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fsynth retrospective


This is a retrospective on the development of fsynth.com (also called Fragment), a free and open source real-time collaborative cross-platform audiovisual live environment with a sound synthesis environment based on pixel data. (oscillator-bank and filter-bank concept)

logo made with GeoGebra

The project was developed from 2016 to 2021, it didn't gain much momentum due to too steep learning curve and weird concept but i had massive fun in the journey (especially the audio server), it is a big project from which i learned a lot, technically overlapping many fields and sub fields.

The whole project can be seen as a big experiment, both in term of synthesizer and in term of software architecture.

Many peoples asked me about this project over the years, it is based on simple concepts but is hard to understand if you are not into sounds or accustomed to the idea of spectral synthesis, it is also quite focused on code so arcane if you don't care about that and especially accelerated graphics code so it is even more arcane and confusing if you try to relate it to audio... !

This project is still maintained if required as i still use it but there is no plan to develop it further, i consider it fully featured going way beyond what i envisioned after the prototype.

I will not talk much about spectral synthesis or how to do stuff with it as all of that is either on the website, YouTube videos or documentation but i will try to talk about what i did with it and the highlights, technical stuff and ideas / architecture / development process.

The beginning / idea / tools

My interest in sound synthesis probably started soon after i started making music in ~2007 (which was mainly triggered by the demoscene and my growing love of electronics music), i started by tinkering with Renoise DAW (also a bit of Reason) and lots of audio plugins progressively modifying patches (aka synth. programming) and making my own sounds, also read plenty stuff about old and new synthesizers and DSP algorithms, i also experimented with many sound synthesis methods or dive into old tech such as tape loop although i mainly stayed with digital stuff.

I don't think i ever liked composing music though, always was too cumbersome for me, what i enjoy the most is definitely jamming, hearing new sounds or reproducing sounds and making up soundscape with lots of layers and some bits of progression.

The continued interest in sounds led me up to read lots of related materials and i eventually reached a point where i knew off many software which worked with sounds in an "uncommon" way such as MetaSynth, Virtual ANS, Photosounder, AudioSculpt, also knew a lot of related synth / interface oddities as well such as UPIC, HighC, Reaktor, Kawai K5000, ANS and sound design languages such as Kyma, SuperCollider, Pure Data, Chuck.

I then tinkered with custom sounds generator although all my early experiments were very crude and i didn't know much what i was doing, the motivation at the time was mostly about real-time sounds for 1k or 4k demos.

An idea that i started being obsessed with is sound synthesis using image-based approach, sculpting the sound by directly editing the spectrogram likes you would do in a graphics program, i especially liked MetaSynth approach early on but this software was only available on Macintosh so i tried alternatives which worked on Windows / Linux such as Virtual ANS which also led me down the rabbit hole of sound synthesis history, discovering oddities such as the ~1950s ANS synthesizer, Daphne Oram Oramics, Hugh Le Cain experiments or Delia Derbyshire work.

early Virtual ANS experiments with ambient sounds and a composition on the right

composition and ambient drone on the right, the original files are available by replacing .ogg by .png in the URL

The concept of this approach is simple :
  • the vertical axis represent the oscillator bank, sine wave oscillators are mapped to the Y axis following a logarithmic scale (because our hearing works on a logarithmic scale)
  • the horizontal axis represent time
  • the pixel brightness is the oscillator amplitude which relate to sound level
The reason i was obsessed with this approach was that it pack both timbre generation and composition through a graphics score, it is intuitive and matched my interest of computer graphics and growing interest of sound synthesis, it is also a fun way to understand one of the simplest and oldest way to generate sounds : additive synthesis

The obsession didn't get away, i wanted to do my own image-based synthesizer akin to MetaSynth with more features than just sketching with brushes, the first synthesizer prototype was made in C for the GP2X in early 2010s, this is where i experimented with the oscillator-bank concept which is basically straightforward additive synthesis, adding huge amount of sine waves together at different frequencies to produce sounds.

