Non Oversampling: How to Achieve the Best Audio Quality

A Non-Oversampling (NOS) DAC is a digital-to-analog converter that processes the audio signal directly at its native resolution and sample rate, intentionally skipping the oversampling and digital filtering stages common in most modern DACs.

Instead of digitally creating additional data points to smooth the signal before conversion, a NOS DAC takes a purist approach, converting only the original samples from the digital file into an analog signal.

Here is a step-by-step breakdown of how a NOS DAC works:

1. Direct Signal Processing: The DAC receives the digital audio signal at its original sample rate (e.g., 44.1 kHz for a CD). This data is fed directly to the conversion chip without any upscaling or interpolation.

2. Digital-to-Analog Conversion: The converter chip transforms each digital sample into a corresponding analog voltage level. This raw conversion results in a "stair-step" waveform that is an unfiltered representation of the digital audio.

3. Analog Smoothing: After conversion, a simple, low-order analog filter is used to smooth the stepped waveform. This filter also helps remove high-frequency noise (aliasing) that can occur above the audible spectrum. This reliance on a gentle analog filter, as opposed to a steep digital one, is a key element of the characteristic "NOS sound."

Primary Attributes of Non-Oversampling (NOS) DACs

• Direct Conversion: The original sample rate and bit depth of the audio file are preserved, ensuring no new data is algorithmically generated.

• Design Simplicity: NOS DACs typically feature more straightforward circuit designs with fewer digital processing stages, which can lead to a cleaner signal path

• No Digital Filtering: True NOS DACs omit the complex digital filters found in oversampling DACs. This avoids potential sonic artifacts introduced by these filters, such as pre-ringing

• Emphasis on the Analog Stage: The design focus shifts to the quality of the power supply and the analog output stage, which are crucial for the final sound quality.

Why Use a NOS DAC?

The primary reason audiophiles are drawn to NOS DACs is the desire to eliminate potential artifacts introduced by the oversampling process and its associated sharp digital filters. These artifacts can include time-domain distortions like pre-ringing (a faint "echo" before transient sounds) and phase shifts, which some listeners find unnatural. A NOS DAC offers a signal path free from this specific type of digital manipulation.

Advantages and Disadvantages of Non-Oversampling DACs

NOS DACs are highly regarded by many audiophiles who often describe their sound as more natural and musical. However, this approach has both technical trade-offs and subjective qualities.

Advantages:

• Natural and Organic Sound: Many listeners find the sound of NOS DACs to be smooth, warm, and less "digital," often comparing it to the presentation of analog sources.

• Elimination of Processing Artifacts: By avoiding steep digital filters, NOS DACs are free from pre-ringing and other time-domain distortions, resulting in a more natural reproduction of transient sounds.

• Simpler Signal Path: A simpler circuit can lead to less electronic noise and a purer, unaltered signal.

• Lower Latency: With minimal digital processing, there is virtually no delay (latency) in the signal conversion.

• Power Efficiency: The absence of complex digital signal processing (DSP) operations results in lower power consumption.

Disadvantages:

• Potential for Aliasing: Without a steep digital filter, there is a higher risk of aliasing artifacts—unwanted high-frequency noise—folding back into the audible range. The quality of the analog filter is critical in mitigating this.

• High-Frequency Roll-Off: The gentle analog filters used in NOS DACs often result in a slight attenuation (roll-off) of the highest audible frequencies. This can lead to a sound that is perceived as softer or less detailed.

• Dependence on Source Quality: A NOS DAC will faithfully reproduce everything in the source file, including its flaws. They are often less forgiving of poorly recorded or low-resolution audio.

• Niche Appeal: The specific sound signature appeals to a subset of audiophiles, while those who prioritize technical measurements and maximum detail may prefer oversampling designs

The Role of Filters in Digital Audio

Filters are essential in digital-to-analog conversion. Their job is to remove the high-frequency images and noise created during the conversion process, smoothing the stepped output into a continuous sound wave.

• Roll-Off: This term describes how sharply a filter cuts off frequencies above a certain point. Oversampling DACs use very steep "brick-wall" digital filters, while NOS DACs use gentle analog filters. A steeper filter is better at eliminating aliasing but can introduce phase shifts and ringing.

Digital Filter Types (in Hybrid Designs)

While true NOS DACs have no digital filters, some designs offer optional digital filtering modes. Understanding these helps clarify what NOS architecture avoids.

• Linear Phase (Fast Roll-Off): Technically precise with perfect phase response. However, it produces both pre-ringing and post-ringing. Often sounds detailed but can be perceived as clinical.

• Minimum Phase (Fast Roll-Off): Eliminates pre-ringing but has some phase shift and longer post-ringing. Many find this sounds more natural and dynamic.

• Slow Roll-Off Filters: These have a more gentle cutoff, which produces much less ringing but is less effective at removing aliasing. Sonically, they are often described as warmer and closer to the NOS sound.

Comparison: NOS vs. Oversampling DACs

Feature	Non-Oversampling (NOS) DACs	Oversampling DACs
Signal Processing	Direct conversion, no upsampling	Upsamples signal to a high rate
Filtering	Relies on a simple analog filter	Uses a complex, steep digital filter
Sound Signature	Warm, organic, natural, smooth	Detailed, precise, analytical, clear
Time-Domain Accuracy	Excellent (no pre-ringing)	Can have pre-ringing (linear phase)
Frequency Response	Potential for slight high-frequency roll-off	Flat frequency response to 20kHz
Aliasing Control	Dependent on analog filter; less effective	Excellent suppression of aliasing
Design Complexity	Simple, focused on the analog stage	Complex, focused on the digital stage
Target Audience	Purists, listeners valuing musicality	Listeners valuing technical precision

Which Is Right for You?

• Choose a NOS DAC if you: Prioritize a natural, "analog-like" sound, are sensitive to the artifacts of digital filtering (pre-ringing), and value design simplicity.

• Choose an Oversampling DAC if you: Value technical perfection, crave maximum detail and clarity, and listen to a wide range of high-resolution audio formats.

Verdict

Audio is highly subjective. The choice between a NOS and an oversampling DAC depends entirely on your personal taste and system synergy. There is no single "better" technology, only different approaches to achieving the same goal: enjoyable music reproduction. The best advice is to listen to both types of converters and decide for yourself which presentation you prefer.