Bit Transparency

Benchmark Media Systems - October 03, 2014

Bit Transparency › Definitions › Wiki ›

Introduction

A digital audio device achieves bit-transparency if it passes digital audio data without affecting the data in any way. This device may be hardware, software, or a combination of both.

Testing for Bit Transparency

To determine if a device is bit-transparent, it can be tested by sending a pseudo-random bit sequence through it and monitoring the digital output. We typically generate the pseudo-random sequence using an Audio Precision digital signal generator. The Audio Precision includes a digital analyzer that is programmed to detect the pseudo-random sequence produced by the generator. The analyzer detects any differences between the generated sequence and the received sequence. The number of differences is tallied by a counter. A digital channel that is bit transparent will show no differences between the transmitted and received pseudo-random sequences.

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Dither

Benchmark Media Systems - October 03, 2014

Definitions › Dither › Wiki ›

Definition

Dither is a type of intentional variation (noise) which is added to a digital audio signal to avoid distortion caused by quantization errors.

Dithering in audio

Dither is often used when an analog signal is being quantized into a finite number of digital levels. It is also often used when a digital signal is being quantized into a fewer number of bits per sample corresponding to a fewer number of digital levels.

Dither randomizes the errors ...

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Computer Audio Playback Setup Guide

Benchmark Media Systems - October 03, 2014

Computer Audio › Wiki ›

Benchmark's Guide for Computer Audio
A simple guide to configure your computer and media player for optimal audio quality

Overview

Why is it important to configure my computer for music playback?

The trend of digital-audio storage and playback is growing and eminent. Since the CD was first introduced, consumers have embraced digital audio. The computer-age of media storage and playback has brought innumerable conveniences and features to the music-lover. With inexpensive hard drives and endlessly configurable media players, your entire music catalog is never further then a few clicks away. As the sonic qualities of digital recording technology has improved, audiophile appreciation for digital audio has grown as well.

However, it is often (incorrectly) assumed that digital-audio "signal-paths" never degrade the audio signal. It is important to realize that digital-audio sources (computers, CD and DVD transports, and other digital media devices) can cause severe distortion if not properly designed and/or configured. These imperfections can be caused by the software and/or hardware being used.

Computer-based playback carries a large set of unknowns and variables that can be very difficult to analyze and trouble-shoot. Beyond the obvious vulnerabilities of computers such as malicious software or hard-drive crashes, problems may exist where you least expect them - such as within the media player!

When using a computer for audio playback, it is very important to configure your software and hardware properly. Settings within the media player, operating system, and/or the device driver can have drastic effects on the quality of the audio being played. Digital distortion can be severely detrimental to the quality of the audio, even if it is DVD Audio, CD Audio, MP3, or any other format. Fortunately, there are simple, proven methods to disarm most of these defects. This article will provide the information needed to determine the best configuration for your computer-based audio.

Bit-Transparency: What is it and why is it important?

’Bit-transparency' (or 'bit-transparent playback') means the digital-audio data is not being modified or distorted as it is being streamed. In other words, every digital 'bit' of audio information remains unchanged throughout a bit-transparent data path.

 

A digital-audio data path is tested for bit-transparency by streaming a special digital audio signal through the data path, then monitoring the resulting data. The special digital audio signal is a pseudo-random sequence of bits. By comparing the resulting data bit-for-bit to the original data, it can be determined whether any bits have been modified.

Factors and settings that affect audio quality

It can be difficult to determine how audio is being affected within a software player, operating system, or other supporting software that runs concurrently. For this reason, Benchmark has extensively tested various media applications and operating systems. Our goal is to determine how the digital audio is being affected along the data path, and examine the resulting sonic quality of the audio.

To maintain quality during digital-audio playback, the following criteria must be accounted for:

These qualities are not completely independent of each other. For instance, 'sample-rate conversion' can be evoked because of 'multiple application mixing'. Also, 'digital volume control capabilities' are closely related to 'word-length capabilities' and 'dither'. Other relationships exist as well, and it is encouraged to read more on each subject.

Some general guidelines for configuring a computer for better audio performance

For the most accurate and specific information, it is recommended that you read the article regarding the specific media player and operating system you are using. However, you can use these general guidelines to begin improving the audio performance of your computer-based playback system:

Keep all volume controls at 100% (or 0.0dB, or 'unity gain', depending on the nomenclature)

If you are using iTunes and/or Mac OS X, set the sample-rate of the audio being played

When possible, keep word-length settings (also known as 'bit-depth settings') at 24 bits. (Even if you are playing 16-bit audio, it is recommended to keep 24-bit settings.)

Turn-off operating system sounds

Keep all DSP and plug-ins turned off

Certain applications, device drivers, etc., have various options such as "Surround Sound Simulations" or "Bass Boost" or "Sound Enhancer". It is highly recommended to disable all of these types of audio DSP and plug-ins.

