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NOS DAC (non-oversampling DAC) is a digital-to-analog converter without an oversampler. The converter may not contain an analog filter sometimes (filterless). Read why NOS DAC is used, its types, advantages and disadvantages, oversampling vs non-oversampling DAC.

June 23, 2023 updated | since March 11, 2018

Author: Yuri Korzunov,
Audiophile Inventory's developer with 25+ year experience in digital signal processing,
author of the articles that make audio easy for beginners

 

 

 

Watch and share: Hi Res Audio [How it works. Sound quality. 7 Myths]

 

How Hi-Res Audio works video
 

What is NOS DAC? Why NOS DAC need

 

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

A digital filter causes ringing audio and passband magnitude and, sometimes, phase deviation. A non-oversampling digital-analog converter is used to rid oversampling artifacts, that the filter makes. It is the usual DAC without an oversampler and digital filter module.

Let's look at the picture.

NOS DAC. Non-oversampled digital-analog converter

NOS DAC. Non-oversampled digital-analog converter

 

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

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


Watch and share: What is DSD audio? Video explanation
video What DSD audio
 

The sample rate of NOS DAC is defined according to the analog filter stopband.

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

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

Digital versus analog filter

Digital versus analog filter

 

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

NOS DAC. Sample rate and analog filter

NOS DAC. Sample rate and analog filter

 

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

 

 

Non-oversampling DAC without analog filter (filterless)

Some NOS DAC may not contain an analog filter. It allows us to rid of the analog filter's frequency and phase distortions. Analog filter may be active (contains operational amplifiers, transistors, tubes). It means that an active analog filter can also cause non-linear distortions.

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

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

DAC output without analog filter

 

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

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

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

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

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

The level of the intermodulation products depends on non-linearity. Aliases have a level same to the 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 limitations. And the maximal level of signal, transmitted thru the analog parts, is defined by summary spectrum energy of the audio signal and its aliases.
I.e. aliases consume part of the dynamic range. In other words, aliases decrease useful dynamic range.

Does dynamic range decreasing give something useful? No. It reduces the maximal signal/noise ratio. And it's not good for lower-level signals especially.

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

Fortunately, analog parts of audio devices have band limitations due to capacitance and inductance of conductors and electronic components. It limits alias spread along the frequency axis via decreasing alias levels with frequency growth.

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

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

 

 

Non-oversampling DAC and upsampling

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

Upsampling on a personal computer (even a 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 in 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 vs offline conversion
Inline Offline
No need pre-convert (upsample) records Records should be pre-upsampled before playback
The required computing power of high-quality algorithms may be limited in real time. Due to a lack of power, the audio stream may be interrupted. Issues are probable with older computers No real-time - almost no computing power limits
For each playback, additional electric energy is consumed for resampling Electric energy for conversion is consumed once
Inline upsampled by the player audio stream should be sent to DAC without any processing (bit-perfect playback) Offline upsampled content of the audio file should be sent to DAC without any processing (bit-perfect playback)

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

Oversampling and/or filtering may be turned on and off, it gives a DAC "NOS mode" (non-oversampling and/or non-filtering).

We must remember, that oversampling may use a digital filter in addition to the analog one.

 

Advantages of a non-oversampling DAC

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

 

 

Disadvantages of a 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 consumption of dynamic range).

 

 

Oversampling vs non oversampling DAC

NOS DAC vs oversampling
Non-oversampling DAC Oversampling DAC
  There are ringing artifacts and other digital filter distortions
External audio file pre-upsampling may need Oversampling / upsampling and filtering in the digital domain before conversion to analog form inside DAC
R2R, sigma-delta modulated R2R, sigma-delta modulated
DSD DSD

 

 

How NOS DAC sounds

It is impossible to guarantee, that NOS DAC gives sound advantages.

As a device with simpler inner works (lesser amount of internal elements), the DAC gives 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 a 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 intermodulation may be more intensive in the audible frequency range due to non-linear distortions and "softer" filtering without a digital filter.

