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What is Audio Converter?

Audio Converter converts audio files from one format to another, using ffmpeg as its engine. You add files, pick a preset, and convert; the result lands next to the originals or in a folder you chose. It reads whatever ffmpeg can decode and writes eighteen formats, from MP3, AAC, FLAC, Opus and Ogg Vorbis to WAV, AIFF, WMA and the iPhone ringtone format.

What sets it apart is that three choices are made separately, each from its own drop-down in the options panel at the right of the window: the preset (what the output is), the tags setting (which tags and cover are kept) and the effects setting (what is done to the sound). One preset can therefore be used with a setting that strips the comments, with another that normalises the volume, or with neither. Tags are also carried from one format to another under the names a tag editor shows, so a field keeps its meaning from FLAC to MP3 to M4A to WMA.

The main window has the list of files, a header with the file actions, a panel at the right with the choices that shape a run - preset, tags, effects, destination and what to do when a file exists - and, at its foot, the number of conversions at once, Convert and the button that opens the output folder, and a bottom bar with the status line.

Adding files

Build the list of files to convert by picking files, picking a whole folder, or dropping them on the window. Every row shows the picture embedded in the file as a small thumbnail, or a music note when there is none.

• Add files opens a file picker. Only audio files ffmpeg can read are offered.
• Add folder adds a folder; the audio files in it and in every folder below it are found. The folder's name is remembered so the structure can be rebuilt in the destination (see Destinations).
• Clear the list empties the list of every row that is not being converted.
• Remove from the list is the button of a single row; while a row is being converted, the same button cancels it instead.

The list

A file is added once; adding the same file again does nothing. A row that is Done, Skipped, Failed or Cancelled stays in the list and is converted again, with the current settings, the next time you press convert.

Converting

Press convert to run every row that is not already queued or running. Each file is its own job, so one failure never stops the others, and a row that failed says why.

• Convert converts the list with the preset, tags setting, effects setting and destination currently selected.
• Stop appears while a run is going and cancels every file that is queued or converting; a partly written output is removed.
• Cancel on a row cancels that one file.

What a row's badge says

A row moves through its states in order, and the badge shows the one it is in.

• Queued waits for a free place.
• Reading is the file being examined.
• Analysing followed by a percentage is the measuring pass of the effects (see Analysing); it only appears when an effect needs numbers.
• A percentage is the conversion itself, measured against the length the output will have.
• Done finished; the row also shows where the file went.
• Skipped the output already existed and the destination says to skip it.
• Failed something went wrong; the reason is shown on the row.
• Cancelled you stopped it.

The status line at the bottom left says how many files are ready before a run, how many are processed during it, and afterwards the result: how many were converted, skipped and failed.

Files at the same time

The number next to Convert, in the last row of the options panel, chooses how many files are converted at once, from one up to the number of physical cores of the machine, which is also the default. Encoding is limited by the processor, so more processes than cores only compete for the same core and the same disk: on an eight-core, sixteen-thread machine sixteen at once measured slower than eight, so nothing above the physical cores is offered. Lower it when the computer must stay responsive.

• Files converted at the same time is its name when you hover over it.

When convert is greyed out

Convert is off when the three choices of the options panel (preset, tags and effects) together would change nothing: the preset is (Original), the tags setting removes no tag and no cover, and no effect is selected. Converting would only re-encode each file, which loses quality for nothing. Pick another preset, a tags setting that removes something, or an effect, and it comes back. It is also off while no file is listed, while a run is going, and when ffmpeg was not found.

Presets: the format and its quality

A preset says what the output is: the format, how good it is, and the options that format has. It is chosen in the first drop-down of the options panel and edited on the Presets tab of Settings, which the button next to the drop-down opens.

• Edit presets opens the Presets tab.
• New, Duplicate and Delete add a preset, copy the selected one and delete it. There is always at least one preset.
• A preset is moved by dragging its row; the same order is used in the panel's drop-down.

A preset

A fresh install comes with a set of ready presets (MP3 at 128, 192 and variable high, AAC, Opus, WMA, AC-3, an iPhone ringtone, Ogg Vorbis, FLAC, WAV 16-bit and AIFF 16-bit). They are ordinary presets: rename, edit or delete them. An iPhone ringtone is cut to 40 seconds, the longest iOS accepts.