Virtual ANS program, one of the main inspiration for Fragment


CSP (Collaborative Sound Painting) prototype was made in mid 2015, one year before the prototype of Fragment, CSP was a web version of the Virtual ANS software with a similar score view, brushes and drawing tools, it support smooth real-time playback (additive synthesis), audio chunks are generated on the fly using web workers, the editor was pretty similar to Virtual ANS editor although it had scriptable (sand-boxed JavaScript) post-processing snippets which could be applied to multiple selections.

screenshot of the app, see also a video of CSP

This program was pushed quite far in term of features, i never managed to release it though, it had most features of Virtual ANS, the code was rushed though and required some refactor at some point, some features such as the multi selection / sand-boxed post-processing broke on later browser versions, a huge factor in its abandon.

The "novel" idea at the time was the flexible post-processing, i wanted to apply image processing algorithms to parts of the sonogram and hear the result, i also could add some features of MetaSynth quickly by doing snippets without having to mess with the UX / code.

The other "novel" idea was the collaborative aspect which may have been perhaps way more interesting than the collaborative aspect on Fragment, the idea was a shared canvas that could be edited by multiple users with a portal to share scores and post process snippets.

This program would have been way more user-friendly than Fragment although less powerful.

CSP was 100% vanilla JavaScript just like Fragment with the same reasons. (see below)

Custom build system

As CSP (and fsynth) started to go beyond a simple prototype i reached the issue of growing code base and generating code for different targets, many bulky build tools were available to address this issue in an optimal way but i still felt it was too much for me as a solo developer, i also didn't like that it felt likes work and i especially didn't want to delve into endless configuration files so i made my own very simple (< 100 lines of code) build system with Anubis / Python which solved my two growing issues :
  • modularity; being able to split the code base into different parts (now solved with JavaScript modules)
  • generating code for different target (optimized code for production etc.)
  • the Anubis build tool also had a build on file change feature which was quite handy
First issue was solved by implementing a simple C style files preprocessor which took an entry point file and looked for #include directives in comments. Second issue was solved by calling external optimization programs on the single file produced by the preprocessor. This was simple to maintain (= didn't had to revisit this at all), required almost zero configuration and produced good enough result for my use case, all of this was also used to ease the development of Fragment.

The code base also required to be quite careful on how parts were structured, it was easy to make a mess with this primitive build system so i restricted the preprocessor to run only on the entry file (no include directives in other files) and i forced all my files around a fixed template with sections such as fields, functions, init. code; it ended up being an okay decision as a solo dev.

What was missing perhaps was some sort of environment variables for different targets which was required for the deployment on my production server (which require a different setup than a local version), i solved this the ugly way instead by running a find and replace tool on all the files, a hacky solution but had few lines to replace anyway...


WUI is a collection of lightweight vanilla (standard ~2010s JavaScript) GUI widgets to build web apps which i purposely started in ~2015 for my web experiments, CSP in particular, the idea was a complete set of independent widgets that i could integrate into my experiments as needed, i never liked big GUI frameworks because they were too big / monolithic for my taste, all major updates were breaking stuff in previous versions and customization was also quite tiresome, they also rely on complex environment which were too bulky (or too much likes work) for my experiments.

There is some innovative features in WUI such as detachable dialogs, dialogs which can be detached from the main window and still works, it is implemented in a hacky way by overriding addEventListener but it provide a way to put all parts of an app into independent window (or tabs) as needed without much issues as long as you use WUI widgets (not guaranteed with something else), i thought this kind of stuff would be cool for "big" monitoring environments where many parts would be monitored over multiple monitors. Modern libraries still don't support this as far as i know !

The other unique feature of WUI is the range slider widget (which also act as an input field) which support MIDI learn feature and works in absolute or relative mode, this allow MIDI interfaces to be easily associated with a slider / input.

most features of WUI at works on fsynth, most dialogs can be detached into individual window

There is now much better native solution to build such framework with Web components API, i still use my library for quick experiments as it works flawlessly albeit with an old fashioned API.

Prototype : GLSL image-based synthesizer

The first prototype of fsynth was a quick weekend project in ~2016 with the idea of computing the spectrogram in real-time with GPU code.

This prototype is still working fine and is basically a stripped down lightweight Fragment (a canvas, a playback marker and a GLSL code editor with real-time update), the stereo audio is generated by the browser so it does not require an audio server.

fsynth prototype

The idea was heavily influenced by "Sonographic Sound Processing" by Tadej Droljc (video) which is a Max/MSP/Jitter experiment of modifying spectrogram in real-time on the GPU, the experiment does not use additive synthesis though but modify the frequencies and then apply an inverse Fourier transform, this can be faster than doing additive synthesis and more accurate due to untouched phase, i thought this was impressive and started to imagine what kind of sounds could be produced by computing the spectrogram in real-time (= throw accelerated graphics stuff around) or heavily mangle spectrograms and hear the result immediately.

