Oversampling, Upsampling Audio [How It Works and Sound]
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Oversampling, Upsampling Audio [How It Works and Sound]

Audio Basis - articles about audio

yuri korzunovOversampling (upsampling) is sample rate multiplication. As rule, oversampling is meant as multiple sample rate increasing. There is opinion, that non-multiple sample rate increasing cause cound degrading. Read how oversampling works and is sound advantages or disadvantages there.
Author: audio software developer Yuri Korzunov.

Why would you upsample?

As rule, you have multiple musical devices: players, DAC, etc. These devices supports different audio resolutions.

And, you are required too adjust audio file resolution to these devices.

In instance, you can optimize audio file resolution to DAC's sound quality abilities.

 

Oversampling vs upsampling

Any digital signal has sample rate that defines its spectrum (low, high frequencies of a sound, in instance).

Upsampling is an increasing of sampling rate on multiple or non-multiple ratio.

Oversampling is a multiple-ratio upsampling.

Though, these terms have no exact definition.

Upsampling may be done via interpolation (spline math function, as example).

But, multiple upsampling (oversampling) may be done by inserting zero-samples. And, it may cause lesser non-linear distortions in proper applications than the interpolation.

In other hand, non-multiple upsampling may be done via multipication and dividing to integer ratio.

You can read details below.

 

Upsampling

Digital signal is sequence of signal values. The values are situated equal time-periods from each other.

When we playback the sequence, a loudspeaker produces oscillations (sound). In the first approach, each value is an air-pressure to eardrum in current moment.

Interpolation is method, that can find "virtual" values between real values. Virtual value is calculated by available real values.

The method has non-linear distortion issues that defined by method type, number of real values, etc. But, interpolation allows non-multiple altering of the sampling rate.

Alternatively, non-multiple rate may be done via multiple oversampling and decimation of the sampling rate.

In instance, 44.1 to 48 kHz may be done as  44.1 kHz x 160 / 147 = 48 kHz.

And, 48 kHz x 147 / 160 = 44.1 kHz.

 

 

Oversampling

Usually multiplexing of sample rate released as inserting ZERO SAMPLES (virtual samples) between "real" source samples.

Oversampling. Upsampling

 

After oversampling spectrum mirrored to upper half of old sample rate fd. So we get mirrored copy of source audio spectrum (upper part of picture).

Oversampling - sample rate miltiplying

 

Resampling filters

It is non-linear distorttions in output (oversampled) audio signal.

For getting non-distorted (almost non-distorted, of course) oversampled audio signal, after oversampling need apply low frequency filtration with band from 0 to fd/2.

Filtering audio signal after oversampling

 

 

Oversampling vs upsampling

As rule, oversampling means multiple sampling rate increasing.

Upsampling means arbitrary sample rate increasing.

Upsampling may be applied via spline function. It means calculation sample values between real samples. Or upsampling may be applied via multiple over- and downsampling.

 

 

Sound quality

Sound quality of oversampling or upsampling defined by resampling filters.

In ideal case, all mirrored audio content should be suppressed, but audio content below [input sampling rate]/2 should be removed.

But there is no ideal resampling filters. And unwanted mirrored audio content folded to audible range and cause distortions.

Also, digital filters cause ringing and frequency distortions.

However, author don't know about serios studies, that prove actual ringing harm. And frequency distortions may be reduced significantly below human perceptions.

Digital filter have several contradictory features. And filter design is balance of the parameters, that is art of development.

So, with proper resampling filters, we get almost clean oversampled source audio signal (music).

 

Read more:

 

How to upsample audio

To upsample audio:

  1. Start audio converter software AuI ConverteR 48x44.

  2. At the main window, click Open files button and select source audio DSD and/or PCM files.
     
  3. At the right of the main window > at Format panel select target audio format (WAV, FLAC, etc.), sample rate, bit depth.
     
  4. Select target directory for converted files
    In the main window (left lower part) > Directory output files field.
    Read more...
     
  5. Push Start button.
     
  6. Wait for conversion end. Converted files are placed into the directory assigned in goal 4.

 

Frequently Asked Questions

Is upsampling audio good?

It's important to remember, that upsampling is information lossy process. However, proper resampling has minimum losses, as rule, below human perception.

On the other hand, we can optimize your music file's resolution to used DAC to reduce playback distortions. Read more...

 

What is upsampling audio?

Upsampling is increasing of the samling rate. Read more...

 

Does upsampling degrade audio?

Upsampling causes losses of information because resampling filter is used. However, high-quality filters causes extremely low distortions. And it may not be considered.

For audio, music-file upsampling may be used as way to reduce DAC distortions. Read more...

 

 


Read about sample rate issues

 

August 3, 2022 updated | since June 19, 2014

 
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