• Name is the name shown in the drop-down.
• Format is the output format. The options below follow it: a format only shows the controls it actually has.

Quality

Every format has one main quality setting, a number field with its unit beside it, and its meaning follows the format: a bitrate in kbps for MP3, AAC, Opus, WMA and AC-3; a variable-quality level for Ogg Vorbis and variable MP3; a compression level for FLAC and WavPack, which changes only how long encoding takes and how small the file is, never the sound. WAV has no quality setting. A higher bitrate or level means a larger, better-sounding file.

• Bitrate mode (MP3 and Ogg Vorbis) chooses how the encoder spends its bits: Constant is the same bitrate throughout, Variable follows the music for a better size-to-quality ratio, and Average (ABR) (MP3 only) aims at an average.
• Bitrate is the bitrate for the constant and average modes.
• Quality level is the level for the variable mode.
• Compression is the compression level of FLAC and WavPack.

Sample rate, channels and bit depth

These describe the shape of the audio. Left on keeping the source, the output has the same values as the input; changing them resamples or mixes. Only the values a format accepts are offered.

• Sample rate is how many samples per second the output has.
• Channels is Keep the source, Mono (both channels mixed into one) or Stereo.
• Bit depth is the size of a sample for the formats that have one (FLAC, ALAC, WAV, AIFF and a few others).

Dither

When a 24-bit recording is made 16 bit, the samples are rounded and the rounding leaves a faint distortion that is audible on quiet passages. Dither adds a very small amount of noise first, which turns that distortion into a steady hiss far below the music. Plain rounding is the default and is fine for most music; dither is for quiet, high-resolution masters. It is never applied to a 24-bit output.

• Dither (when the output is 16 bit) appears when the output is 16 bit.

The ID3 tag version

MP3, AIFF and WAV outputs carry an ID3 tag, in version 2.3 or 2.4. Version 2.3 is read by every player and is what Mp3tag writes by default; 2.4 is newer and stores text as UTF-8. Both keep a repeated field as separate values. Leave it on 2.3 unless something needs 2.4.

• ID3 tag version chooses the version.

Advanced options

Below the usual controls a collapsed section holds the switches an encoder has beyond its quality setting. Left alone, the encoder decides, and a preset that never opens the section produces the same file as before. Leave them alone unless you know what you are after.

• Advanced options is the section; each entry is Automatic until you set it.
• MP3: Joint stereo, Encoder quality (0 best, 9 fastest), Low-pass filter (Hz), High-pass filter (Hz), Use the bit reservoir, Set the copyright flag and Set the original flag.
• Ogg Vorbis: Minimum bitrate (kbps) and Maximum bitrate (kbps) switch it to a bitrate-targeted mode.
• Opus: Optimise for (music, voice, or low delay), Bitrate mode, Frame size, Complexity (0 to 10), Bandwidth and Intensity-stereo phase inversion.
• FLAC: Block size, Stereo mode, Maximum LPC order, LPC coefficient precision, Minimum partition order, Maximum partition order and Exhaustive model search.
• AAC: Bandwidth limit (Hz) limits the highest frequency kept.

(Original)

The first choice of the preset drop-down is not a preset: (Original) converts each file to the format that keeps its own audio codec, at the bitrate it had. It is for jobs where only the tags or the sound should change, not the format. A codec that no output format holds (APE, for instance) fails that one file, with a message on its row. It cannot be edited or deleted.

Destinations: where the files go

A destination is a folder plus a pattern that names the files, chosen in the options panel and edited on the Destinations tab of Settings.

• Edit destinations opens the Destinations tab.
• Where the converted files are written is the destination drop-down of the options panel.
• Name is the name shown in the drop-down.
• Folder is where the files are written, and Browse picks it.

Source Folder

The first entry, Source Folder, is built in: each output is written next to its original and keeps its name, with the new extension. It needs no folder and cannot be edited.

File name pattern

The pattern names each output from the tags of its own file. Click a tag to add it; a slash makes a sub-folder. The tags are the name of the original file, the title, the artist, the album artist, the album, the track number, the disc number, the year and the genre.

• File name pattern is the pattern. A tag the file lacks makes an empty part that is dropped, except that a missing artist or album reads "Unknown Artist" or "Unknown Album" and a missing title falls back to the original file name.
• Example shows what a made-up file would be called, so you can check a pattern before using it.