I experimented with my prototype for some times and was amazed at all the noise it was able to produce even though the spectrogram was all computed (didn't have any bitmap import features) so i started a second version to add the features it lacked, mainly multiple playback marker so i could compose sounds through spectrum slices and shader inputs so i could import bitmap images.

The audio was generated on the main thread with a WebAudio ScriptProcessorNode by adding sine waves together computed from a wave-table, the audio processing done in the main thread was a well known burden of the Web Audio API at the time so the sound quality was not that good, this was improved to some extents with fsynth.

fsynth v1: The launch

The first version of Fragment was released ~4 month after the prototype, it was an improved version of the prototype with better UX and some more features but was still using the not so polished web audio engine, it was still quite rough in some aspect, lacking documentation and having bugs.

A second update came 3 month after the initial release with many features plus complete documentation, many update followed with the final v1 update being released a year after launch, at this point Fragment had matured with good audio quality and an improving audio server with support for additive synthesis and granular synthesis.

It was well received i believe but the steep learning curve, weird concept and usability issues fend off many peoples so in the end i don't think much peoples used it.

fsynth.com v1 client


Fragment is a complex software with several independent parts and tools all centered around the core of Fragment which is the web client, the client can be used either locally or online from the official website, it use a global sessions mechanism handled by strings, data is shared between users of the same session.

The other parts are :
  • a main server which handle the chat and hold session data (slices) it use a Redis database back-end
  • a sharedb server which handle session data (settings and code) and the collaborative feature / OT for the code edition component, it use a MongoDB database back-end
  • a native high performance audio server which provide on-demand smooth high quality sound synthesis
  • NGINX as a reverse proxy (online version), the local version use an independent web server
The reason for using two type of database back-end was isolation and scalability, slices were also supposed to be updated more often than the rest, i think it was a valid choice but it was not really necessary for the current features state so i am kinda mixed in the end.

The idea for the architecture was to be highly flexible, highly scalable and robust, the audio server is the part that manage all of that ideal.

The scalability is handled by independent programs and most back-end parts are clustered; most parts of this project is able to leverage multiple cores or multiple machines even the audio server.

There is also many tools that were made alongside Fragment :
  • an OSC relay to control external or internal stuff through a simple messages mechanism
  • a relay to distribute the client data over multiple instances of the audio server "distributed multi-machines/multi-core realtime sound synthesis"
  • an external GLSL code editor
  • a file server which provide a convenient API to manipulate files remotely for the audio server
  • a sonogram player web widget
  • the audio server
Most of the project was made in vanilla JavaScript (ECMAScript version 5), most back-end tools were made with NodeJS.

The vanilla JavaScript choice for the client was mainly due to mitigate the issue of software rot and to simplify future development, i think it was an excellent choice in the end for a solo developer over a 5 years project time frame because i didn't have to fight with the tooling nor with dead, deprecated or outdated dependencies, it is also fairly easy for a newcomer to understand the parts of this project and to hack it.

For comparison the back-end parts required several refactor over the years due to deprecated / outdated dependencies.

The vanilla JavaScript drawback is mainly related to the code which is a bit rushed, many parts of the client probably require heavy refactoring to cope with side effects and some parts are probably a mess to understand / a pain to manage. (especially all the UI related code !)


Earliest issue was the performance of the graphics loop which was intertwined with sound generation, reading the canvas surface back from the GPU was kinda slow (even if slices were 1px wide) and could produce stall issues which was partially addressed with careful optimizations of the graphics loop notably doing minimal allocations (re-using buffers), can perhaps be more optimized by using fences now.

The first version also supported two WebGL version which was quite a chore to maintain as some features were not available with WebGL 1 so i had to address them differently such as being limited to one draw buffer with WebGL 1 which meant no independent output for sonogram and graphics on the fragment shader, all the differences induced by this had an impact on the fragment shader code which was probably very confusing from an user perspective, it also imposed some limits. It was a mistake to handle both versions since WebGL 2 had increasing support and WebGL 1 support was quickly shunned with the v2 in the end.


