(Non) Oversampling
Oversampling came into existence because engineers needed a way to overcome the limitations of steep analog filters and the inaccuracies of early DACs. Back then, converting digital signals to analog without oversampling required filters that were hard to build and often damaged the sound with phase issues and unnatural ringing. Oversampling moved these challenges to a much higher frequency range, allowing designers to use gentler filters while reducing noise and distortion, making digital audio more practical and affordable.
Yet, in this pursuit of measurement perfection, something was lost. Non-Oversampling (NOS) DACs, while technically imperfect, have a way of preserving timing and transients that many listeners find more engaging and lifelike. By avoiding the digital filter stages used in oversampling, NOS designs eliminate certain artifacts that can make music feel processed or clinical, letting the natural flow of the recording shine through.
Today, NOS designs benefit from advances in circuit design, cleaner power supplies, and the availability of high-resolution files that push the drawbacks of NOS far outside the audible range. This means listeners can enjoy the organic and fluid qualities of NOS without the limitations that once held it back. As a result, NOS has become a viable and often preferred choice for those seeking a more natural sound, while others continue to favor oversampling for its technical precision. In the end, the choice between NOS and oversampling has become less about which is superior, and more about what kind of listening experience you value most.
What is NOS and Oversampling?
-
NOS (Non-Oversampling):
The DAC converts the digital signal to analog without increasing the sample rate. For example, a 44.1 kHz signal remains 44.1 kHz during D/A conversion. -
Oversampling:
The DAC increases the sample rate (e.g., 44.1 kHz → 352.8 kHz by 8x) using digital filters before D/A conversion, followed by conversion at this higher sample rate.
Why was oversampling invented?
Early digital audio faced:
- Steep analog filter problems: To avoid aliasing, a steep "brick-wall" analog filter near 20 kHz was needed, which was hard to design without phase distortion and ripple.
- Limited DAC linearity: Early DAC chips had inaccuracies, causing distortion.
Oversampling was invented to:
- Move the need for steep filtering to higher frequencies (e.g., 352.8 kHz instead of 44.1 kHz), allowing gentler analog filters with fewer phase issues.
- Reduce quantization noise within the audible band by spreading it over a larger bandwidth.
It made DACs cheaper, easier to design, and improved measured performance.
What was used before oversampling?
Early CD players (e.g., Philips TDA1540, TDA1541) used:
- NOS DACs, relying entirely on steep analog filters.
- Large, heavy, expensive analog filter stages with phase issues near the cutoff.
Oversampling became the standard in the mid-to-late 1980s for cost, performance, and practicality.
Why is NOS popular today?
Audiophiles discovered that:
- NOS DACs can sound more natural, fluid, and engaging, despite having measurable high-frequency roll-off and aliasing.
- They avoid the pre-ringing (pre-echo) artifacts introduced by digital oversampling filters, which can sound unnatural to some listeners.
- They retain timing and transient characteristics that some perceive as more realistic.
Why is NOS "better" now than before?
- Modern NOS DAC designs benefit from high-precision resistor networks, better clocking, and low-noise power supplies, improving linearity and noise compared to early designs.
- With modern high-resolution files (96/192/384 kHz), NOS becomes more practical since the high-frequency roll-off is pushed far beyond human hearing, reducing the drawbacks of NOS.
- Designers sometimes use gentle analog filtering to reduce aliasing without harming transients.
Is NOS often preferred over oversampling today?
Not universally:
- NOS is popular in high-end, purist audio circles prioritizing naturalness over measurement perfection.
- Oversampling DACs often have lower measured distortion and noise, making them a better fit for those prioritizing transparency and accuracy.
- Some modern DACs (e.g., Holo Audio, Audio-GD) offer NOS modes alongside oversampling so users can choose based on preference.
Overview of (Non) Oversampling DACs from Magna Hifi