A pattern of artist, then album, then track and title with a dash between them puts a track tagged Björk, Homogenic, 3, "Jóga" at Björk/Homogenic/03 - Jóga.mp3; the track number is padded to two digits, and characters a file system forbids are replaced.

Keep the folder structure

When a folder is added, its sub-folders can be rebuilt under the destination, which keeps an album tree an album tree. The added folder's own name is kept too, so two added folders stay two folders. A file added on its own has no folder to keep.

• Keep the folder structure when converting a folder is on by default.

If the file exists

What happens when an output with that name is already there. An output is never the input file itself.

• What to do when the output file already exists is the drop-down in the options panel.
• Skip it (the default) leaves the existing file and marks the row Skipped.
• Overwrite it replaces it.
• Keep both writes the new file beside it under a numbered name, name (2).mp3.

Open the output folder

• Open the output folder opens the folder of the newest finished file, and before anything is finished the destination's own folder.

Tags: what is kept and what is removed

Tags are the title, artist, album, year, track number and the rest that travel inside a file, along with its cover picture. The tags setting decides which of them reach the output. It is chosen in the second drop-down of the options panel and edited on the Tags tab of Settings; the encoding and the metadata to drop are chosen independently, so one preset serves both.

• Tags is the drop-down.
• Edit tags opens the Tags tab.
• Name is the name of an entry.
• Entries are added, copied, deleted and dragged into order the way presets are.

Keep all, or delete all

An entry has a mode and a list of exceptions.

• Keep all tags keeps every tag; the list holds the ones to delete (Exceptions: tags to delete).
• Delete all tags deletes every tag; the list holds the ones to keep (Exceptions: tags to keep).
• Tags is the choice between them.

A fresh install has one entry, Keep all tags, with an empty list. To strip a noisy field, keep the default mode and list it; to keep only the basics, switch to deleting everything and list TITLE, ARTIST, ALBUM and TRACK.

Naming the tags

Write one name per line, or separated by a semicolon, as your tag editor shows it. Names are not case sensitive and an asterisk matches any text, so REPLAYGAIN_* matches every ReplayGain field. A name matches however a format spells the field: the editor's name (ENCODERSETTINGS), the ID3 frame id (TSSE), the Vorbis name, or the description of a custom field. While you type, the known field names that fit are offered under the box; click one to use it. Any name is accepted, offered or not.

The cover picture

A picture is not a tag name, so it has a switch of its own, and a mode that deletes all tags never touches it.

• Remove the cover picture leaves the cover out of the output.

How tags travel between formats

ffmpeg alone would rename the fields it does not know and drop some others. Audio Converter writes each field under the name a tag editor uses for that format, so the album artist, the sort fields, the composer, the BPM, the lyrics, the ISRC and custom fields keep their meaning from format to format. Fields with several values keep them as separate values, a track or disc number is written with its total (3/10) in the formats that do so, and a year that a tag stored badly is not lost. The cover goes along to every format that can hold one; the exceptions are FLAC in Ogg, the iPhone ringtone and AU, which carry none. Raw streams (AC-3, raw AAC, MP2, Wave64) hold no tags and no cover at all.

(None)

The first choice of the tags drop-down, (None), is not an entry: it keeps every tag and the cover with no exception. It cannot be edited or deleted.

Effects: what is done to the sound

An effects setting is a named list of things done to the audio while it is converted: cut, fade, level, clean up, reshape. It is chosen in the third drop-down of the options panel and edited on the Effects tab of Settings.

• Effects is the drop-down.
• Edit effects opens the Effects tab.
• Name is the name of an entry.
• Add an effect is a menu that offers the effects not yet in the list, grouped under Time, Level and Sound with each group in alphabetical order; choosing one adds it with its default values.
• Remove this effect removes an effect from the list.

An effects setting

Being in the list means an effect is on, and removing it turns it off; there is no separate switch. Each effect appears at most once. On the page each effect is a title that folds away, so a long list can be reduced to its titles. A fresh install has one empty entry, No effects, that changes nothing, and entries are added, copied, deleted and dragged into order like presets.

The order effects run in

The order is fixed by the kind of effect and shown on the page: trim silence, keep a range, limit the duration, channels, repair, low cut, noise, de-esser, equaliser, bass and treble, dynamics, tempo, volume, fade in, fade out, silence at the end. Everything that changes the length or the sound comes before the volume, so the level is set from what the track will really be, and the fades and the silence come last so nothing moves them.