The web audio engine was not so good at first due to the audio processing being done on the same thread as the UX which interrupted the audio process and ended up in crackling sounds (can be heard in the early videos), it had too many oscillators which the Web Audio API wasn't really suited for, it worked ok on Chrome at some point but was on the edge of being usable on anything else and still required a lot of computing power.

The audio processing being done in the same thread as everything else was a well known burden of the Web Audio API at the time, it can be solved now with AudioWorklet.

A temporary fix to lessen the crackles was to build a standalone code editor since audio interruptions were worse on code change due to browser reflow and shader compilation, this helped quite a lot on some of my early videos to produce clean sounds.

I did some more fixes to the audio engine to support OscillatorNode (WebAudio oscillators) instead of a ScriptProcessorNode, this was better on Chrome since an OscillatorNode was implemented natively in an optimized manner, it was implementation dependent though and still worked poorly on Firefox...

I tried many things to improve the performance with OscillatorNode, first creating oscillators on the fly for each events, second by having a pool of pre allocated oscillators and messing with the oscillator gain value on note event.

An idea i didn't try was to pre allocate oscillators progressively instead of having a big number of already allocated oscillators, finding a good balance between ready to use oscillators and real time allocation to alleviate the needs of maintaining lots of ready but unneeded oscillators which was also a burden for the browser to maintain.

Other fixes were to minimize browser re-flow and improve the algorithms but there was limits to that as well.

The complete fix to the crackling audio issue without going for restrictions was to ditch WebAudio entirely and externalize it by creating a high performance audio server from the ground up in C that would be linked to the client with Websockets, the motivation for this was also the prospect of embedded uses so i could use the synthesizer on dedicated computers or a network of computers, something done for example by Kyma (with much better latency !) or actually all modern digital synthesizers.


Fragment has some collaborative aspect because i ported the idea of CSP collaborative aspect for fun, the code and most data is shared across session users, the global clock and media data is not shared though so there is no real synchronization of audio / visual happening between users, it is just a mean to quickly get the same session data which is probably sufficient for simple jams.


The Fragment website was built with the Skeleton CSS framework and Font Awesome icons as a single page with bits of JavaScript (using localStorage) for sessions handling, Skeleton has a clean modern design and remained very simple under the hood, i added some customization such as the fixed header and social buttons and added a Flarum powered forum which was perhaps the heaviest part of all this because it turned hard to upgrade later on because it rely on bulky tech...

FAS: Fragment Audio Server

The audio server was already available at launch albeit in a somewhat unstable form but it was a good idea as the sound quality quickly improved, it also paved the road for features that would have been tough to add with Web Audio alone such as all the goods of externalizing the synth part which could now be reused for other apps, embedded and computed across many computers.

The first version of the audio server still had issues with crackling and many crashes because i was learning audio software architecture in the process, i iterated on it with many structural changes for many months until i had a solid architecture which i believe is pretty similar to "professional" instruments / DAW although with a different data source.

A very helpful book (along with web articles and papers) for the audio server was L'audionumérique (don't know the english title) by Curtis Roads, most concept covered by this book were implemented in the audio server.

The first version of the audio server was rough and was only good for additive synthesis although it had support for FM and granular synthesis as well. (granular was ok but heavy computationally)

The audio server is fully documented here with lots of details about its architecture.

a clustered FAS diy environment with RPI (running netJACK 2 / FAS) and NanoPI NEO 2 boards (running FAS instances)

Latency issues

Latency is an important factor in audio, Fragment never did well on this part even with the audio server, it remains highly sensitive with heavy latency variations due to GPU / CPU reliance, the web stack adds to this burden, i also didn't care much about the client UI code at first so it is probably not helping much, latency is also due to the lengthy process by which the audio is generated : GPU -> reading/decoding back GPU content on CPU -> data transfer to the audio server over websocket -> decoding note events -> sound generation

There is even more latency issues with multiple audio servers since it adds a relay program which will distribute the data.

There is some ways to mitigate this by carefully tuning the audio server / browser / machine / network but it will never match a native app unless the client is also native.


