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Audio Format

SPX

Speex (Voice Codec)

Speex is an open-source, royalty-free audio codec optimized for voice. Developed by Xiph.org from 2002, Speex was used in early VoIP applications, voicemail systems, and ham radio software. The Xiph.org Foundation officially deprecated Speex in 2012 in favor of Opus, which provides better quality across the full audio range. Speex files (.spx) are now a rarely-encountered legacy format.

What is SPX?

Speex uses Code-Excited Linear Prediction (CELP) — a speech-specific compression technique. The codec offers narrowband (8 kHz, optimized for voice up to 4 kHz audio) and wideband (16 kHz, voice up to 8 kHz) modes. Bitrates range from 2.15 kbps (intelligible voice) to 24.6 kbps (high-quality voice). Speex files use the Ogg container by default, though raw Speex frames can also be stored.

SPX pros and cons

Advantages

  • Open-source and patent-free
  • Very low bitrate operation — 2.15 kbps still intelligible
  • Multiple sample rates (8 kHz, 16 kHz, 32 kHz)
  • Built-in noise suppression and acoustic echo cancellation
  • Was free alternative to commercial voice codecs (G.729)

Limitations

  • Officially deprecated since 2012
  • Poor music quality — voice-only optimization
  • Replaced by Opus, which is better in every metric
  • Limited modern software and hardware support
  • Almost no use in current applications

When should you convert SPX files?

Convert Speex to Opus for modern use — Opus is the official replacement and supports all of Speex's use cases at higher quality: `ffmpeg -i input.spx -c:a libopus -b:a 32k output.opus`. Convert to MP3 for general playback compatibility: `ffmpeg -i input.spx -c:a libmp3lame -b:a 64k -ac 1 output.mp3`.

Convert SPX files

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SPX FAQ

Why did Speex get deprecated?
Opus (released 2012) provides better quality at the same bitrate, supports both voice and music, and has lower latency. The Speex project officially recommended migration to Opus for all new applications. Existing Speex files remain playable but no new applications adopt Speex.
Where would I encounter Speex files in 2026?
Older voicemail systems, archived ham radio recordings, legacy VoIP archives, some older voice-recording apps. Modern smartphones and PCs almost never produce Speex files.
What software plays Speex?
VLC plays Speex on every platform. Audacity opens .spx files for editing. foobar2000 with Speex plugin plays them on Windows. Most other modern audio players don't support Speex natively.

More formats

HEICWebPAVIFEPUBWebMOpusMP3WAVM4AFLACAACMP4AVIMKVMOVJPGPNGGIFSVGTIFFICOBMPEPSARWOGGWMVPPTXPagesPDFDOCXRAWHEVCH.264AIFFM4VWMA3GPDNGRTFPSDOGVTSFLVAIXLSXCSVZIPAPNGMIDIJPEG XLMPG/MPEGVOBM2TSAC3ALACDTSJP2HEIFAPECBZ3G2SRTDOCPPTXLSODTODSM3URAR7ZM4BAMRTXTMOBITARGZJSONXMLYAMLKEYMTSCR2NEFMXFTGACAFASSVTTDSFRMF4VWVASFRMVBORFRW2EAC-3TTAISODMGXZDjVuFB2AUM4RAZW3MKAEXRVCFICSAAXPRCMP2XSPFM3U8