Convert SLN Files Free

What is SLN format in Asterisk?

SLN (Signed Linear) is raw PCM audio format used internally by Asterisk open-source PBX (phone system). It's uncompressed 16-bit signed linear audio at 8kHz sample rate (telephone quality) stored with absolutely no header - just raw audio samples. SLN exists because Asterisk needs common internal format for processing calls from different codecs (G.711, GSM, Opus, etc.). Everything converts to SLN internally, then back to output codec.

Technical details: SLN is headerless - file is pure audio data, nothing else. 8kHz mono is standard (telephony bandwidth), though higher rates exist for wideband/HD voice (16kHz SLN16). Each sample is 2 bytes (16-bit), little-endian byte order on x86 systems. Format is efficient for Asterisk's internal processing but useless for playback - players need headers to know sample rate and format.

Should I convert SLN to WAV or MP3?

Converting SLN makes sense for these reasons:

Playback Requirement

SLN has no header. Media players can't detect format. Convert to WAV for universal playback.

Asterisk-Specific

SLN is internal Asterisk format. Outside Asterisk, files are unreadable. Conversion enables external use.

Archival Access

Call recordings in SLN need conversion for long-term storage and future access. WAV is archival standard.

Analysis Tools

Audio analysis software expects WAV/MP3. Convert SLN to analyze recordings in standard tools.

Convert SLN to WAV for compatibility. WAV adds header that players/editors need. Use MP3 only if storage is limited and quality loss acceptable.

How do I convert SLN to WAV?

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What sample rate do SLN files use?

Standard SLN is 8kHz (narrowband telephony). This is traditional telephone quality - sufficient for voice intelligibility but limited fidelity. Frequency response cuts off at 4kHz (Nyquist limit), so no high-frequency detail. Sounds like landline phone call - acceptable for communication, poor for music or high-quality voice.

SLN16 variant: Asterisk supports SLN16 format at 16kHz sample rate (wideband or HD voice). Better quality than standard SLN - more natural sounding voice with improved high-frequency clarity. Used in HD voice VoIP when endpoints support wideband codecs. File extension might be .sln16 to distinguish from 8kHz .sln.

Higher rates possible: SLN32 (32kHz) and SLN48 (48kHz) exist for ultra-wideband and full-bandwidth audio in specialized Asterisk deployments. Rare in practice - most VoIP is narrowband (8kHz) or wideband (16kHz). When converting SLN, verify sample rate - wrong rate causes chipmunk/slow-motion playback. Check Asterisk configuration or test conversion with 8kHz first (most common).

Why doesn't SLN have a file header?

Efficiency reasons: Asterisk processes millions of audio frames during call routing, mixing, transcoding. Adding/removing headers for every internal operation would waste CPU cycles. Raw SLN format eliminates header overhead - pure audio data enables fastest possible processing. Speed matters in real-time telephony where latency is quality issue.

Internal use assumption: SLN was never designed for external consumption. It's intermediate format inside Asterisk processing pipeline. Asterisk knows sample rate/format through context (configuration, dialplan) - no header needed. Exposing SLN externally (call recordings, voicemail) is side effect of internal format, not original intention.

Simple implementation: No header means no parsing, no format detection, no complexity. Just read/write raw samples. This simplicity reduces bugs and improves reliability in telephony system where stability is critical. Header-free design is feature for Asterisk, not bug - external users must adapt.

Can media players play SLN files directly?

Generally no - headerless format confuses players. VLC might play if you force raw audio import with correct parameters (Audio > Open Media > Advanced Options > specify codec, rate, channels). This is tedious and impractical. Windows Media Player, iTunes, Chrome, Firefox - none handle raw SLN natively. They need headers to identify format.

Audacity can import: File > Import > Raw Data, then specify signed 16-bit PCM, 8000Hz, mono, little-endian. Audacity visualizes waveform and allows export to WAV/MP3. Good for one-off conversions with GUI. Not practical for hundreds of SLN files - use command-line tools for batch work.

Practical advice: Don't fight format limitations. Convert SLN to WAV immediately after exporting from Asterisk. WAV files play everywhere, SLN files don't. One-time conversion removes headaches. Store recordings as WAV, not SLN, for future accessibility.