Analysing

An effect that needs numbers - where the sound starts and stops, how loud it is - makes the file be read once or twice before it is converted, and the row shows Analysing with a percentage. The loudness is measured on the part that is kept, through the effects above the volume. The conversion that follows gets exact values, and its percentage is measured against the length of the output, so a trimmed or limited file still reaches one hundred percent. Fades, a range and a fixed gain need no reading.

(None)

The first choice of the effects drop-down, (None), is not an entry: it applies no effect. It cannot be edited or deleted.

Time

These decide which part of each file is kept and how long it is. They work one after the other on what is left.

Keep a range

Keeps only the part between two times, counted from what the effects above it left. It removes an intro or an outro, not only the tail. A range that falls outside the track changes nothing.

• Keep a range is the effect.
• From is where the kept part starts.
• To (0 = to the end) is where it ends; 0 means to the end.

From 10 to 40 of a four-minute track keeps thirty seconds.

Limit the duration

Keeps at most the given number of seconds from the start, after the range. It is what makes a preview or a sample.

• Limit the duration is the effect.
• Seconds is the length.

Silence at the end

The song is cut where its trailing silence begins, and then exactly the number of seconds you set follow. Whatever the track's own ending was, a tail of a third of a second, of three seconds or none at all, every track ends the same way, so an album played in order is separated in any player without a player setting. The silence is real samples in the file.

• Silence at the end is the effect.
• Seconds of silence after the song is how much silence follows the song.
• Keep this much of the old ending is a little of the old ending that is kept so a quiet decay is not clipped; it counts in the seconds above.
• The level that counts as silence is Counts as silence below, as for trimming.

With two seconds set, a song of three seconds followed by 3.5 seconds of silence comes out as three seconds and two seconds.

Tempo

Plays the track faster or slower with the pitch kept, from half speed to double. The track becomes shorter or longer, and the fades, the silence at the end and the progress all follow the new length. A change of a quarter is already clearly audible.

• Tempo is the effect.
• Change (negative = slower) is the change: 25 is a quarter faster, -20 is a fifth slower.

A ten-minute track at +100 % is five minutes.

Trim silence at the ends

Exports often carry silence before the sound and after it. This cuts both, or one of them, and never touches silence inside the song: a gap between two movements is part of the music.

• Trim silence at the ends is the effect.
• Trim the start and Trim the end choose which end is cut.
• Counts as silence below is the level below which sound counts as silence.
• Keep this much silence at each cut keeps a little silence at each cut so a quiet attack or decay is not clipped.

A recording of two seconds of silence, a song, and two seconds of silence comes out as the song with two small margins.

Level

These change how loud the sound is, and how that changes over time.

Dynamics

A compressor lowers what is louder than a threshold by a ratio, making a wide-ranging track easier to listen to at one volume; a dynamic normaliser evens the level out over the whole track. Either one can ruin a track if pushed, so use small amounts.

• Dynamics is the effect.
• Compressor and Dynamic normaliser are the two modes.
• Threshold is the level above which the compressor works.
• Ratio is how strongly: 3 means three decibels in become one out.

Fade in and fade out

A fade in raises the sound from silence at the start of what is kept; a fade out lowers it to silence at its end. AI-made tracks and cut tracks often begin or end abruptly. The fades come after the volume so no gain moves them, and a fade longer than the track is the whole track.

• Fade in is the effect.
• Fade out is the effect.
• Length is the length, in milliseconds, up to 9999: 2000 is two seconds.

Volume

Volume is one effect with four modes, set in the parts below; the limiter and the album gain belong to it.

Four ways to set the volume

The volume is one effect with four modes, never stacked, because two gains in a row have no use.

• Volume is the effect.
• Mode chooses the way.
• Normalise to a loudness brings the integrated loudness, in LUFS, to a target, so tracks sound equally loud. Target loudness is the target: 16 means -16 LUFS, a common level for music.
• Normalise to a peak brings the highest sample to a level, which makes a file as loud as it can be without clipping but says nothing about how loud it sounds. Peak level is that level.
• ReplayGain: apply to the audio applies the ReplayGain gain to the sound, the same figure the tags would carry.
• Fixed gain is exactly the gain you ask for: Direction is Louder or Quieter and Gain is how much.