Media Player Software Updates

Different Results from Different Versions and/or Updates of Media Player and OS

Benchmark has been investigating the performance of computer-based media playback platforms since the DAC1 USB was in design-phase. However, these tests have a ‘shelf-life’. The behavior of computer systems is incredibly dynamic because of the vast network of interaction between all the software and hardware involved. As such, every time a media player or operating system is updated, the previous tests can be disregarded. Also, it should not be assumed that the newer versions are better, or that any problems with the previous version were corrected.

Notes on Media Player Software Updates

Apple has made great improvements with their newest versions of iTunes. In fact, it performed excellently in all categories except one: following the sample rate of the audio file. Ideally, the software would operate at the sample rate of the audio file. It would automatically change sample rates whenever a file with a different sample rate was played. In other words, if a 44.1 kHz file is succeeded by a 96 kHz file, the software would switch from 44.1 kHz to 96 kHz when that file began. As it stands, iTunes will only play at the sample rate specified in the ‘QuickTime Preferences’ control panel found in QuickTime. Therefore, if iTunes is playing an audio file with a different sample rate from that set in QuickTime, the audio file will be sample-rate converted. Although the quality of the sample-rate conversion in iTunes is surprisingly good, it is an unnecessary DSP process which is best avoided.

Many Windows-based media players are capable of dynamically adjusting the sample-rate to that of the audio file. This is not the case with Mac, however, which has the same problem as iTunes. (This should not be surprising, however, since Apple makes iTunes and QuickTime).

On that note, the newest versions of iTunes on Mac also perform well. Its performance is satisfactory, even when sample-rate conversion is engaged.

Please continue to check these pages for updates.

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iTunes for Windows Setup Guide

Benchmark Media Systems - October 03, 2014

Audio Setup Guide › iTunes › Wiki ›

Benchmark's Guide for Audio Playback using Mac
A Simple Guide to Configure Your Mac for Optimal Audio Quality

Overview

iTunes uses the audio engine built into QuickTime to play its audio (and video) files. In other words, iTunes is a sort of media file manager that plays its media using QuickTime, just like a website might play its media with QuickTime. Therefore, the information in this section will apply to both players, and we will address both players simultaneously.

Certain information within this article is very specific to the version of the software being used. Please check your software before acting on this information.

Recommended Player Settings

Set word length and sample rate to the highest settings of the audio interface

Word length should be set to the highest that the audio interface is capable, regardless of the word-length of the audio file being played.

Sample rate can be set to match the sample rate of the media or to the highest that the audio interface is capable, since the upsampling in iTunes is harmless

The sample-rate conversion in iTunes 7.X or 8.X will not add significant distortion to the audio. The user should not be discouraged from setting the sample-rate to 96 kHz as a permanent setting, even when the audio is less then 96 kHz.

These settings can be set in QuickTime (even when using iTunes) by going to:
QuickTime->Edit->Preferences->QuickTime Preferences->Audio->Sound Out

For more information about why this is important, read more about sample-rate and word-length.

Set iTunes volume to "Full"

Volume attenuation may cause severe distortion, depending on which version of iTunes you are using.

The volume controls in iTunes versions 7.X or 8.X will not cause significant distortion. The user should not hesitate to use the volume control in iTunes v7.x or 8.X

This does not refer to the track-specific "Volume Adjustment" settings found in the "Get Info" menu. The "Volume Adjustment" setting should always be set to "None" for all tracks.

Read more about how digital volume controls affects audio

Benchmark's Guide for Audio Playback using Mac
A Simple Guide to Configure Your Mac for Optimal Audio Quality

Overview

Set the correct sample-rate and set word-length to 24 bit

These are set in the ’Audio MIDI Setup' control interface, which is in the 'Utilities' folder:

(Applications -> Utilities -> Audio MIDI Setup).

In the drop-down menu titled "Properties for:", select the output device which you are using

Under "Audio Output", the "Format" should be set to the appropriate sample rate and '2ch - 24 bit’.

For iTunes versions earlier then 7, we recommend setting the sample rate to match the sample rate of the media (music) being played

For iTunes versions later then 7, we recommend setting the sample rate to the highest sample rate that your device is capable of

For iTunes versions later then 7, iTunes must be launched after the sample rate is set in AudioMIDI. Any sample rate changes made in AudioMIDI while iTunes is open will not change the sample rate of iTunes until iTunes is re-launched. Consequently, it will cause CoreAudio to sample-rate convert the audio coming from iTunes. The result of CoreAudio sample-rate conversion is significant distortion.

Set iTunes volume to "Full"

Volume settings below "full" may cause severe distortion, especially on version 6 and earlier.

