Stereo Noise Reduction Circuit Working and Application

A stereo noise creates a huge disturbance, especially when you are listening to weak radio stations. Peaks of unwanted background noise take over the broadcast signal, making it unpleasant. The noise is always disturbing in the intervals when the music stops. This article discusses the Stereo Noise Reduction Circuit Design and Applications. Different noise filters, noise removal strategies.

A stereo noise reduction circuit is used to overcome the problem by limiting the noise peaks. It will attenuate the signal output by 45dB when the input music signal is very weak.

Classification of Noise

Broadband Noise: The noise in which the acoustic energy is distributed over a wide range of frequencies is called as broadband noise. The spectrum of this noise is continuous and smooth, hence it is also called as continuous noise.

The measure of this noise is called as a signal to noise ratio (SNR). It is the ratio of the average level of the program material to the average level of noise.

Narrowband Noise: This noise is limited to a narrow level of frequencies. This kind of noises has a constant level and are normally caused by incorrect grounding and poorly shielded cables. It includes single frequencies such as 50 to 60Hz.

Impulse Noise: This noise includes sharp sounds like clicks and pops. High amplitude noise impulses are called as pops and they persist for long durations of more than 2m/s. Similarly, clicks are lower in amplitude, high in frequency and persist for short durations.

Irregular Noise: It includes noises from background conversations, traffic and rain. These noises are made of random sounds that vary in frequency and loudness.

Noise Removal Strategies

Minimizing noise before recording: The process of noise reduction requires the best possible transfer of signal from analogue to a digital medium. Before digitizing an LP record, clean the record to remove dirt and dust.

The stylus and cartridge of turnable should be in good condition. Good quality of shielded cables further reduces the noise interference due to electricity.

Computer Noise: When we record through a sound card, analog to digital conversion process creates distortion from quantization error and the electrical noise can be picked up from other components in the computer. Low priced sound recorders are not shielded properly, which makes more noise.

Set Appropriate Level: It is important to set the recording levels to at high to obtain a good SNR and maximum dynamic range. Noise removal tools are effective when they are used correctly.

Types of  Noise Filters

Filters are used to test the frequency dependent nature of the impedance of inductors or capacitors. As the frequency changes, the value of both reactive impedance and voltage divider ratio changes. This operation produces the change in input/output transfer function depending on the frequency which is known as frequency response.

The filtering is carried out in three methods, namely

• Butterworth Filter
• Chebyshev Filter
• Elliptical Filter

Butterworth Filter

A Butterworth filter is the best method to obtain attenuation and phase response. It does not have a ripple in the passband or the stopband, hence this is called a maximally flat filter.

The Butterworth filter achieves its flatness because of the relatively wide transition region from pass band to stop band, with average transient characteristics.

The normalized poles of this filter fall on the unit circle. The poles are equidistantly spaced on the unit circle, which means the angle between the poles is equal. The Butterworth filter is normalized for a -3dB response.

Chebyshev Filter

Chebyshev filters have a narrower transition region between stop band and phase band. The sharp transition between the pass band and stop band of this filter produces smaller absolute errors and has fast execution speeds than a Butterworth filter.

Elliptical Filter

Butterworth and Chebyshev filters are all-pole designs, which means that the zeros of the transfer function are at one of the two extremes of the frequency range For a low pass filter, the zeros are at f=8. If finite frequency transfer function zeros are added to poles an elliptical filter.

Stereo Noise Reduction Circuit

The below circuit gives the stereo noise reduction circuit.

The stereo noise reduction circuit uses MCP6001 is a single purpose op-am-amp offering rail to rail input and output. It operates in the range of 1.8V to 6V. This amplifier has a GBWP of 1MHz with a typical quiescent current of 100 microamps.

The sensitivity of the noise reduction circuit can be regulated using a potentiometer. If it is higher, noise suppression decreases rapidly.Thus the sensitivity of the circuit can be

Thus the sensitivity of the circuit can be adapted to different sources of music. The maximum audio signal that can work is 210mV with distortion less than 0.01%.

The time delay of the noise reduction circuit is determined by using time constant R7C4. For the schematic values, it is 1 second, but can be changed if necessary.

The electronic circuit can be supplied with a voltage ranging from 12V to 30V and current consumption will be2 to 3mA. This design consists of active noise cancellation solution addressing feedforward architecture for stereo devices.

Applications

• Noise reduction circuits are used in radio system to overcome the noise produce due to electrical signals, poor shielding effect.
• Stereo noise reduction circuits are used in various music systems to suppress unwanted sounds.

Do you want to know in detail about  Stereo Noise Reduction Circuit Design? If you are interested in designing electronics projects then, share your views, comments, queries, and suggestions in the comments section below.