NOS DAC. Non-Oversampling DAC [Advantages, Disadvantages]
NOS DAC (non-oversampling DAC) is a digital-to-analog converter without oversampler. The converter may not contains analog filter sometimes (filterless). Read why NOS DAC is used, its types, advantages and disadvantages, oversampling vs non-oversampling DAC.
Before reading the article see R2R, SDM PCM and DSD DAC types >
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. Non-oversampling digital-analog converter is used to rid oversampling artefacts, that the filter makes. It is the usual DAC without oversampler and digital filter module.
Let's look to the picture.
NOS DAC. Non-oversampled digital-analog converter
- At the 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 of PCM and one of DSD DACs.
Sample rate of NOS DAC is defined according to analog filter stopband.
But 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
NOS DAC has no digital filter. Thus for non-oversampling DAC sample rate should be high enough, to provide more effective suppressing of the first (by frequency) group of aliases by the 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 analog filter. It allows us to rid of 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 output. Let's consider the spectrum of the "stairs".
DAC output without analog filter
(it is spectrum of the "stairs")
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 analog filter. And it is really so.
But there are no absolutely linear audio systems. It causes intermodulation distortions of ultrasound components. And intermodulation products by ultrasound components appear into 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, thansmitted 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.
Do 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 to dynamic range, it should be increased.
Fortunately, analog parts of audio devices have their own band limitation due to capacitance and inductance of conductors and electronic components. It limits aliase 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 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 into a musical player (software or hardware);
- offline into sample rate conversion software.
| Inline | Offline |
|---|---|
| No need pre-convert (upsample) records | Records should be pre-upsampled before playback |
| Required computing power of high quality algorithms may be limited in real time. Due lack of the power, audio stream may be interrupted. Issues are probable with older computers | No real-time - almost no computing power limits |
| 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 processings (bit-perfect playback) | Offline upsampled content of audio file should be sent to DAC without any processings(bit-perfect playback) |
In Windows operation system, to avoid processing between the player and DAC (bit-perfect playback), ASIO and WASAPI drivers are used.
Advantages of a non-oversampling DAC
- No digital filter ringing artefacts,
- Simpler inner workings,
- Potential ability to lesser distortions.
Disadvantages of a NOS DAC
- As rule, audio file pre-upsampling is necessary,
- Probable issues by insufficient alias filtering (audible products of intermodulation distortions and additional consumption of dynamic range).
Oversampling vs non oversampling DAC
| Non-oversampling DAC | Oversampling DAC |
|---|---|
| There are ringing artefacts and other digital filter distortions | |
| External audio file pre-upsampling may need | Oversampling / upsampling and filtering in 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 itermodiulation 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 contains mistakes and irrelevant information. Obligatorily ask DAC vendor/manufacturer about actual NOS ability.
| Manufacturer | Model |
|---|---|
| DigiMaster | |
| Audial | |
| Model S USB | |
| Audio-GD | |
| R2R 1 | |
| R2R 2 | |
| R2R 7 | |
| Audio Note | |
| DAC 0.1x | |
| DAC 1.1x/II | |
| DAC 2.1x | |
|
|
|
| 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
- NOS DAC allows riding the ringing artefacts of a digital filter.
- 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.
- Availability issues of records on their native sample rate of the non-oversampling DAC are probable.
-
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).
- For some audio systems, to reduce intermodulation distortions, ultrasound filtering during upsampling records may be applied.
- Also, DAC has other issues, that impact final quality (clock stability, power supply, etc.). So non-oversampling digital-analog converter scheme don't improve sound alone.
Frequently Asked Questions
What is an oversampling DAC? What does oversampling do in a DAC?
Oversampling DAC increases sampling rate (oversample) of input digital audio signal to better filtering with an analog filter.
Does a DAC improve sound quality?
DAC can't improve sound quality of a played back recording. It can convert recording to analog with higher or lesser distortions.
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.
Does oversampling sound better?
Potentially, oversampling allows distortion reduction. However, it's not means that any oversampled DAC sounds better any NOS one. Because, the oversampling is not single factor.
Is oversampling good or bad?
Oversampling causes distortions (noise, ringing). However, for accurate algorithms, it's almost transparent for using.
Theoretically, ringing may be matter for audio stuff with sharp level altering (piano, guitar, etc.). At the time of the article writing, the author know nothing about ringing-listening impact studies from safe sources.
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Author: Yuri Korzunov,
October 11, 2021 updated | since March 11, 2018,
Audiophile Inventory's founder, developer.