Measured against a file of -22 LUFS, a target of -16 raises it by six decibels.

The limiter

A louder track must not clip, so every gain that could push peaks over full scale is followed by a peak limiter that holds them under a ceiling. The peak mode needs none because its result cannot exceed its target.

• Peak ceiling is the ceiling, as a distance under full scale. Leave it at 1 dB unless a delivery specification says otherwise.

Album gain

ReplayGain figures can describe a whole album, so tracks keep their loudness relative to each other, which a track gain would flatten. The album is the files of one run that sit in the same folder and carry the same album tag, measured as one continuous program, once per album. A file with no album tag has no album and uses its track gain.

• Use the album's gain (same folder, same album tag) makes the ReplayGain mode use the album's gain.

ReplayGain tags

Writes the track gain and peak as tags and leaves the sound untouched. Players such as MusicBee level the volume from them. The figures describe the file that is written, after any gain the volume effect applied, and they are added after the tags setting has done its work, so a tags entry that deletes REPLAYGAIN_* removes the source's old figures and cannot remove the new ones. They are written in every format that holds tags.

• Write ReplayGain tags is the effect.
• Also write the album's gain also writes the album's gain and peak.

Sound

These change what the sound itself is like: its balance, its frequencies and its noise.

Bass and treble

Two controls instead of ten bands: raise or lower the low end and the high end, in decibels. Small moves first.

• Bass and treble is the effect.
• Bass and Treble are the two gains.

Channels

Swaps left and right, plays one channel on both sides (a mono source heard on both sides too), or mixes both into one. It fixes a swapped recording or makes a mono file. The preset's own channels option can also produce a mono or stereo file; this one is for the cases the preset cannot express.

• Channels is the effect.
• Swap left and right, Left channel on both sides, Right channel on both sides and Mix into one channel (mono) are the four ways.

De-esser

Softens harsh "s" sounds in a voice. Raise the intensity until the harshness goes; it is meant for speech and vocals.

• De-esser is the effect.
• Intensity is how strong it works.

Equaliser

Ten bands, an octave each, from 31 Hz to 16 kHz, drawn as ten vertical sliders on one scale from -12 to +12 dB, with a line across them at zero. Drag a band to set its gain: the fill between the line and the handle shows a boost above it and a cut below it, so the shape of the curve is the shape you see. The arrow keys move a focused band by one dB, Page Up and Page Down by three, Home and End to the ends, and a double-click puts a band back at zero. A band left at zero changes nothing.

• Equaliser is the effect.

Low cut

Removes everything below a frequency: rumble, mains hum and the constant offset of old transfers. Only worth using on such material.

• Low cut is the effect.
• Cut below is the frequency.

Reduce noise

Lowers steady noise, such as hiss or a fan, by an amount you give, assuming a noise level. It is good on steady noise and can leave artefacts on music, so try a small reduction first.

• Reduce noise is the effect.
• Reduction is how much the noise is lowered.
• Noise level is the level the noise is assumed to sit at, as a distance under full scale.

Repair clicks and clipping

Removes short clicks and rebuilds peaks that were cut flat by an overloaded recording, each on its own switch. It is real restoration, so it takes longer than the other effects; use it on files that need it.

• Repair clicks and clipping is the effect.
• Remove clicks removes the clicks.
• Repair clipped peaks rebuilds the clipped peaks.

The window: language, theme, notifications, updates

The gear in the header opens Settings, which holds the Presets, Effects, Tags and Destinations tabs described above, and the tabs of the window itself.

• Open settings opens Settings.
• Language is the language of the window.
• Theme is Light or Dark.

The notice when a run ends

When nothing is queued or converting any more, one operating-system notice says how many files were converted, skipped and failed. A run that was only cancelled shows none. It appears whether or not the window has focus.

• Show notification when conversion is finished on the Notifications tab turns it off.

A new version

On launch the app asks whether a newer version exists and, if so, shows a banner. It only notifies: nothing is installed by itself, and a failed check is silent.

• New version available is the banner, with Download, What's new and Skip this version. Skip remembers the version, so the banner returns only for a newer one.