What quality is SLN audio?

Telephone quality (8kHz SLN): Acceptable for voice communication, poor for everything else. Frequency range 300-3400Hz (PSTN telephone bandwidth). Intelligible speech but lacks naturalness - no low bass frequencies, no high-frequency sibilance crispness. Comparable to traditional landline phone call quality. Sufficient for business calls, voicemail, IVR systems.

HD voice (16kHz SLN16): Noticeably better than narrowband. Frequency range extends to 7kHz. More natural voice timbre, clearer consonants, less robotic sound. Modern VoIP with wideband codecs (G.722, Opus wideband) uses 16kHz internally. Significant improvement over traditional telephony but still not hi-fi.

Uncompressed advantage: SLN is lossless within its bandwidth - no compression artifacts. Quality limitations come from sample rate (8kHz/16kHz), not compression. Converting SLN to WAV preserves quality perfectly. Converting to MP3 adds compression but doesn't worsen inherent quality limitations. For call recordings, SLN quality is appropriate - higher fidelity unnecessary for voice.

How do I batch convert SLN recordings to WAV?

Batch conversion strategies:

SoX Bash Script

`for f in *.sln; do sox -t raw -r 8000 -e signed -b 16 -c 1 "$f" "${f%.sln}.wav"; done` converts all SLN in directory.

FFmpeg PowerShell

`Get-ChildItem -Filter *.sln | ForEach-Object { ffmpeg -f s16le -ar 8000 -ac 1 -i $_.Name "$($_.BaseName).wav" }` for Windows.

Asterisk CLI

Use Asterisk file format conversion if on server. Asterisk can transcode between formats through dialplan or CLI commands.

Parallel Processing

GNU Parallel or xargs -P for faster batch conversion: `parallel 'sox -t raw -r 8000 -e signed -b 16 -c 1 {} {.}.wav' ::: *.sln`.

Verify Sample Rate

If 8kHz sounds wrong (too fast/slow), try 16kHz (-r 16000). SLN16 files need different rate parameter.

Scripted batch conversion handles hundreds of SLN files efficiently. Test one file first to confirm correct parameters.

Why does Asterisk use SLN instead of WAV?

Processing efficiency: Asterisk's core job is audio processing - mixing conferences, transcoding between codecs, applying effects. Raw SLN eliminates repeated header parsing/writing. Every audio operation works on raw samples directly. For high-volume PBX handling hundreds of simultaneous calls, this efficiency matters. CPU savings accumulate.

Codec neutrality: Calls arrive in various codecs (G.711, GSM, Opus, G.729). Asterisk converts all to SLN, processes in common format, converts to output codec. SLN is lingua franca - simple, uncompressed intermediate representation. Using WAV wouldn't improve this - SLN's headerless simplicity is optimal for internal processing.

Historical reasons: Early Asterisk (early 2000s) ran on limited hardware. Every CPU cycle mattered. SLN's raw format was practical choice for software PBX on commodity servers. Modern Asterisk could handle WAV easily, but changing core format would break compatibility with decades of deployments. SLN works well - no compelling reason to change.

Can I use SLN files in other VoIP systems?

SLN is Asterisk-specific with limited external use:

Not Standard Format

SLN isn't ITU or IETF standard. Other VoIP systems (FreeSWITCH, Cisco, Avaya) don't use SLN. It's Asterisk internal format.

Convert for Interop

To use Asterisk audio in other systems, convert to standard formats: WAV (PCM), G.711 (μ-law/A-law), or Opus.

FreeSWITCH Alternative

FreeSWITCH uses similar concept (raw PCM) but different implementation. Not directly compatible with Asterisk SLN.

No Hardware Support

VoIP phones, ATAs, SIP clients don't support SLN codec. It's server-side processing format only.

Migration Requires Conversion

Migrating from Asterisk to other PBX means converting SLN recordings/prompts to standard formats first.

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What's the difference between SLN and RAW PCM?