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

 

 

NOS DAC list

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

NOS digital-analog converter list
Manufacturer Model Sample rate, bit depth Remark
Acoustic Plan      
  DigiMaster 192kHz / 24 bit R2R, USB/SPDIF
Audial      
  S5 up to 384 kHz [Manual]
Audio-GD      
  R2R 1    
  R2R 2    
  R2R 7    
Audio Note      
  DAC 0.1x    
  DAC 1.1x/II    
  DAC 2.1x    
  DAC 4.1x    
  DAC 5 Special    
dB Audio Labs      
  Tranquility USB DAC    
Junilabs      
  Jundac Two    
Metrum      
  AMETHYST    
  ONYX    
  JADE    
  PAVANE    
  ADAGIO    
MHDT Labs      
  Atlantis    
  Pagoda    
  Stockholm 2    
  Orchid    
  Canary    
  Istanbul    
Mojo Audio      
  Mystique    
Promitheus Audio      
  Solid State DAC    
  Tube DAC    
Sakura Systems      
  Model 4705 Gemini    
TeraDak      
  TDA1543 NOS DAC-V2.6D    
  TDA1543 NOS DAC-V3.1D    
  TDA1543 NOS DAC-V4.0    
  TDA1543 NOS DAC-V4.1D    
  TDA1543 NOS DAC-V4.5    
Vinnie Rossi      
  LIO DSD/PCM DAC (with LIO DAC 2.0)    

 

 

Conclusions

Non-oversampling DAC. NOS DAC
  1. NOS DAC allows riding the ringing artifacts of a digital filter.
  2. Filterless NOS DAC allows riding analog filter distortions (phase, frequency, non-linear). But there are issues with high-frequency aliases, that waste dynamic range and can cause intermodulation distortions.
  3. Availability issues of records on their native sample rate of the non-oversampling DAC are probable.
  4. 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).
  5. For some audio systems, to reduce intermodulation distortions, ultrasound filtering during upsampling records may be applied.
  6. Also, DAC has other issues, that impact final quality (clock stability, power supply, etc.). So non-oversampling digital-analog converter scheme doesn't improve sound alone.


 

Frequently Asked Questions

What is the purpose of DAC? What does a DAC digital to analog converter do? What is the function of DAC?

DAC converts a digital signal to analog form (numbers to electrical oscillations). Analog signal is converted to acoustic waves.

Read more about:

 

Does a DAC make a difference?

DAC may significantly impact sound quality. Read more...

 

What is a NOS DAC?

NOS DAC is non-versampling DAC. Read more...

 

What is an oversampling DAC? What does oversampling do in a DAC? What is DAC oversampling? What is the purpose of oversampling?

Oversampling DAC increases sampling rate (oversample) of input digital audio signal to better filtering with an analog filter.

Read more...

 

What is the disadvantage of oversampling?

Oversampling uses analog and, sometimes, digital filters. Analog filters cause frequency and phase distortions. Digital filters cause also ringing distortions.

Read more...

 

What is non oversampling?

Non-oversampling is a DAC feature. It's the absence of oversampling before output analog filter.

Read more...

 

What is non oversampling DAC?

Non-oversampling DAC is a digital-to-analog converter without oversampling at the device output.

Read more...

 

 

 

 

What is NOS in DAC?

NOS in DAC is a non-oversampling ability. I.e., such DAC converts a digital signal to analog one, but oversampling before the analog filter isn't used. Also, the analog filter may be absent.

Read more...

 

What is a NOS R2R DAC?

NOS R2R DAC is a digital-to-analog converter based on resistor matrix (ladder) principle. R2R is a subtype of the matrix DACs, that is designed on resistors with 2 values: R and 2R.

Read more...

 

Are NOS DACs better?

NOS with analog filtering may work the same way, as the oversampling, at high sampling rates.

It's a matter of design of using DAC and sampling rate.