The command line

Next to Audio Converter the installer puts a small program, yac (yaiol audio converter), that does the same conversion from a terminal. It reads the same saved presets, tag settings, effects and destinations as the window, so a preset made in the window is a preset on the command line, and the files it writes are the same.

yac -p "MP3 320 kbps" -d Export Music/
yac -p FLAC -t Clean -o D:/Converted Album/
yac --list

• Give it files and folders; a folder is searched, with its sub-folders.
• -p names a saved preset (original keeps each file's own format), -t a saved tag setting, -e a saved effects entry and -d a saved destination. A name is matched without regard to case, and the id works too.
• Left out, -t and -e mean nothing is changed: every tag and the cover are kept, no effect is applied, whatever the window has selected. Left out, -p and -d are the window's current choice.
• -o writes into a folder. Together with -d it is that destination's file-name pattern and folder structure, written into that folder.
• -j is how many conversions run at once, up to the machine's physical cores, --overwrite is skip, overwrite or rename, and -q prints only failures and the summary.
• --list shows what each of these can name, with the window's current choice marked.

It prints one line per file and the same summary as the window, such as 3 converted · 1 skipped · 2 failed. The exit code is 0 when nothing failed, 1 when a file failed and 2 for a mistake in the command, so a script can wait for it and act on the result.

To run it from any terminal, tick the last option of the installer, which adds its folder to your PATH, or call it by its full path in the install folder. The option is off by default and only changes your own PATH.

Technical reference

Facts for anyone who needs to know exactly which name a field gets in which format. None of it is needed to use the app. The tables below are generated from the table the app itself uses, so they cannot drift from it.

Field names by tag standard

One row per field, one column per standard. Each cell is the name the field has there: an ID3 frame id, an MP4 atom, a WMA attribute, a Vorbis comment or APE item name, or a WAV INFO id. A dash means the standard has no name of its own for the field; the app then keeps it as a custom field, under the field's own name, so the value is not lost.

Field            ID3v2.3  ID3v2.4  MP4 atom  WMA attribute               Vorbis / APE     WAV INFO
---------------  -------  -------  --------  --------------------------  ---------------  --------
ALBUM            TALB     TALB     ©alb      WM/AlbumTitle               ALBUM            IPRD
ALBUMSORT        TSOA     TSOA     soal      WM/AlbumSortOrder           ALBUMSORT        -
ALBUMARTIST      TPE2     TPE2     aART      WM/AlbumArtist              ALBUMARTIST      -
ALBUMARTISTSORT  TSO2     TSO2     soaa      -                           ALBUMARTISTSORT  -
ARTIST           TPE1     TPE1     ©ART      Author                      ARTIST           IART
ARTISTSORT       TSOP     TSOP     soar      WM/ArtistSortOrder          ARTISTSORT       -
BPM              TBPM     TBPM     tmpo      WM/BeatsPerMinute           BPM              -
COMMENT          COMM     COMM     ©cmt      Description                 COMMENT          ICMT
COMPILATION      TCMP     TCMP     cpil      -                           COMPILATION      -
COMPOSER         TCOM     TCOM     ©wrt      WM/Composer                 COMPOSER         -
COMPOSERSORT     TSOC     TSOC     soco      -                           COMPOSERSORT     -
CONDUCTOR        TPE3     TPE3     ©con      WM/Conductor                CONDUCTOR        -
CONTENTGROUP     TIT1     TIT1     ©grp      WM/ContentGroupDescription  CONTENTGROUP     -
COPYRIGHT        TCOP     TCOP     cprt      Copyright                   COPYRIGHT        ICOP
DISCNUMBER       TPOS     TPOS     disk      WM/PartOfSet                DISCNUMBER       -
ENCODEDBY        TENC     TENC     ©enc      WM/EncodedBy                ENCODEDBY        -
ENCODERSETTINGS  TSSE     TSSE     -         WM/EncodingSettings         ENCODERSETTINGS  -
GENRE            TCON     TCON     ©gen      WM/Genre                    GENRE            IGNR
INITIALKEY       TKEY     TKEY     -         WM/InitialKey               INITIALKEY       -
ISRC             TSRC     TSRC     -         WM/ISRC                     ISRC             -
LANGUAGE         TLAN     TLAN     -         WM/Language                 LANGUAGE         ILNG
LYRICIST         TEXT     TEXT     -         WM/Writer                   LYRICIST         -
MEDIATYPE        TMED     TMED     -         -                           MEDIATYPE        -
MIXARTIST        TPE4     TPE4     -         -                           MIXARTIST        -
MOOD             -        TMOO     -         WM/Mood                     MOOD             -
ORIGALBUM        TOAL     TOAL     -         WM/OriginalAlbumTitle       ORIGALBUM        -
ORIGARTIST       TOPE     TOPE     -         WM/OriginalArtist           ORIGARTIST       -
ORIGLYRICIST     TOLY     TOLY     -         WM/OriginalLyricist         ORIGLYRICIST     -
ORIGYEAR         TORY     -        -         WM/OriginalReleaseYear      ORIGYEAR         -
PUBLISHER        TPUB     TPUB     ©pub      WM/Publisher                PUBLISHER        -
SUBTITLE         TIT3     TIT3     -         WM/SubTitle                 SUBTITLE         -
TITLE            TIT2     TIT2     ©nam      Title                       TITLE            INAM
TITLESORT        TSOT     TSOT     sonm      WM/TitleSortOrder           TITLESORT        -
TRACK            TRCK     TRCK     trkn      WM/TrackNumber              TRACKNUMBER      IPRT
UNSYNCEDLYRICS   USLT     USLT     ©lyr      WM/Lyrics                   UNSYNCEDLYRICS   -
YEAR             TYER     TDRC     ©day      WM/Year                     DATE             ICRD

