SLN to FSSD Converter

Move Asterisk SLN telephony audio into FSSD audio format

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Legacy Compatibility

Convert SLN telephony audio into FSSD — satisfying the requirements of older audio systems and preservation projects.

No Installs Needed

Run the SLN to FSSD conversion directly in your browser — no legacy software installation required.

Secure Conversion

Telephony recordings are deleted after processing. FSSD output files purged from servers within 24 hours.

How to convert SLN to FSSD

1

Select files from Computer, Google Drive, Dropbox, URL or by dragging it on the page.

2

Choose fssd or any other format you need as a result (more than 200 formats supported)

3

Let the file convert and you can download your fssd file right afterwards

About formats

SLN (Signed Linear) is a headerless raw audio format storing 16-bit signed linear PCM samples at 8000 Hz mono, most closely associated with Asterisk — the open-source PBX framework developed by Digium (now Sangoma Technologies). Within Asterisk, SLN serves as the native internal audio representation: every codec transcoding operation passes through signed linear as an intermediate step. This makes SLN the backbone of Asterisk's codec translation architecture. The format contains nothing but raw samples — no headers, no metadata, no framing — so parameters must be known in advance. While this lack of self-description might seem limiting, it is actually an advantage in telephony where sample format is fixed by convention and every overhead byte matters across thousands of simultaneous channels. The 8000 Hz rate aligns with the G.711 standard for traditional telephony, capturing the full 300-3400 Hz voice band. Asterisk also supports extended variants (sln16, sln32, sln48) for wideband audio. SLN files require no decoding — just direct memory mapping — making them ideal for real-time mixing, conferencing, and prompt playback in high-density VoIP environments.
Initial release: 1999
FSSD is a raw audio format that originated in the classic Macintosh ecosystem, where Farallon Computing's MacRecorder hardware (1988) stored digitized sound as unsigned 8-bit PCM in resource fork entries tagged with the 'FSSD' type code. In modern audio processing tools such as SoX, FSSD is treated as an alias for the u8 (unsigned 8-bit) raw format — headerless files containing a flat stream of single-byte amplitude samples, where each value from 0 to 255 represents an audio level with 128 as the center point. Because there is no header, playback parameters like sample rate and channel count must be provided externally. The original MacRecorder typically captured at rates up to 22 kHz in mono, though any sample rate is valid when interpreting the raw data. FSSD and its compressed companion format HCOM (which adds Huffman compression to the same underlying data) were the standard audio formats for early Mac multimedia: HyperCard stacks, educational CD-ROMs, and system alert sounds of the late 1980s and early 1990s relied heavily on this encoding. One advantage of the raw FSSD format is trivial parseability — with no container overhead, the audio data begins at byte zero and can be read by any tool capable of processing unsigned 8-bit PCM. The format's historical significance also makes it practically relevant for digital archivists: converting FSSD recordings to modern containers like WAV preserves the original audio content losslessly, since the raw samples only need a header prepended, not any form of transcoding.
Developer: Farallon Computing
Initial release: 1988

Frequently Asked Questions

Why convert SLN to FSSD?

FSSD is a legacy audio format needed by specific older systems. Converting SLN to FSSD satisfies compatibility with those platforms.

What software supports FSSD?

SoX and certain vintage audio applications can read and process FSSD format files.

Is FSSD a common format?

No — FSSD is a niche legacy format. It is mainly needed for specific historical audio systems or preservation work.

Can I convert several files at once?

Upload multiple SLN recordings and process them all to FSSD in a single batch conversion.

Is my data kept private?

SLN uploads are erased after conversion, and FSSD outputs are removed from servers within 24 hours.