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What is AU?

Sun AU is the native audio format for Sun Microsystems workstations, using simple mu-law encoding or uncompressed PCM.

What is SPX?

Speex, open-source speech codec for VoIP and voice — stored as audio/speex — uses lossy encoding to handle voip telephony, voice chat, speech compression. Typical file sizes: varies by content.

When Should You Use AU vs SPX?

Use AU When...

AU shines in legacy unix audio, sun workstation sound scenarios. The pixel-perfect output meets the demands of professional production environments.

Use SPX When...

Choose SPX for voip telephony, voice chat, speech compression. It balances quality and file size better than most alternatives in its class.

AU vs SPX: Key Differences

  • •AU and SPX diverge on compression: lossless compression for the source, lossy compression for the target.
  • •The switch from lossless compression to lossy compression means you gain significant space savings while accepting minor quality trade-offs.
  • •Alpha channel support: SPX handles transparency, so semi-transparent pixels from the source will be lost.
  • •Storage footprint: Small to medium, similar to WAV for AU vs varies for SPX.
  • •AU is optimized for legacy unix audio, sun workstation sound while SPX is designed for voip telephony, voice chat, speech compression — choose based on your specific workflow needs

Advantages of AU

  • ✓AU uses lossless compression tailored for Legacy Unix audio, Sun workstation sound.
  • ✓AU is supported natively on virtually every device, browser, and operating system.
  • ✓AU files are bit-for-bit perfect, preserving every pixel and sample for professional work.
  • ✓AU integrates seamlessly with all major creative tools and workflows.

Advantages of SPX

  • ✓SPX compresses data using lossy methods, keeping file sizes manageable for VoIP telephony, voice chat, speech compression.
  • ✓Industry-wide support means SPX files open anywhere — from legacy systems to the latest devices.
  • ✓Small file sizes make SPX ideal for email, web uploads, and cloud storage.
  • ✓From professional software to open-source alternatives, SPX works out of the box.

Common Use Cases for AU to SPX Conversion

  • Conference call recordings
  • Field recording preservation
  • Music production and mixing
  • Podcast distribution
  • Voice recordings
  • Streaming services

Industry Applications

Medical TranscriptionRadio BroadcastingMusic ProductionPodcastingBroadcastingAudio Engineering

Best Practices for AU to SPX Conversion

  • →For AU to SPX: match the sample rate and bit depth to your delivery platform — 44.1 kHz / 16-bit is the standard for music
  • →Apply loudness normalization (LUFS) before conversion to ensure consistent volume across your audio library
  • →SPX prioritizes quality — ensure your storage can accommodate the resulting file sizes
  • →Check that metadata tags (artist, album, track number) survive the conversion
  • →Convert a short segment first to verify quality at your chosen bitrate
  • →Keep a lossless master copy in case you need to re-encode for a different platform

Related Article

Video Compression Guide: Reduce File Size Without Losing Quality

Learn more about AU and SPX formats in our detailed guide.

Troubleshooting AU to SPX Conversion

  • !SPX audio quality degraded vs AU → use a higher bitrate (at least 256 kbps for music)
  • !SPX file won't play on your device → check format compatibility with your player or platform
  • !Metadata (artist, album, title) lost after AU to SPX conversion → some formats like SPX may not support ID3 tags
  • !Volume changed after converting AU to SPX → normalize audio levels before conversion for consistent playback

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About AU and SPX

AU

AU (AU audio) - Sun AU, legacy Unix audio format for Sun workstations. Best for legacy unix audio, sun workstation sound.

SPX

SPX (SPX) - Speex, open-source speech codec for VoIP and voice. Ideal for voip telephony, voice chat, speech compression.

Format Comparison

CompressionLossless → Lossy
TransparencyN/A
Use CaseConvert from legacy unix audio, sun workstation sound to voip telephony, voice chat, speech compression
File Size~Standard file sizes after conversion

FAQs

Is converting AU to SPX free?

Yes, GoConvertFile offers free conversion of AU to SPX for files up to 100MB. No registration required.

How long does it take to convert AU to SPX?

Most AU to SPX conversions complete within seconds for standard-sized files. Larger files may take longer depending on the conversion complexity.

What is SPX best used for?

SPX is ideal for voip telephony, voice chat, speech compression. Speex, open-source speech codec for VoIP and voice

Does converting AU to SPX lose quality?

Yes, converting AU to SPX uses lossy compression, which reduces file size by discarding some visual data. Set quality to 85-95% for a good balance of quality and file size.

What file size should I expect after converting AU to SPX?

SPX files are typically 50-80% smaller than AU files because SPX uses efficient lossy compression. The exact reduction depends on quality settings.

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