Fragment v2 was released in March 2021 and was a labor of love spanning three years, i was not as focused as before though, this version brought refinement to almost every parts of the program, it addressed usability issues with lots of quality of life improvements as well and i also perfected the audio engine; audio stability and quality, effects chain and new instruments / custom ones with Faust was the major focus for the audio server.

Perhaps i could have put some features in the v1 but releases were already tough to manage as a single developer so i preferred a quieter approach.

The v2 development was chaotic with long period of doubts about the direction i should takes with Fragment, mostly thinking about an embedded product approach and sharing platform which slowed down the development, in the end i decided that there was too little interest to push it further or that it required too many changes with a more intertwined audio / graphics part concept so i ditched all these ideas and focused on improvements and things i wanted to experiment with.

Another idea of the v2 was to improve the longevity of Fragment by adding features such as desktop capture and disabling audio (to makes Fragment focus on visual experiments) but most importantly by adding Faust support on the audio server, with the addition of Faust anybody can add new algorithms with an elegant language without messing with the audio server code, this feature could also be used to replace all custom algorithms of the audio server by robust equivalents so that it can be used in other products, it can also reduce the complexity of the audio server code, it makes the audio server quite generic.

all the new instruments and channel effects

On the audio server the new features i definitely wanted was ASIO / Jack support because i was limited by hardware device channels with PortAudio, using Jack allowed "unlimited" output channels and flexible routing / instruments, also wanted to improve instruments and add new ones notably wave-table (additive but with different single cycle waveform) and band-pass synthesis (to do the inverse of additive; carving out noise), another huge feature which was missing was virtual channels (a mixing layer) and sound effects.

On the client part the new features were desktop capture which would enable Fragment to become easy to experiment with because you can use any tools you are accustomed with to drive the synthesizer, MIDI out was also kinda important as i wanted to test triggering external synths from Fragment events.

For maintenance reasons and because the audio server was finalized i dropped Web Audio along with WebGL 1.

There is many other new features i don't discuss here, they were added because i wanted to experiment with them.

Adding many types of sound synthesis algorithm was fun to do and experiment with but many types don't blend well with the way Fragment works so some of them are difficult to use or have limits, a different approach (UX issue ? parameters encoding ?) may be required for some of them.

My experiments

Parts of Fragment development was testing / using it (kinds of poor man QA), this was a fun part with many random lengthy jam sessions.

Note that plenty early videos are messy with bad audio quality and most of them without direction except the synthesis type or some features constraint, i was stressing the synth with lengthy session to find bugs or things to improve and sometimes trying ideas that passed by.

Most videos focus on sounds since it is what needed testing but i also tinkered with the live graphics aspect of Fragment on some videos.

There is many other videos here : https://www.youtube.com/@FragmentSynthesizer

Bandpass filter-bank

Mix of synthesis type

Additive synthesis

Early :


Phase / Frequency modulation

Themed jams (with visuals)

Did many themed jams with nature videos or demoscene visuals, i sometimes tried to sync the audio jam to the visuals with limited success.
Early :


Some demo songs made with Fragment :

Modal synthesis


Few experiments of live interaction where i interact with the spectrum through visual or other means, i didn't explore this part much and most the experiments are crude but i think it has potentials.
Early :

Browser is not a professional music platform

I had some criticism of the project over the years due to the web tech stack being "not professional" for audio, i don't think this was a concern since Fragment is just an experiment, it is true though that its biggest issue is latency and that the web stack doesn't help to improve this.

Still, there was many successful web audio projects such as AudioTool which proved that web audio projects can be as accessible as native and reach some level of "professional" audio. (whatever that word means, for me it is related to audio quality / stability / latency)

Another criticism was mainly associated to the fact that you could not use Fragment to go along your day to day DAW as a plugin, this is kind of important nowadays since digital music peoples have a DAW / software that they use daily and associate any kinds of instruments or effects to it through a standardized plugin system.

I don't think this would works with Fragment as it is akin to a standalone alternative DAW / performance tool already than an instrument, it is a tool of its own with graphics / code being a major part of it, i doubt it would works in tight integration, i can imagine some parts to be used as a VSTi but it would then become just like any other synthesizers which was never the point, it would also induce quite a lot of development / maintenance on my part that i was not interested with.

