How to Improve Sound Quality [Article]

Tutorials

In this article term "to improve sound quality" mean "to decrease sound playback distortions at analog output of digital-analog converter". Read below how to try to decrease the playback distortions for your audio equipment or portable device (digital audio player, mobile phone, etc.).

Disclaimer: Result of sound quality improving method, that described below, depend on used playback hardware and software and sound quality improving isn't guaranteed.

Digital-analog converter audio hardware may have different distortion level for different modes. Here DAC mode is combination of sample rate, bit depth, PCM/DSD.

Different distortions for different DAC modes

One or several modes may have minimal distortions. Below such mode is called as "the best sounding mode".

Audio file resolution may be converted to values (sample rate, bit depth, PCM/DSD) of "the best sounding mode" to improve playback sound quality (optimization of audio files).

Example:

DAC can be able provide lesser distortions for mode 192 kHz/24 bit/PCM than for 96 kHz/24 bit/PCM.
Therefore, conversion of audio file resolution to 192 kHz/24 bit/PCM is recommended for playback at the DAC.
 

Theoretically, higher sample rate and bit-depth cause lesser noise/distortion level. However, it is implementation matter rather.

"The best sounding mode" may be found via measurements of the test signals (see at picture above). However it demands precise measurement equipment and skills in the measurements.

In the article described step-by-step how to found "the best sounding mode" by ears. It is not precise way, unlike measurements. But ears allow approximately estimate the DAC's modes at home without additional measurement equipment.

In contrast, audio-file-resolution changing, that work together with DAC, dithering allow to increase sound quality of audio file as itself in some cases. Because non-linear distortions of quantization, initially correlated with audio signal, are smoothed and transformed to uncorrelated noise after dithering.

Dither can improve sound quality of audio file via correlated distortion decreasing

If DAC have level noise above quantization noise of audio file, dithering impact may be decreased or disappeared even. But modern DACs, as rule, have noise level below 16-bit error quantization and dithering noise. Therefore, for such DAC dithering may impact to perceived sound quality.

Below pro audio conversion software is suggested to convert audio files. It have low distortion noise. Theoretically, we can neglect quality losses of conversion (including non-multiple resampling) for pro class of audio converters.

However, there are may be different modes of the audio converter's resampling filters, that can cause different ear perception due different features:

• steepness of transient band,
• ringing,
• flatness of frequency response,
• phase response linearity,
• etc.

Different converters also have different features of the resampling filters.

Currently the author know nothing about reliable results of researches of resampling-filter-features impact to sound perception.

Also band width of filtration may impact to intermodulation distortions at analog output of DAC.

For purer experiment, audio playback software (audio player and system audio driver) should transfer digital audio data from file to DAC without changes of binary content. As example, preferable way for audio player software is using of ASIO or WASAPI output and turned off processing.

For hardware (devices) adjusting of settings for minimal/without sound processing is recommended.

About digital room acoustic correction read below.

Described below testing algorithm may be used for DAC, digital audio player, music server, any audio device, that have DAC chip.

1. Found several samples of audio files with different bit depth (16, 24 bit, DSF, DFF, ISO, etc.) for testing of your audio equipment.
    
2. Launch AuI ConverteR 48x44.
   For sound checking may be used AuI ConverteR 48x44 Free. Despite its limitations, it allow to estimate abilities of audio file optimization for your equipment or device.
    
3. Open source audio file(s) from step #1.
    
4. Go to AuI ConverteR's Settings > General:
    
5. Uncheck "Don't make DSP for unchanged sample rates" switch.
   
   
    
6. In Filter Mode list select "Optimized resampling filter".
   
   
    
7. Uncheck "Use minimum phase filter (without pre-ringing)" switch.
   
   
    
8. Click OK button.
    
9. Select one of output format: WAV, FLAC, AIFF. If DAC support DSD mode only, go to step #19.
   
   
    
10. Select output sample rate, bit depth.
    Recommended to begin for maximal sample rate and bit depth, that is native for your DAC.
    However, higher sample rate and/or bit depth don't guarantee better sound quality.
     
11. In AuI ConverteR's main window set Dither On.
    For both input and output bit depth upper 16 bit "Dither" button will be ignored.
     
12. Select new target directory
    Advice: use special name directory for recognizing of used conversion parameters during further testing.
    Example: "44100-16bit-DthOn", "DSD64-DthOn", etc.
     
13. Push Start button and wait for end of conversion.
     
14. In main window set Dither Off.
     
15. Select new target directory.
     
16. Push Start button and wait for end of conversion.
     
17. Repeat steps #10 ... 16 for all available sample rate and bit depth combinations, that available for your DAC or device.
     
18. Repeat steps #10 ... 17 for all filter modes (Settings > General > Filter Mode).
    For "Optimized resampling filter" test both checked and unchecked "Use minimum phase filter (without pre-ringing)" switch.
    For other filter modes this switch is ignored.
    
    
     
19. If your DAC support DSD mode, repeat steps #10 ... 18 for output format DSF.
     
20. Listen and compare similar files for all conversion modes: compare files with similar audio content, that placed in the different target directories (see steps #12 and #15).
    Conversion parameters of converted audio files, that have most preferable sound, you may use for conversion (optimization) of your library to playback on tested DAC.

In general, for the DAC mode comparison, author recommend turn off digital room acoustic correction. But you can compare results with the room correction and without.

Resume

If optimization of audio files give audible sound advantages in your case, you can convert your audio library to "the best sounding mode"'s parameters: sample rate, bit depth, DSD/PCM.

If in your case no audible sound advantages by optimization, there is not necessary to optimize your library.

It is strongly recommended, original audio files be backup. Because you, probably, will buy new devices or/and new resolution conversion algorithms will be released.

Yuri Korzunov,
Audiophile Inventory's developer,
09 May 2017

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