Nyquist filter

A Nyquist filter is an electronic filter found in television receivers, used to balance the video signal's frequency response. It takes its name from Harry Nyquist (1889–1976), an engineer of Swedish and American origin. The filter operates before demodulation to fix amplitude differences that result from vestigial sideband (VSB) transmission.

Vestigial Side Band (VSB)

In analog TV broadcasting, the visual radio frequency signal is created through amplitude modulation, where the video signal varies the carrier's amplitude. This process generates two symmetrical sidebands, each carrying the same information, so the radio frequency bandwidth ends up being twice the video frequency bandwidth. For System M, a video signal with a 4.2 MHz bandwidth requires an 8.4 MHz radio frequency bandwidth.

To use the broadcast spectrum more efficiently, one sideband can be partly suppressed. Completely removing one sideband without disturbing the other is not feasible, and a filter with an extremely sharp cutoff introduces unacceptable delay. As a compromise, a standard filter—called a vestigial sideband filter—removes a large portion of one sideband (typically the lower one) while keeping delays manageable.

For example, in System B, the video bandwidth is 5 MHz. Without suppression, the visual radio frequency bandwidth would be 10 MHz.

Example of a VSB

Using a VSB filter reduces this to 6.25 MHz: 5 MHz in one sideband and 1.25 MHz in the other. In the suppressed sideband, frequencies from 0 to 0.75 MHz are left unsuppressed. This saves 3.75 MHz, allowing roughly one and a half times more TV services within the same allocated band.

Demodulation problems

Demodulation becomes problematic with VSB filtering. The region from 0 to 0.75 MHz retains both sidebands (double sideband), while frequencies above 1.25 MHz have only one (single sideband). As a result, the demodulated signal in the 0–0.75 MHz range is 6 dB stronger than in the higher range. Since high-frequency video components correspond to fine details and the color subcarrier, this imbalance causes loss of detail and reduced color saturation compared to coarser parts of the picture.

Nyquist filter

To correct this, a Nyquist filter is placed in the receiver before demodulation. It acts as a low-pass filter that applies 6 dB of attenuation at the intermediate frequency carrier.