In this blog post, we’ll explore the principles of basic filters used in electronic circuits and how they’re applied in sound quality enhancement.
Have you ever seen a “BASS” option in the settings menu when listening to music on an MP3 player? Although it varies by device, this feature allows you to emphasize the low-frequency range for a more immersive listening experience. At first glance, it may seem like a simple feature, but BASS is one of the most fundamental functions of music players, and many manufacturers have developed and implemented their own proprietary BASS features, such as Mega BASS and Extra BASS. So, what is the basic principle behind these sound enhancement features?
This can be understood through the concept of a “filter,” as used in basic circuit theory. The word “filter” means “to sift” or “to strain.” In this sense, even a sieve used in everyday life can be considered a filter. Filters composed of actual electronic circuits also perform functions not significantly different from those of a sieve. Let’s examine one of the most basic filters: the LPF (Low-pass filter). As the name suggests, an LPF is a filtering device that allows only low frequencies to pass through. So, what is the criterion for “low” here? Generally, this criterion can be explained in terms of frequency. Frequency is a value that indicates how many times a particular state repeats per unit of time in a periodic phenomenon. Imagine two people holding the ends of a long rope and shaking it. The faster they shake the rope, the more frequently the S-shaped waveform repeats, resulting in a higher frequency. Frequency is a characteristic of all waves; in sound, lower pitches correspond to lower frequencies, while higher pitches correspond to higher frequencies. Therefore, if an LPF is designed based on a specific frequency, it becomes a filtering device that allows only sounds with frequencies lower than that threshold to pass through.
Using a resistor (R1) and a capacitor (C1), you can implement the most basic form of an LPF. In a simple circuit, a resistor is a component that limits the current flowing through the circuit, while a capacitor is a component that stores electrical energy in the form of an electric field. However, in a filter circuit, they perform functions that are more critical to implementing the filter’s characteristics than their basic roles; this can be likened to the role that the differential symbol plays in mathematical equations. Therefore, by appropriately configuring resistors and capacitors, various types of filters can be created. In the filter described earlier, the frequency that serves as the design criterion is called the cutoff frequency, and this value is determined by the values of the resistor and capacitor. For example, let’s assume the cutoff frequency is set to the frequency of “Fa” in the scale “Do-Re-Mi-Fa-Sol-La-Si.” In this case, if a 7-second music file consisting of “Do-Re-Mi-Fa-Sol-La-Si” is input, the result will be a 7-second music file containing only “Do-Re-Mi.”
In practice, LPFs are not as simple as the circuit described above. Various electronic components, including resistors, are added to increase the filter’s accuracy and achieve superior results—that is, better sound quality. One method of increasing accuracy is to connect identical circuits in series. A series connection refers to connecting identical circuits in succession, where the output of the first circuit is used as the input for the next circuit. For example, imagine a ride that only allows passengers who are at least 160 cm tall to board. If checking height once at the entrance is considered a first-stage filter, then checking height again immediately before boarding can be viewed as a second-stage filter connected in series. There are various other methods as well, and today’s BASS function can be seen as a practical application of these advanced filter circuits.
Filters range from relatively simple types, such as LPFs and HPFs (High-pass filters), to highly complex circuits, and their applications are extremely broad. Filters can be broadly categorized into analog filters and digital filters. The LPF and HPF introduced earlier are both analog filters and are constructed using actual electronic components. In contrast, digital filters convert the input signal into digital data and then use mathematical operations to achieve the desired characteristics. Both methods have their own advantages and disadvantages. Although analog filters are subject to errors caused by physical components such as resistors and capacitors, they are still widely used at very high frequencies. Digital filters generally offer high precision and excellent flexibility, but since they require specific signal processing capabilities and circuit configurations, their implementation varies depending on the application environment. In actual systems, it is common to use both analog and digital filters together.
Basic filters are a crucial part of circuit theory, and understanding their principles is not particularly difficult. The experiment mentioned earlier—filtering a music file consisting of the notes “Do, Re, Mi, Fa, Sol, La, Si” using an LPF—is frequently used in circuit theory education. Even today, various filter circuits are utilized not only in complex electronic devices but also in the process of building vacuum tube amplifiers and audio equipment from scratch. If you are interested in the fundamentals of electronics, such as current and voltage, designing and building your own filters can be a very rewarding experience.
