１．What is audio distortion?
Figure 2 Conceptual diagram of audio distortion
Filters that remove electromagnetic noise without affecting the audio quality are suitable for use as noise suppression products in audio circuits.
First, we will briefly explain what it means when audio distorts.
The audible range of the human ear is said to be from 20 Hz to 20 kHz. Audio distortion can be considered to affect this audible range. To explain this in simple terms, let’s use a 1 kHz sound as an example. When there is no audio distortion, a sine wave can be observed over the time domain as shown in Figure 2, while the frequency spectrum shows only one fundamental frequency.
However, the shape of the sine wave is altered when distortion occurs in the audio. Looking at the frequency spectrum, harmonic spectrum lines appear in addition to the fundamental frequency.
Expressing the measurement as an audio parameter gives us THD + N (Total Harmonic Distortion + Noise), which indicates the harmonic ratio that is produced. For a noise filter, it is important that harmonic distortion does not occur when an audio signal flows.
２．Comparison with conventional ferrite beads of the BLM series
Figure 3 A comparison of the audio characteristics (THD+N) of conventional ferrite beads and an audio line filter
Figure 3 compares the audio characteristics (THD+N) of conventional ferrite beads with an audio line filter.
THD+N is typically displayed as a percentage or as a dB value, with lower values indicating less audio distortion and higher audio quality. Looking at conventional ferrite beads, we can see that the audio distortion value is low when the audio output is low, but the distortion increases as the output increases. However, when an audio line filter is applied, audio distortion does not occur even if the output is increased, and there is no degradation of the audio characteristics.
In addition, the impedance curve is adjusted to increase noise removal within the cellular band to achieve impedance characteristics that make the filter well suited to suppressing noise in smart phones.