Audio line noise filter

NFZ_SM series・NFZ_SG series・

LQW_CA series

The NFZ series of audio line noise filters was developed as filters that combine noise suppression and audio quality.
These filters provide Class D amp noise suppression, high frequency isolation, and TDMA noise suppression while maintaining audio quality in speaker and earphone lines.

Murata Icon X Necessity for audio noise filters

Suppression products:
NFZ_SM / SG series
LQW_CA series

Suppression products:
[For speakers]
  NFZ_SG series(High-Z products)
  LQW_CA series
[For earphones]
  NFZ_SG series (Large current products) 

Suppression products:
NFZ_SG series(High-Z products)
LQW_CA series
    Audio line noise suppression solution page

Murata Icon X Audio line problems in smart phones

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.

Murata Icon X Smart phone audio distortion

Figure 4 Measurement results of earphone audio distortion

In recent years, smart phone audio quality has continued to improve, but what are the actual differences in audio quality? Using earphones as an example, Figure 4 shows the results of audio distortion (THD+N) measurements.

As we can see in this figure, the audio quality differs depending on the manufacturer.

Smart phones with poor THD+N results typically use ferrite beads to suppress noise within the internal audio circuits, while models that emphasize audio quality use the NFZ series.

The human ear is particularly sensitive in cases such as an earphone where sound is delivered directly, and even slight distortion can be detected.
As the shift toward high-resolution audio continues to advance not only in specialized audio equipment, but in smart phones as well, filters that take the impact on audio quality into account will become more important.

Audio line noise suppression solution page