SLN to M4A Converter

Transform Asterisk SLN telephony audio into M4A for Apple

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Settings

Set the AAC audio bitrate per channel. For example, stereo audio with 128 kbps set here will produce a 256 kbps file. If set to "Custom", the recommended range is ≥64 kbps.
Set the number of audio channels. This setting is most useful when downmixing channels (e.g., from 5.1 to stereo).
Set the sample rate of the audio. Music with a full spectrum (20 Hz — 20 kHz) requires values not lower than 44.1 kHz to achieve transparency. More info can be found on the wiki.

sln

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.
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m4a

M4A is Apple's preferred file extension for audio-only content inside an MPEG-4 Part 14 container, widely adopted after the launch of the iTunes Music Store in 2003. The extension distinguishes pure audio streams from video-capable MP4 files, signaling to players that no video track is present. Under the hood, an M4A file most commonly wraps an AAC-LC (Advanced Audio Coding, Low Complexity) bitstream, though Apple Lossless (ALAC) payloads also use the same extension. AAC-encoded M4A files deliver better sound quality than MP3 at equivalent bit rates, thanks to improved spectral band replication, temporal noise shaping, and a refined psychoacoustic model. Sample rates up to 96 kHz and bit depths up to 24-bit are supported. Apple ecosystem integration is seamless — iTunes, Apple Music, iPhone, iPad, and macOS all handle M4A natively — while third-party support spans VLC, foobar2000, Android, and most car infotainment systems. Three tangible benefits define the format: superior coding efficiency over older lossy codecs, rich metadata through the MP4 atom structure (artwork, chapters, lyrics), and dual-mode flexibility serving both lossy and lossless workflows.
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Apple Ecosystem Ready

M4A plays natively on iPhones, iPads, Macs, and iTunes. Convert your SLN telephony audio for seamless Apple integration.

Adjustable Quality

Control the M4A bitrate before converting — optimize for quality, file size, or a balance between the two.

Private Processing

Telephony recordings are sensitive by nature. Your SLN uploads are purged after conversion, and M4A files are removed within 24 hours.

How to convert SLN to M4A

1

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

2

Choose m4a or any other format you need as a result (more than 200 formats supported)

3

Let the file convert and you can download your m4a 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
M4A is Apple's preferred file extension for audio-only content inside an MPEG-4 Part 14 container, widely adopted after the launch of the iTunes Music Store in 2003. The extension distinguishes pure audio streams from video-capable MP4 files, signaling to players that no video track is present. Under the hood, an M4A file most commonly wraps an AAC-LC (Advanced Audio Coding, Low Complexity) bitstream, though Apple Lossless (ALAC) payloads also use the same extension. AAC-encoded M4A files deliver better sound quality than MP3 at equivalent bit rates, thanks to improved spectral band replication, temporal noise shaping, and a refined psychoacoustic model. Sample rates up to 96 kHz and bit depths up to 24-bit are supported. Apple ecosystem integration is seamless — iTunes, Apple Music, iPhone, iPad, and macOS all handle M4A natively — while third-party support spans VLC, foobar2000, Android, and most car infotainment systems. Three tangible benefits define the format: superior coding efficiency over older lossy codecs, rich metadata through the MP4 atom structure (artwork, chapters, lyrics), and dual-mode flexibility serving both lossy and lossless workflows.
Developer: Apple Inc.
Initial release: 2001

Frequently Asked Questions

Why convert SLN to M4A?

M4A is the native audio container for Apple devices. Converting SLN recordings to M4A makes them immediately playable on iPhones and iPads.

What is the difference between M4A and MP3?

M4A typically uses AAC encoding, which delivers better quality than MP3 at the same bitrate. It is the preferred format in the Apple ecosystem.

Will the file size shrink?

Yes — M4A uses efficient AAC compression, producing significantly smaller files compared to raw SLN data.

Can I convert a batch at once?

Upload multiple SLN files simultaneously and convert them all to M4A in one pass — no repetitive individual uploads needed.

Is the conversion secure?

SLN files are removed right after conversion, and M4A outputs are deleted from servers within 24 hours.

Do I need special software?

No. The entire conversion runs in your browser — compatible with any operating system or device.