The web tech stack came with many challenges regarding real-time audio processing with many black boxes to deal with (dealing with plenty abstractions), the audio issues were all addressed by moving the audio part to a native application, this part could become a plugin of its own but there is little interest in doing that as it don't bring anything new that VSTi brings already.

There was many advantages of the web tech stack though : ease of development, faster iterations, flexibility (UI etc.)

Anyway, i never thought of Fragment as a professional tool, it is just some toy / oddity / experimental synth / DAW with a "what if ?" concept, it can be used professionally i guess but it was never meant to.

Fragment can still be used alongside a DAW the old fashioned way, many of my videos were made with Renoise DAW being used to compose (sending MIDI events) and apply effects.

There was also some criticism on the lengthy QA videos but the point of these videos were just me QA testing my project and having fun with it as well perhaps in goofy way sometimes because i quite sucks at playing an instrument !


As a synthesizer / audiovisual environment

I think of Fragment as a maximalist takes of the oscillator-bank and filter-bank concept, a big what if ? i took the original concept and pushed it as far as i could, the result is an experiment of sort; massive amount of oscillators and filters driven by a big matrix of on/off switches triggered by GPU accelerated code.

It can do all sorts of noise, i just scratched a tiny amount of what is possible with it i believe but its main issue is its bulkiness with lots of intertwined tech that is sometimes poorly related, this makes it quite unfriendly to most peoples that would be interested, it have a very steep learning curve (graphics / audio programming) and the web stack / audio server also adds a lot of burden although i think it works reasonably well for all it does.

It is alright if ones use shader inputs with small amount of code and stay with sound synthesis types which does not require too many parameters (eg. additive synthesis, FM, wave-table ?), in this way i can see it as an interesting platform / companion for sound design, ambient stuff, experiments etc. it can do quite a lot of cool stuff with a stock shader and some inputs.

On the audiovisual / performance side i didn't do much with it so i don't know how far it can go but i believe it is where it shines, especially with the desktop capture or webcam feature which allow the outside world to interact with Fragment with minimal setup / knowledge, some of my ideas were things such as some kind of prepared lights show, making up some kind of organic "live composition", the other cool thing is to be able to add visuals (generated or playback) and mix them live.

Even though its inner working has a maximalist approach the synthesizer interface is very minimal and i love its cold presentation as a black "monolith", a big black opaque canvas which feels whatever is sketched on it and spout it back through speakers, this was also my original feels when i first watched pics or videos of the ANS synthesizer which is quite a cold beast.

ANS synthesizer, ~1957 photo-electronic musical instrument

As a composer tool i think vector graphics approaches likes HighC or UPIC are better concepts since they have a thoughtful / elegant symbolic aspect whereas Fragment is more free-form, there is a good explanation of this on the HighC webpage.

As an audio editor it is not quite there yet due to relying on straightforward additive synthesis instead of manipulating the spectrum directly and do an inverse transform, this result in a diminished audio rendition so there is much better image-synth to do that such as PhotoSounder but it could be addressed with some more development i guess.

As a Shadertoy like

Fragment is good as a Shadertoy alternative (focused on real-time graphics), you can disable all the synth features with a checkbox in the settings panel so it focus on graphics, it is quite advanced compared to similar platforms on the web (at the time i worked on it at least) due to all kinds of supported shader inputs, especially the Processing input and desktop capture, the collaborative aspect and MIDI / OSC support may also be interesting for some applications, compared to Shadertoy it lacks the social / sharing aspect and some concepts such as in depth control of multiple buffers although support for Processing can replace this.

As an education platform

There is perhaps some use cases for the education sector due to the collaborative aspect but this was never tested.


It was an interesting and formative experience to tackle a big project like this all alone, also was a good approach to learn all kinds of in depth details about audio, WebGL and real time audio software architecture, going for a simple additive synthesizer up to the architecture of a DAW with effects chain, modulations and implementing many well known sound synthesis algorithms.

Nowadays i prefer minimalist approaches which can be as effective and fun without a big bulky all in one environment that weight quite a bit in term of maintenance and usability.

Even though still unfriendly to most i still think Fragment can be useful, it has matured and has huge amount of flexible features for an image-synth (or even a Shadertoy clone) and it may allow some peoples to probe what works and what does not.

It is also quite fun as a conceptual noise-of-all-kinds software and its unexplored performance aspect may be interesting to reveal as well.

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