[Fire TV Q&A, Help & Troubleshooting] knreddy:
As on April, 2023 there are following methods:

The avialble applications can be categorised as follows'

I) Real Time Normalization

(a) Across the system: Viper4Android-Android, Realtek driver v6.0.1.6767, Date 30-10-2012-Windows, Realtek-Dolby Audio

(b) Application level: AIMP-Android, AIMP-Windows, double Twist v3.2.1-Android, Cloud Player v1.8.5-Android, many online streaming services

II) Replay Gain identification, linking and saving as separate file without even disturbing meta data: Neutron Audio Player - Android

III) Replay Gain Normalization (Replay gain writing at Meta Data it can be undone, if player is not compatible it will ignore Replay gain): mp3gain, foobar2000, R128Gain_1.0.11_win64_gui.

IV) Loudness conversion (can not be undone to original, such processes generally cannot be lossless): Audacity, etc many apps for windows as well as android.

Note: Perceived volume level or Digital “loudness” is measured as LUFS and it’s negative. Mp3gain 89dB = -18LUFS

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I.a.1 The following is experience sharing of Android with Viper4Android 2.7.2.1 on spare phone Xiaomi mi5 lineage 17.1 (Android 10) (Phone Volume 40 to 70%)

i) Enable "Master limiter"

Output -0db

Output pan 50:50

Threshold limit 0dB

ii)Enable "FET Compressor" with the following settings

operator threshold to -60dB (or -50 to -60dB)

Compression ratio to 10.00:1, (or higher like 20:1 upto infinite)

Automatic Knee untick it,

inflection: 10.8dB

inflection point gain 2.00x,

Automatic Gain-Untick,

Automatic Gain 25.6dB (or 20 to 40dB it will be more effective but in relation with operator threshold )

Automatic attack, untick

Attack to 10ms. (0.1 to 12ms)

Max attack 200ms, (10 to 250ms)

Automatic Release untick

Release 20 ms (0.2 to 25 ms)

Max Release 250 ms (2 to 300ms)

Crest 20dB,

Adapt 2639

Clipping prevention Tick

it is able to normalize loudness automatically tested qith audio volume 88.8dB (-14LUFS) and 105.2 db (-3LUFS), -36LUFS (dB as identified by MP3Gain PC software).

As normalization standard is available so many parameters are not required for audio normalization, once selected it can target -18 LUFS like Foobar plug in /double Twist.

Note:

(i) Even after Audio Modification libraries (AML) is installed, a4v needs SoundFX to be disabled. It is working on Legacy Mode. it works along with Dolby Atmos also.

(ii) James DSP is having Dynamic Range Compressor, but not having options to provide necessary inputs /it is not having them built in, it is not able to work as auto volume normalizer.

(iii) Any DSP expected to impure or introduce artifacts to the audio, we may not be able compensate by tweaking with v4a etc to the original. If you enjoy your own way, you may like it. Further if the function of Auto loudness / Auto volume level /auto volume normalization is at top priority , it succeeds to great extend android across the system. Tested with pink noise with digital loudness from -3LUFS to -23LUFS. It is noticed if bass is very high in original the above settings may not be suitable or difficult to adjust to universal setting.

(iv) There is no auto loudness normalization in "Dolby Atmos" in current versions. Dolby Audio is effective which is available at speaker control panel on windows 10.

in my case:

The Viper4Android 2.7.2.1 rootless type installed from TWRP (as magisk module is infinitely trying for installation of driver) even after installing Selinux 11, AML 4.2, Lineage's SoundFX disable or all in one type available at Github.

at present UNINSTALLER for above rootless type is not readily available.

There is noticeable deterioration in sound quality of low frequency with fet compressor of v4a.

I.a.2. or If you are on windows 10 with Realtek audio , install 2012 driver v6.0.1.6767, Date 30-10-2012 (old driver) it will give options “Enhancements" select loudness equalization ’ (sometimes it miss few milliseconds at start of track). This is real-time type as well as across the system, but it's effectiveness cannot be checked with pink noise. It is also noticed that later drivers having Dolby Audio , when made it on , it is also acting as loudness normalizer. However Dolby Atmos is not having any such feature.

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I.b.1 or use "double Twist v3.2.1.apk" (i.e classic) Android {for real-time normalization: Settings>Loudness Normalization>on} but it is not able to recognize all folders / audio files.

I.b.2. or "Cloud Player v1.8.5" for Android by double Twist Corp {for real-time normalization: Settings>Loudness Normalization>on (after buying)} but it is not able to recognize all folders / audio files.

I.b.3. Or use "AIMP" for Android or PC {for Real-time auto loudness normalization: options >sound effects > options > Volume Normalization> Replay Gain > select / Tick Replay Gain, Untick at Use values from tags (as not required for real time mode) , Tick on the fly Analysis> tick (=ok)}. Some times it misses by a few milliseconds at new track.(PC version's UI is not user friendly at present)

I.b.4 or Jet Audio HD Player Android {for Real-time auto loudness normalization: Menu > Preferences > Playback> select "Adjust Replay gain with AGC"}

I.b.5 Foobar2000 PC

Use "EBU or R128 Normalizer" plugin for Real-time auto loudness normalization ( install the plug in , add the same at DSP from preference. It is working ok. Tested with pink noise with digital loudness from -3LUFS to -33LUFS. Working perfectly when started with -33LUFS and going up , it's output was normalized to -18LUFS and verified with Youlean_Loudness Meter 2 vst3 plug-in installing to Foobar.