For low sampling rates 44.1 and 48 kHz, oversampling is required for proper analog filtering.

Read more...

 

Does a DAC improve sound quality?

DAC can't improve sound quality of a played-back recording. It can convert the recording to analog with higher or lesser distortions.

Read more...

 

How important is a DAC?

DAC is important and its "quality share" may be audible. The author would place DAC's importance after a loudspeaker with amplifier.

Read about sound file optimization for your DAC...

 

Does a DAC really make a difference?

Yes. The difference between DACs may be audible. Also, it's necessary to account a recording quality and other equipment in your audio setup.

Read more...

 

Are DACs good for movies?

In AV receiver or a home-theater unit, a built-in DAC is installed. It may be better or worse than stand-alone digital-to-analog converter. Such analysis requires special technical skills.

In general case, an audio comparison of the AV receiver with and without the DAC may be recommended.

Proper testing is described here...

But, it's too sophisticated and usual home comparison is enough to choose the decision.

 

Why do you oversample?

Oversampling can help to achieve better sound quality. Read details...

 

How much should you oversample?

Oversampling degree needs are depending on your DAC. Read details...

 

Does oversampling sound better?

Potentially, oversampling allows distortion reduction. However, it does not mean that any oversampled DAC sounds better than any NOS one. Because, oversampling is not a single factor.

Read more...

 

Is oversampling good or bad?

Oversampling causes distortions (noise, ringing). However, for accurate algorithms, it's almost transparent for use.

Theoretically, ringing may become a matter for audio stuff with sharp level altering (piano, guitar, etc.). At the time of the article writing, the author knows nothing about ringing-listening impact studies from safe sources.

Read more...
 

What is a DAC filter?

DAC applies low-frequency filtering of aliases. The aliases are excessive high-frequency products, that rise after conversion of digital signal to analog.

Read more...

 

Does oversampling reduce noise?

Oversampling in DAC fits analog signal for better filtering. It allows for eliminating intermodulation distortions in the audible range.

For higher sampling rate, quantization noise energy is distributed in a wider band. Noise level in the audible band is lower.

However, oversampling can't improve  signal-to-noise ratio of original signal.

Also, DAC's electrical noise in the output circuits may be upper than the quantization noise of digital signal.
And we don't get noise reduction at the DAC output.

Read more...

 

What is oversampling in ADC?

ADC (analog-to-digital converter) captures an analog electrical signal and transforms it into a digital one.

Quantization noise is combined with the electrical noise of the input circuits of the ADC. It's an input noise.

As rule, the capturing is provided at a sampling rate several times above captured range (oversampling).

It allows the distribution of the input noise energy in a wider band.

After it, in the digital domain, we can cut all frequencies above the useful captured signal band.

Part of the total noise energy will be cut too.

All these things improve the signal-to-noise ratio in the useful captured band. In instance, in the audible band.

Read more...

 

Are R2R DACS better?

R2R DACs have issues with non-linear distortions.

Read more...

 

Do I need a preamp with a DAC?

You can use pre-amp that built-in DAC or external amplifier. The result is individual for different device combinations.

 

Do I need a DAC or just an amp?

You can use an external DAC, if you find it sounds better than built-in one.

If you are satisfied with your DAC, you can use an amplifier for headphones or speakers connected to the DAC.

 

Will a DAC make YouTube sound better?

DAC defines sound quality. YouTube will sound better with a more qualitative DAC.

Read details...

 

Is Schiit Multibit R2R?

As the author knows, Shiit calls R2R DACs "true multibit". Here you can find the answer about Schiit Multibit R2R DAC...

 

Is the Schiit Bifrost an R2R DAC?

Yes. Schiit Bifrost is based on TI DAC8812 which is multiplying digital-to-analog converter.

Read more R2R DACS...

 

Who makes R2R DACs?

Check out who makes R2R DAC...

 

Why is oversampling bad?

Oversampling requires a digital filter application. The filter causes ringing audio.

However, ringing is not critical in many cases.

Read more...