Nyquist filter
A receiver filter that equalizes video after VSB transmission.
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.
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. 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 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.
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.
- Named after
- Harry Nyquist (1889–1976)
- Type
- Low-pass filter
- Suppression at if carrier
- 6 dB
- Application
- TV receivers, before demodulation
- Associated systems
- System B, System M
Lore & Background
In analog television broadcasting, the visual radio frequency signal is produced by amplitude modulation, which generates two symmetric sidebands containing identical information. To use the broadcast band more efficiently, one sideband is partially suppressed using a vestigial sideband (VSB) filter. However, this creates a demodulation problem: the region from 0 to 0.75 MHz retains both sidebands (double sideband), while the region beyond 1.25 MHz has only one sideband (single sideband). As a result, the demodulated signal in the 0–0.75 MHz region is 6 dB higher than in the region beyond 1.25 MHz, causing high-frequency components—corresponding to fine details and color subcarrier—to fade relative to low-frequency components.
The Nyquist filter is a low-pass filter applied before demodulation that suppresses the double-sideband portion by 6 dB at the intermediate frequency (IF) carrier. This equalization reconstructs the original band characteristic at the output of the demodulator, restoring proper balance between low- and high-frequency video components. Tolerance masks for the Nyquist filter are specified for different broadcast systems, such as System B (used in most countries, including G or H in UHF band) and System M (used in America), with the sound trap off.
Reader's Guide
The Nyquist filter is significant because it directly addresses a practical limitation of VSB transmission, which is widely used in analog television to conserve bandwidth. Without this filter, the inherent 6 dB level difference between double-sideband and single-sideband regions would degrade picture quality, particularly in fine details and color saturation. By providing a precise 6 dB suppression at the IF carrier, the filter ensures that the demodulated video signal has a flat frequency response, preserving image fidelity.
Its legacy lies in its role as a standard component in analog TV receivers, enabling efficient use of the broadcast spectrum without sacrificing picture quality. The filter's design is tied to specific broadcast systems—System B and System M—each with its own tolerance mask. The Nyquist filter remains a textbook example of how a simple, carefully tuned filter can solve a complex signal-processing problem in mass-market consumer electronics.
Did You Know?
- The Nyquist filter is named after Swedish–US engineer Harry Nyquist (1889–1976).
- It is a low-pass filter applied before demodulation in TV receivers.
- The filter provides 6 dB suppression at the intermediate frequency (IF) carrier.
- It equalizes the level difference between double-sideband and single-sideband regions caused by VSB transmission.
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