Which standard each format uses

Every output format writes its tags as one of the standards above. The last column says which column of the table above names the fields there.

Format                 Extension  Tags are written as                                          Names from
---------------------  ---------  -----------------------------------------------------------  ------------------------
AAC (M4A)              .m4a       MP4 atoms                                                    MP4 atom column
AAC (raw ADTS)         .aac       none (a raw stream)                                          -
AC-3                   .ac3       none (a raw stream)                                          -
AIFF                   .aiff      an ID3v2.3 or v2.4 chunk (the preset)                        ID3 columns
ALAC (M4A)             .m4a       MP4 atoms                                                    MP4 atom column
AU                     .au        an annotation field: title, artist and album only            -
FLAC                   .flac      Vorbis comments                                              Vorbis / APE column
FLAC in Ogg            .oga       Vorbis comments                                              Vorbis / APE column
M4R (iPhone ringtone)  .m4r       MP4 atoms                                                    MP4 atom column
MP2                    .mp2       none (a raw stream)                                          -
MP3                    .mp3       ID3v2.3 or v2.4 (the preset)                                 ID3 columns
Ogg Vorbis             .ogg       Vorbis comments                                              Vorbis / APE column
Opus                   .opus      Vorbis comments                                              Vorbis / APE column
TTA                    .tta       APEv2 tag                                                    Vorbis / APE column
WAV                    .wav       an ID3v2.3 or v2.4 chunk (the preset), and a RIFF INFO list  ID3 columns; INFO column
Wave64                 .w64       none (a raw stream)                                          -
WavPack                .wv        APEv2 tag                                                    Vorbis / APE column
WMA                    .wma       WMA attributes                                               WMA column

Notes

• Fields without a name of their own are kept as custom fields: a TXXX frame in ID3, a free-form atom (----:com.apple.iTunes:NAME) in M4A, an item of the same name in a Vorbis or APE tag, an attribute of the same name in WMA.
• One ID3 version without a name. ID3v2.3 has no mood frame, and ID3v2.4 keeps the original release as a timestamp (TDOR) rather than a text frame, so those two cells are dashes and the value is kept as a custom field in that version.
• The date. ID3v2.3 stores a date as the year (TYER) and a day and month (TDAT); ID3v2.4 has one timestamp (TDRC). A Vorbis or APE tag keeps it as DATE.
• Track and disc numbers. The total is part of the number in ID3 and MP4 (3/10); a Vorbis tag keeps TRACKNUMBER and TRACKTOTAL as two fields.
• WAV INFO. Only the ids with a name in the last column are carried; an id without one is dropped, because the same value is in the ID3 chunk when a tagger wrote one.
• Sort fields and the conductor. Names such as ALBUMSORT and CONDUCTOR are written the same way in every Vorbis output; a conductor read from an ID3 TPE3 frame is written as CONDUCTOR, not PERFORMER.
• Covers. A cover is carried by every format that writes tags except FLAC in Ogg, the iPhone ringtone and AU; the raw streams hold none.