NOS DAC. Non-Oversampling DAC [Advantages, Disadvantages]

NOS DAC (non-oversampling DAC) is digital to analog converter without oversampler. And there may be absent analog filter sometimes. Read why NOS digital analog converter is used, its types, advantages and disadvantages, oversampling vs non-oversampling DAC.

Before reading of the article read about R2R, SDM PCM and DSD DAC types >

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Why need NOS DAC

Traditionally, PCM digital-analog converters have oversamplers to provide better work of analog filter. Because analog filter is mandatory part of DSD and PCM DAC.

Non-oversampling digital analog converter is used to rid oversampling artefacts (digital filter's ringing audio and pass band magnitude deviation). It is usual DAC without oversampler and digital filter module.

NOS DAC is not separate DAC type, in author's opinion. Let's look to the picture.

NOS DAC. Non-oversampled digital analog converter

• At part A non-oversampling R2R PCM DAC is shown.
• Sigma-delta-modulation PCM NOS DAC is drawn at part B.
• DSD DAC (part C) is NOS DAC too.

There are different converter types: two of PCM and one of DSD DACs.

Sample rate of NOS DAC is defined according analog filter stop band.

But analog filter have no exact border between bands. Because it is gradual.

At the picture we can see difference between digital and analog filters.

Digital versus analog filter

NOS DAC have no digital filter. Thus for non-oversampling DAC sample rate should be high enough, to provide more effective suppressing of first (by frequency) group of aliases by analog filter.

NOS DAC. Sample rate and analog filter

Therefore recording should be done on the DAC's high sample rate.

Non oversampling DAC without analog filter

Some NOS DAC may have no analog filter. It allow us rid of analog filter frequency and phase distortions. Analog filter may be active (contains operational amplifiers, transistors, tubes). It is mean that active analog filter can also cause non-linear distortions.

But "no analog filter" mean that well known "stairs" are present at output. Let's consider the spectrum of the "stairs".

DAC output without analog filter
(it is spectrum of the "stairs")

Alias spectrum is copy of audio signal. Here audio signal is all in frequency range between 0 and [sample rate]/2.

The aliases are repeatable along frequency axis from [sample rate]/2 to infinity. Also part of them is flipped by frequency.

Could we listen such signal without issues? Yes. Under certain conditions.

If half of sample rate above 20 kHz, all aliases are located in ultrasound range. And we can't hear it. Some people say, that ear works as analog filter. And it is really so.

But there are no absolutelly linear audio systems. It cause intermodulation distortions of ultrasound components. And intermodulation products by ultrasound components appear into audible frequency range. And these products correlate with useful audio signal. It is not good.

Level of the intermodulation products depend on non-linearity. Aliases have level same to useful audio signal. The more level of ultrasound, the more level of its intermodulation products.

Also need to remember that analog parts of apparatus have dynamic range limitation. And maximal level of signal thansmitted thru the analog parts defined by summary spectrum energy of the audio signal and its aliases.
I.e. aliases consume part of dynamic range. In other words, aliases decrease useful dynamic range.

Do dynamic range decreasing give something useful? No. It reduce maximal signal/noise ratio. And is not good for lower level signals especially.

To compensate alias impact to dynamic range, it should be increased.

Fortunatelly, analog parts of audio devices have own band limitation due capacitance and inductance of a conductors and electronic components. It limit aliase spread along frequency axis via decreasing of alias levels with frequency growth.

Resume: NOS without analog filter allow to rid its distortions. But such DAC may cause:

• audible intermodulation issues,
• additional reducing of dynamic range of useful musical signal.

Non-oversampling DAC and upsampling

Practically, we have limited range of available resolutions (sample rates) of recordings that downloaded from online stores or ripped from CD. To prepare music downloads for NOS DAC, these audio files should be upsampled to sample rate compatible with the converter.

Upsampling on personal computer (even laptop) may give audio developers more resources for precise digital signal processing calculations. But audio quality is defined not only by computing resources. Because design of the resampling filters may be done many ways.

Upsampling issue, considered in this part of the article,  may be applied to both PCM and DSD DACs.

The preliminary upsampling may be provided:

• inline into musical player (software or hardware);
• offline into sample rate conversion software.

In Windows operation system, to avoid processing between the player and DAC (bit perfect playback), ASIO and WASAPI drivers are used.

Advantages of non-oversampling DAC

1. No digital filter ringing artefacts,
2. Simpler inner workings,
3. Potential ability to lesser distortions.

Disadvantages of NOS DAC

1. As rule, audio file pre-upsampling is necessary,
2. Probable issues by insufficient alias filtering (audible products of intermodulation distortions and additional consuming of dynamic range).

Oversampling vs non oversampling DAC

How NOS DAC sounds

It is impossibly to guarantee, that NOS DAC give sound advantages.

As device with simpler inner works (lesser amount of internal elements), the DAC give potential abilities for better sound (lesser distortions).

But there are other factors, that impact to work of any kind of digital analog converter:

• power supply quality (noise and deviation),
• analog filter and other elements issues,
• clock frequency and amplitude deviation,
• edge blurring of clock signal,
• other.

For non oversampling DAC, quality of external upsampling is very important.

If high resolution audio records are used to playback at NOS DAC, products of ultrasound itermodiulations may be more intensive  in audible frequency range due non-linear distortions and "softer" filtering without digital filter.

Resume: sound quality of NOS DAC vs other types depend on device implementation.

NOS DAC list

Warning: Here are no recommendations. It is not full list. It is not advertizing or offer. It is NOS DAC example list only. The list can contains mistakes and irrelevant information. Obligatorily ask DAC vendor/manufacturer about actual NOS ability.

Conclusions

1. NOS DAC allow to rid the ringing artefacts of digital filter.
2. Availability issues of a records on native sample rate of the non-oversampling DAC are probable.
3. Upsampling records may have issues of:
   • sound quality of different sample rate conversion tools,
   • bit-perfect abilities of the music player,
   • additional consumption of electric energy (for inline resampling).
4. For some audio systems, to reduce intermodulation distortions, ultrasound filtering during upsampling records may be applied.
5. Also DAC have other issues, that impact to final quality (clock stability, power supply, etc.). So non-oversampling digital analog converter scheme don't improve sound alone.

Author: Yuri Korzunov, March 11, 2018,

Audiophile Inventory's founder, developer.

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