The volume controls in iTunes versions 7.X and 8.X will not cause significant distortion. The user should not hesitate to use the volume control in iTunes v7.x and 8.X

This does not refer to the track-specific "Volume Adjustment" settings found in the "Get Info" menu. The "Volume Adjustment" setting should always be set to "None" for all tracks.

Read more about how digital volume controls affects audio

Bypass all audio DSP and plug-ins (EQ and any other audio 'enhancer')

For iTunes, disable 'Sound Enhancer' and 'Sound Check'

These settings can be found in iTunes by going to:

iTunes->Preferences->Playback

How to play FLAC files in iTunes

 

Benchmark's guide for configuring Windows-based media players General strategies for setting up your computer for high quality audio playback

Overview

Windows Media Player (WMP) has extremely inconsistent behavior. Several issues have arisen among users, several of which remain unresolved. There is very little information available from Microsoft about the operation of this software. The information in this article is accurate to the best of our knowledge at the time of this printing. When further information is confirmed, this article will be updated appropriately.

Operation

If Windows Media Player does not stream audio to the appropriate output device, try restarting Windows Media Player. If this does not resolve the problem, open the following menu:

Tools -> Options -> Devices -> Speakers -> Properties -> Sound Playback

Choose the appropriate output device in this menu.

New info on Windows Media Player 12

Windows Media Player 12 (WMP12), which is only available on Windows 7 at the time of this writing, has the interesting quality of playing 24-bit audio files

New info on Windows Media Player 11

Recent tests with Windows Media Player 11 (WMP11) have brought to light some interesting qualities:

  • WMP11's volume control outputs (un-dithered) 16-bits with 'Library' playback
  • WMP11's volume control outputs 24-bits with HDCD playback
  • WMP11's volume control outputs 24-bits with playback of 24-bit audio files

This means that the volume control will cause distortion with 16-bit playback. It is recommended to keep the volume control at 100% for 16-bit audio playback.

Recommended Player Settings

SET VOLUME TO 100%

This volume control can be used in moderation without seriously affecting the quality of the audio. We recommend using an analog volume control (post D-to-A conversion) instead, as it will result in the best signal-to-noise ratio.

Read more about digital volume control, and how it affects the quality of your audio.

BYPASS ALL AUDIO PLUG-IN'S AND AUDIO 'ENHANCEMENTS'

Plug-in's should be bypassed by default, but to make sure, go to the plug-in menu uncheck any active plug-in's. Also, in this menu, choose 'Other' and make sure no Plug-In's are checked. The menu can be found by going to:

Windows Media Player -> View -> Plug-ins -> Options -> Plug-ins -> Audio DSP

Audio enhancements should be bypassed. Open the "Enhancements" window and scroll through to make sure each one is turned off. The menu can be found by going to:

View -> Enhancements -> Show Enhancements  

Read more about how DSP and plug-ins affect your audio

Known Problems

Will not play 24-bit audio

As mentioned above, older versions of Windows Media Player was not able to play 24-bit audio files without a third-party plug-in/codec.

The Nyquist Frequency is the highest frequency that can be represented by a digital system. It is exactly 1/2 of the sample rate. Frequencies above the Nyquist Frequency will be incorrectly represented in the digital system. Frequencies above the Nyquist Frequency alias (or fold back) into the region below the Nyquist frequency. A frequency 1 kHz above the Nyquist Frequency will fold back to 1 kHz below the Nyquist Frequency. Likewise a frequency 2 kHz above the Nyquist Frequency will fold back to 2 kHz below the Nyquist Frequency.

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This article describes the process of creating an 'Aggregate Device' with the Audio MIDI Setup utility in Mac OS X computers.

Creating an aggregate device is a way to group multiple audio interfaces into one virtual device so that an audio application can speak to a single, solitary virtual audio interface.

Certain audio software cannot interface to multiple audio device drivers. In these cases, it is necessary to create an aggregate device if you wish to use multiple interfaces. For example, interfacing with Logic simultaneously via a device using optical and another device using USB requires an aggregate device to be created encompassing these two devices. This new group of devices appears as a single aggregate device to the audio application.

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Windows 7 Audio

Windows 7, like Windows Vista, requires the user to set the sample-rate for streaming audio. In contrast, Windows 2000 and XP simply streamed at the sample-rate inherent to the audio file and/or media player being used.

In our testing, we found evidence that Windows 7 converted the audio to the sample-rate that is set in the 'Properties' menu for the playback device, unless the rate is set to the sample-rate of the audio being played. However, the sample-rate conversion within Windows 7 performs extremely well, causing no detectable amounts of distortion (below -140 dBFS).

Unfortunately, certain computers seem to have trouble maintaining a steady USB stream when running Windows 7 (even computers that worked perfectly with XP and/or Vista). We have found that turning off the 'energy saving' options will reduce the amount of interruptions in the USB stream.

This article will guide you in properly configuring your operating system and media player.

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