Very similar - both are headerless PCM audio. Difference is context and naming. SLN is Asterisk's name for its specific raw PCM format (8kHz/16-bit signed/mono by default). RAW PCM is generic term for headerless PCM with parameters specified externally. Technically nearly identical - SLN is branded RAW PCM variant.

Practical implications: Converting SLN uses same tools as RAW PCM (SoX, FFmpeg) with same parameter specifications. SLN files ARE raw PCM files - extension and context differ, format is equivalent. Understanding this helps troubleshooting - techniques for handling RAW PCM apply to SLN.

Convention in Asterisk: .sln extension signals "this is 8kHz Asterisk audio" - implies format parameters even though header lacks them. .raw extension is more ambiguous (could be any PCM configuration). SLN naming convention reduces guesswork in Asterisk environments about audio specifications.

How do I create SLN files from WAV?

SoX method: `sox input.wav -t raw -r 8000 -e signed -b 16 -c 1 output.sln`. This converts WAV to raw 8kHz/16-bit mono PCM (SLN format). Useful for creating Asterisk prompts or music-on-hold files from standard audio sources.

FFmpeg method: `ffmpeg -i input.wav -f s16le -ar 8000 -ac 1 output.sln`. Similar result - strips header, resamples to 8kHz if needed, outputs raw signed 16-bit little-endian PCM. Both tools work identically for WAV-to-SLN conversion.

Quality considerations: If source WAV is higher quality (44.1kHz, stereo), downsampling to 8kHz mono loses fidelity. This is appropriate for telephony (no point preserving quality phone system can't reproduce), but be aware conversion degrades audio. For Asterisk prompts, record at 8kHz mono from start or accept quality reduction.

Are there legal/compliance issues with SLN recordings?

Call recording regulations apply: Many jurisdictions require notification/consent for call recording (two-party consent states in US, GDPR in EU, various international laws). SLN files from Asterisk call recordings are subject to same laws as any recording. Format doesn't matter - content and context create legal obligations.

Retention requirements: Regulated industries (finance, healthcare, emergency services) have specific retention periods and security requirements for call recordings. SLN format is fine for retention IF properly secured and maintained. However, converting to documented standard formats (WAV, FLAC) with metadata is better archival practice for compliance.

Accessibility for legal discovery: Courts and regulators expect standard formats. Producing SLN files in litigation requires expert testimony explaining format and providing conversion. Preemptively converting to WAV/MP3 simplifies legal discovery. Consider compliance needs when choosing storage format for call recordings - proprietary/obscure formats create unnecessary complications.

What happens if I specify wrong sample rate converting SLN?

Chipmunk effect (rate too high): If you convert 8kHz SLN with 16kHz parameter, audio plays at double speed with high-pitched voices. Easily recognizable - sounds cartoonish. Indicates sample rate mismatch.

Slow-motion effect (rate too low): If you convert 16kHz SLN as 8kHz, audio plays at half speed with deep, slow voices. Also obvious error. Both rate mistakes are immediately audible - no subtle degradation, complete playback failure.

Finding correct rate: Try 8kHz first (most common). If wrong, try 16kHz. Less common rates (32kHz, 48kHz) exist but rare. Listen to converted file - correct rate produces normal voice speed and pitch. One file test tells you rate for entire batch. Document rate for future reference.

Should I preserve SLN files or convert to WAV for archival?

Convert to WAV for archival. SLN's headerless format is preservation risk - future tools might not know how to interpret raw data without sample rate documentation. WAV embeds all necessary parameters in header. If Asterisk documentation is lost, SLN files become archaeological puzzle. WAV is self-describing and universally supported.

Add metadata: When archiving converted WAV files, document source ("Asterisk call recording, converted from SLN"), date, parties involved (if applicable/legal), and any relevant context. Call recordings without metadata lose evidentiary value. Sidecar JSON or CSV files work well for bulk recording archives.

Retention policy: Don't archive recordings indefinitely unless legally required. Call recordings contain private conversations - indefinite storage creates privacy risks and storage costs. Convert SLN to WAV, keep per retention policy (90 days, 7 years, whatever applies), then securely delete. Format conversion is preservation tool, not excuse to hoard data unnecessarily.