science

Nyquist filter

Nyquist filter is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Nyquist filter rather than just read about it. In short: A Nyquist filter is an electronic filter used in TV receivers to equalize the video characteristics. The filter is named after the Swedish–US engineer Harry Nyquist (1889–1976).

Key takeaways

Nyquist filter belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Nyquist filter to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Nyquist filter from memory before moving on to harder problems.

Reference excerpt

A Nyquist filter is an electronic filter used in TV receivers to equalize the video characteristics. The filter is named after the Swedish–US engineer Harry Nyquist (1889–1976).

Vestigial Side Band (VSB) In analogue TV broadcasting the visual radio frequency (RF) signal is produced by amplitude modulation (AM) i.e., the video signal (VF) modulates the amplitude of the carrier. In AM two symmetric sidebands appear, containing identical information. So the RF bandwidth is two times the VF bandwidth. For example, the RF bandwidth of a VF signal with a bandwidth of 4.2 MHz, is 8.4 MHz. (System M) In order to use the broadcast band more efficiently, one sideband can be suppressed. However, it is impossible to suppress one sideband completely without affecting the other. Furthermore, a very sharp edge filter characteristic causes intolerable delay problems. So as a compromise, a standard filter is used which reduces a considerable portion of one side band (lower side band in RF) without causing extensive delay problems. Such a filter is known as vestigial side band filter (VSB).

Example of a VSB In System B the VF bandwidth is 5 MHz. Without any suppression, the corresponding visual RF bandwidth must be 10 MHz. (Here, presence of aural signal is omitted for the sake of simplification.) But by using a VSB filter, the visual RF bandwidth is reduced to 6.25 MHz; 5 MHz in one sideband and 1.25 MHz in the other sideband. (The filter characteristic in the suppressed sideband is such that between 0 and 0.75 MHz there is no suppression.) By this method, 3.75 MHz is economised, which means that for the same band allocated for broadcasting, the number of TV services increases approximately one and half fold.

Demodulation problems When VSB filter is used in broadcasting, a problem arises during demodulation. While 0 - 0.75 MHz region has two side bands, the region beyond 1.25 MHz has only one side band. ( i.e., 0 - 0.75 MHz region is double sideband and the region beyond 1.25 MHz is single sideband ) Thus, the level of the demodulated signal in 0 - 0.75 MHz region is 6 dB higher than the level in the region beyond 1.25 MHz. Since high frequency components of the VF signal correspond to fine details and color subcarrier, the demodulation results in fading the detailed portions and color saturation of the picture with respect to less detailed portions of the picture.

Nyquist filter In order to equalise the low frequency and high frequency components of the VF signal, a filter named a Nyquist filter is used in receivers. This filter, which is used before demodulation, is actually a low-pass filter with 6 dB suppression at the intermediate frequency (IF) carrier. Thus, the level of double sideband portion of the VF signal is suppressed and the original band characteristic is reconstructed at the output of the demodulator.

Tolerance masks of the Nyquist filter The specifications below are given for sound trap off case. System B (G or H in UHF band) refers to broadcast system used in most countries. System M refers to broadcast system used in America.

System B

System M

See also Broadcast television systems Sideband TV transmitters

References

Worked examples

Example 1 — a first encounter with Nyquist filter

Start with the simplest possible case. Write down what Nyquist filter claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Nyquist filter before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Nyquist filter ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Nyquist filter

In research: Nyquist filter appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Nyquist filter in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Nyquist filter is common in secondary-school and first-year university syllabi. It links to neighbouring topics Radio electronics, Television technology, so understanding it makes those chapters shorter.
In everyday life: Look for Nyquist filter outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Nyquist filter in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Nyquist filter means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Nyquist filter out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Nyquist filter in simple terms?

A Nyquist filter is an electronic filter used in TV receivers to equalize the video characteristics. The filter is named after the Swedish–US engineer Harry Nyquist (1889–1976).

Why does Nyquist filter matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Nyquist filter?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Nyquist filter.