What is noise? Its impact on audio quality.

Noise refers to unwanted components that are mixed into an audio signal. In audio equipment, this noise is not only perceived as hiss or hum, but also affects the overall sound quality. Specifically, an increase in noise lowers the signal-to-noise ratio (S/N ratio), impairing the resolution and detail of the sound. It also narrows the dynamic range, making it difficult to hear subtle sounds and nuances.

Furthermore, since this type of noise is affected by circuit design, power supply, and layout, it is important to be aware of it from the early stages of design. Thus, noise is not merely "noise," but a crucial factor that affects audio quality itself.

Main types of noise in audio

Audio equipment can generate noise due to various factors, but the following types are particularly prone to causing design problems:

■ Power supply-derived noise (ripple switching)
Noise generated from the power supply circuit superimposes on the audio signal as a periodic component. Switching power supplies, in particular, are prone to generating high-frequency noise, which is a major cause of sound quality degradation.

■ EMI (Electromagnetic Interference Noise)
This noise is caused by electromagnetic waves emitted from surrounding digital circuits and external devices. It is susceptible to interference from clock signals and high-speed communication lines, and becomes more apparent in miniaturized and high-density designs.

■ Quantization noise (digital)
This noise is caused by quantization errors that occur during A/D and D/A conversion. The fewer bits used, the greater the impact, and it also affects the accuracy of signal processing.

On the other hand, some types of noise cannot be completely eliminated through design.

■ Thermal noise
This noise is caused by the thermal motion of electrons and is unavoidable in principle.

Therefore, in noise countermeasures, the concepts of "suppressing generation" and "reducing impact" are important.

Why is noise reduction important?

Noise reduction is a crucial element in the design of audio equipment. Even with high-performance DACs and amplifiers, significant noise can prevent them from performing at their full potential. Furthermore, recent equipment is becoming smaller and more highly integrated, creating an environment where noise is more likely to occur due to the influence of power supply circuits and digital circuits. Such noise directly impacts sound quality evaluation and significantly affects product quality and user experience.

Furthermore, noise often becomes apparent as a problem during the evaluation phase, and in many cases, it is difficult to implement countermeasures later, such as requiring revisions to the board layout or power supply design. Therefore, noise countermeasures should not be adjusted in later stages, but rather be considered an important element from the initial design phase.

Where should noise reduction measures be implemented?

The effectiveness of noise reduction measures varies greatly depending on when they are implemented. Generally, the most important approach is to "suppress noise generation with hardware," and then make final adjustments with software (digital signal processing).

Hardware countermeasures

The most important aspect of noise reduction is hardware design. Power supply design, board layout, and wiring methods directly affect the generation and intrusion of noise, so careful consideration during the design phase is essential.

• Power supply design (utilization of decoupling capacitors, LDOs, and filters)
• Layout design (GND isolation, wiring optimization)
Shielding and EMI countermeasures

These measures can suppress the generation of noise itself. The most effective noise countermeasure is "designing to prevent noise generation in the first place."

Examples of major noises that occur on the circuit boards of audio equipment and their locations.

Software-based countermeasures (digital signal processing)

For software-based noise reduction, digital signal processing techniques are widely used. Filtering and noise reduction algorithms can suppress unwanted components in the signal, thereby improving sound quality.

- Frequency control using low-pass filters, etc.
• Noise reduction algorithm
- Bandwidth adjustment and sound quality tuning

However, these methods do not completely eliminate noise; rather, they play a role in correcting and optimizing aspects that could not be suppressed by hardware. By utilizing a dedicated development environment, these processes can be efficiently designed and verified.

The fundamental approach in audio design is to suppress noise generation with hardware and then ultimately optimize it with software.

Summary

Noise in audio equipment is a crucial factor that significantly impacts sound quality, and understanding its types and causes is essential for appropriate countermeasures. Hardware design, particularly power supplies and layout, directly contributes to noise generation, making countermeasures from the initial design stages indispensable. On the other hand, software (digital signal processing) is an effective means of correcting residual noise and optimizing sound quality.

Thus, it is important to think of noise reduction as a division of labor: suppressing it with hardware and refining it with software. Being mindful of noise from the design stage will lead to improvements in the final product quality and sound quality.

Related Information

As we have seen, in noise reduction in audio development, it is important to understand the causes of noise generation and to appropriately divide the roles of hardware and software. In particular, software-based noise reduction and sound quality optimization play an important role in post-design tuning.

DSP Concepts' audio development platform, "Audio Weaver," provides an intuitive GUI-based environment for designing and verifying signal processing that deals with sound, including filtering and noise reduction.

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