I.b.6 PC through installation of VST adapter: adding this adapter to DSP compnent to Foobar and using available VST plug in. At present volume normalizer for play purpose yet to be developed.

I.b.7 Many online streaming services

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II.1 "Neutron Audio Player" Android It writes replay gain information to a separate file with a reference for location and file name, so even meta data is also preserved. However getting into folder browsing is outdated {Neutron v2.12 Menu (3 horizontal lines) icon> Sources (do not select folders available in options, the following folders will linked /show up at the folders of neutron at later stage )> + > File system > Devices > SD card> now it will come to nearly modern browser mode..browse to required folder having music files...., select folder > select Tick > it will scan files}

(i) Addition of music folder to Neutron

{Neutron v2.12 Menu (3 horizontal lines) icon> Sources (do not select folders available in options, the following folders will linked /show up at the folders of neutron at later stage )> + > File system > Devices > SD card> now it will come to nearly modern browser mode..browse to required folder having music files...., select folder > select Tick > it will scan files

, if not hold on folder to get option to scan.(The files inside above folder is not responding/not scanning , delete above folder in above settings of neutron click and hold to get delete option (it is not real deletion at internal drive) , make back and forth , to get refreshed and re-create. If required

select other than previously used folder or super folder or some other sub folder. For using any other than sub folder you have to repeat above process.

{Neutron v2.21.9 Menu , just after fresh installation : select Audio Source as Manual>File System> + > Audio files folder or subfolder (as per your plan and scheme how you want them appear later) >Use This folder > ALLOW >select this folder > click on Tick Icon> Folder on drive will be added to folders of Neutron, if not successful add another folder or subfolder consisting audio files.

if above skipped or at later stage some more folders to be added : (3 horizontal lines with arrow) icon> Sources > + > File system > + > now it will come to nearly modern browser mode..browse to required folder having music files...., select folder > >Use This folder > ALLOW > select Tick >.....

(ii) (i) Deletion of music folder to Neutron

To delete folder in neutron app (No physical deletion on inernal drive) : (3 horizontal lines with arrow) icon> Sources > tap and hold on the folder > delete

(iii)Normalize the volume level all files in a folder to -18LUFS

{Menu> Sources> Folders > Click and hold on the required folder to get normalize option >+EQ Preset> Normalize > select Tick}

(Source file will not be affected , even metat data)

(iv) To remove above normalization

To delink above Neutron assigned Replay Gain through Normalize: {Menu> Sources> Folders > Click and hold on the required folder >click again on +EQ Preset}

Note: Some times icons /menu are relocated, please find them to get navigation options.

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III.1 mp3gain.exe {It is the most compatible with Windows as well as Android apps, it is a defacto standard for Replay Gain, until mp3 normalization standard is formed}

or III.2 Foobar2000 v1.6.16 {compatible with Foobar2000 player v.1.16.16, AIMP v3.30.1250 and VLC player v3.5.4 , Jet Audio HD v11.2.3, both windows / Android , USB Audio player pro trial 6.0.3 (Android) but not v6.1.0.0, Power Amp v3b957 (Android), FIIO Music v3.1.7 (Android) }

or III.3 R128Gain_1.0.11_win64_gui {compatible with AIMP and VLC player both windows / Android, USB Audio player pro trial 6.0.3 (Android) but not v6.1.0.0, Power amp v3b957 (Android)} program on PC (freeware) to write replay gain data at meta data of track (or album) without changing any music data i.e music will be as original. If the player is compatible with this replay gain, data stored at metadata will play the music file with normalized volumes.

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IV.1 Normalization open freeware DAW like Audacity etc.

Procedure to Loudness Normalization by Audacity

1)file>Browse to the required files > Open all required files,

2)Select all files,

3)Effect > Volume & Compression>Loudness Normalization>Perceived Loudness (Digital)>.....LUFS>Apply

4)File > Export >...... (processed file will be outlet folder)

IV.2 PC through VST3 for 5.1

Install VST3 plug-in e.g The Normalizer by FinalLoud by TBPro Audio VST3 manager. They are in advanced audio creation (DAW) etc. It will do permanent changes to music.

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In case of real time loudness normalizer, if it is not programmed to know track to track to information or analysis of sufficient length of buffer etc , it may intervene in between / intermittently causing a feeling of unnaturalness or distortion. Same difficult is similar with across system loudness normalizer.

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Due to any reason , if you don't want to listen to full length of song, but initial few seconds (like 30 sec or 120 sec or 180 sec or any) , automated preview of various varieties are available at "Automate Community" a forum of Automate by llamalab.