever fraction of a low-frequency component is attenuated in the surviving sideband, the same fraction survives in the vestige, so the two add back to unity. This is the same Nyquist half-amplitude-at-the-edge condition that governs pulse shaping in digital systems, applied here to an analog spectrum. Get the receiver’s complementary shaping wrong and the near-DC content — flat, bright picture areas in video — comes back distorted while the fine detail is fine.
Variants
Analog television video used AM-VSB with the lower sideband trimmed and the upper sideband full, fitting a ~6 MHz channel that would otherwise need far more for double-sideband video. The digital successor, 8VSB, keeps the same single-sideband-with-vestige spectral shape but replaces the analog video with an eight-level amplitude-modulated digital symbol stream plus a small pilot tone at the vestigial carrier frequency. 8VSB carries roughly 19.4 Mbit/s in a 6 MHz channel and is the physical layer of North American ATSC 1.0 digital television. (The later ATSC 3.0 standard abandons VSB for OFDM.)
Relevance to SDR
VSB is the signal you see when a wideband SDR sweeps the old TV bands: a strong pilot at the lower channel edge with energy trailing off across the channel is the 8VSB signature. Demod requires a pilot-locked carrier recovery, an adaptive equalizer to fight the long multipath echoes that plague terrestrial TV, and a trellis decoder. GopherTrunk is a land-mobile trunking decoder and does not process television signals, so VSB is out of its scope; its interest here is conceptual, as the middle point on the DSB → VSB → SSB bandwidth-versus- filtering spectrum, and as a reminder that practical modulation choices are driven as much by realizable filters as by theory.
In practice
Analog NTSC video fit a full-detail luminance signal, a color subcarrier, and an FM-modulated audio carrier into a single 6 MHz channel precisely because the video used VSB rather than double-sideband — DSB video would have needed roughly 8 MHz for the picture alone. The digital 8VSB successor keeps the vestigial shape but is notoriously demanding on the receiver: terrestrial multipath produces long delayed echoes that smear the eight-level symbols, so an 8VSB demodulator leans on a long adaptive equalizer and a trellis decoder to recover the data, and the small pilot at the vestigial carrier frequency exists specifically to give the receiver a robust phase reference to lock onto before the equalizer converges. That sensitivity to multipath was a recurring complaint against 8VSB, and it is one reason ATSC 3.0 abandoned VSB entirely in favor of OFDM.
Sources
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Vestigial sideband — Wikipedia, for the partial-sideband definition, Nyquist slope, and TV/8VSB applications. ↩