Nyquist filter
Encyclopedia
A Nyquist filter is an electronic filter
used in TV receiver
s to equalize the video characteristics. The filter is named after the Swedish–US engineer Harry Nyquist
(1889–1976).
(RF) signal is produced by amplitude modulation
(AM) ; i.e., the video signal (VF) modulates 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 untolerable 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).
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.
(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.
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.
Electronic filter
Electronic filters are electronic circuits which perform signal processing functions, specifically to remove unwanted frequency components from the signal, to enhance wanted ones, or both...
used in TV receiver
Television set
A television set is a device that combines a tuner, display, and speakers for the purpose of viewing television. Television sets became a popular consumer product after the Second World War, using vacuum tubes and cathode ray tube displays...
s to equalize the video characteristics. The filter is named after the Swedish–US engineer Harry Nyquist
Harry Nyquist
Harry Nyquist was an important contributor to information theory.-Personal life:...
(1889–1976).
VSB
In analogue TV broadcasting the visual radio frequencyRadio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
(RF) signal is produced by amplitude modulation
Amplitude modulation
Amplitude modulation is a technique used in electronic communication, most commonly for transmitting information via a radio carrier wave. AM works by varying the strength of the transmitted signal in relation to the information being sent...
(AM) ; i.e., the video signal (VF) modulates the carrier
Carrier wave
In telecommunications, a carrier wave or carrier is a waveform that is modulated with an input signal for the purpose of conveying information. This carrier wave is usually a much higher frequency than the input signal...
. 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 untolerable 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 BSystem B
System B is the analog broadcast television system used in the most countries. There are several systems in use and letter B is assigned for the European system which is also used in the majority of Asian and African countries. - Specifications :Some of the important specs are listed below.Frame...
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 frequencyIntermediate frequency
In communications and electronic engineering, an intermediate frequency is a frequency to which a carrier frequency is shifted as an intermediate step in transmission or reception. The intermediate frequency is created by mixing the carrier signal with a local oscillator signal in a process called...
(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.
The tolerance masks of the Nyquist filter
The specifications below are given for sound trapSound trap
A sound trap is a special acoustical treatment of Heating Ventilating and Air-Conditioning ductwork designed to reduce transmission of noise through the ductwork, either from equipment into occupied spaces in a building, or between occupied spaces....
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
| Frequency, MHz | Level, dB |
|---|---|
| -1.65 | -34 |
| -1.65 to -1.35 | -42 |
| -1.0 | -21.4 |
| -0.5 | -16.4 to -12.1 |
| 0 | -6.4 to 5.7 |
| 0.5 | -2.5 to -1.4 |
| 1 | -0.75 to 0.12 |
| 1.4 | -0.44 to 0.44 |
| 1.5 | 0 |
| 1.6 to 5.0 | -0.44 to 0.44 |
| 5.0 to 10.0 | 0.44 |
System M
| Frequency, MHz | Level,dB |
|---|---|
| -1.65 | -34 |
| -1.65 to -1.35 | -42 |
| -1.0 | -21.4 |
| -0.5 | -16.4 to -12.1 |
| 0 | -6.4 to - 5.7 |
| 0.5 | -2.5 to -1.4 |
| 1 | -0.75 to 0.12 |
| 1.4 | -0.44 to 0.44 |
| 1.5 | 0 |
| 1.6 to 5 | -0.5 to 0.5 |