CONVERT
SPX → GSM
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Fast, secure SPX to GSM conversion. No registration required.
Opening note — SPX is an audio format with specific trade-offs between file size, bitrate flexibility, and device support. The GSM you want is two clicks away. Converting SPX to GSM changes the audio container without re-recording anything. Whether you are moving from a studio master to a distribution format or just making a file playable on an old car stereo, KaijuConverter re-encodes the audio with FFmpeg at your chosen bitrate and preserves sample rate, channels and ID3 tags. The source SPX file stays untouched. A quick refresher — SPX is an audio format with specific trade-offs between file size, bitrate flexibility, and device support. By contrast, GSM is an audio format with specific trade-offs between file size, bitrate flexibility, and device support.
Speex Audio
Source formatSpeex is an open-source audio compression format specifically designed for speech encoding. It uses Code-Excited Linear Prediction (CELP) and supports narrowband, wideband, and ultra-wideband modes for different speech quality requirements.
GSM Audio
Target formatGSM 06.10 is a speech compression standard designed for the Global System for Mobile Communications. It encodes speech at 13 kbps using Regular Pulse Excitation with Long Term Prediction, optimized for voice intelligibility over cellular networks.
Why convert SPX to GSM
Speex Audio is great in its own niche, but GSM Audio is either more universally playable or better suited to the device you are targeting. Converting lets you ship the audio without asking listeners to install a codec. The loss in quality between the two is negligible at sensible bitrates.
HOW TO CONVERT
SPX → GSM
Upload the SPX
Drop or select your SPX file. The upload is encrypted and the file is queued for conversion.
Transcode via FFmpeg
FFmpeg decodes the SPX stream to PCM internally, then re-encodes as GSM at the bitrate you select.
Download the GSM
The GSM is delivered as a direct download; metadata and cover art transfer automatically where possible.
Common Use Cases
Podcast distribution
Podcast hosts (Spotify, Apple, Acast) publish audio as GSM when the workflow requires it; converting upfront skips server-side transcoding.
DAW ingestion
Pro Tools, Logic, Ableton and Reaper pull GSM into projects without decode overhead, so scrubbing and waveform display are snappy.
Portable players
GSM plays reliably on old iPods, car stereos, Bluetooth speakers and fitness trackers where SPX support is spotty.
Voice memo sharing
Voice notes recorded as SPX travel to phones and desktops as GSM without recipients installing extra codecs.
SPX vs GSM — Strengths and limitations
What each format does best, and where it falls short.
SPX Strengths
- Patent-free voice codec.
- Three sample-rate modes for voice.
- Low CPU decode.
Limitations
- Deprecated in favor of Opus.
- No music support.
- Rarely used in new projects.
GSM Strengths
- Tiny bitrate (13 kbps) — hours of speech in a few MB.
- Speech-optimized — clear voice reproduction.
- Universal cellphone decoder adoption 1991-2015.
- Stable since 1987.
Limitations
- Speech-only — music sounds distorted.
- 8 kHz sampling — narrowband, muffled by modern standards.
- Legacy — LTE VoLTE moved to AMR-WB, Opus, or EVS.
SPX vs GSM — Technical specifications
Side-by-side comparison of the technical details.
SPX
- MIME type
- audio/speex
- Extension
- .spx
- Container
- Ogg
- Modes
- Narrowband/Wideband/Ultra-wideband
- Successor
- Opus
GSM
- MIME type
- audio/gsm
- Extension
- .gsm
- Codec
- GSM 06.10 (RPE-LTP)
- Sample rate
- 8 kHz
- Bitrate
- 13 kbps
| Specification | SPX | GSM |
|---|---|---|
| MIME type | audio/speex | audio/gsm |
| Extension | .spx | .gsm |
| Container | Ogg | — |
| Modes | Narrowband/Wideband/Ultra-wideband | — |
| Successor | Opus | — |
| Codec | — | GSM 06.10 (RPE-LTP) |
| Sample rate | — | 8 kHz |
| Bitrate | — | 13 kbps |
SPX vs GSM — Typical file sizes
Approximate file sizes for common scenarios.
SPX
- 1 min voice (wideband 24 kbps) ~180 KB
GSM
- 1 min of voice ~100 KB
- 1 hour voicemail archive ~6 MB
Quality & Compatibility
Lossy-to-lossy transcoding (most cross-format audio jobs) loses a tiny amount of quality on each pass — usually inaudible at our default VBR ~190 kbps for music or 96 kbps for speech. Lossy-to-lossless conversions freeze the existing quality but cannot improve it; lossless-to-lossy is only as good as the target bitrate you choose.
Tips for Best Results
- Pick 128 kbps for podcasts and voice, 192–256 kbps for music, 320 kbps only if the audio will be edited further downstream.
- Keep the SPX master alongside the GSM — re-encoding a lossy format twice accumulates audible artefacts.
- For mono voice content, convert to mono GSM explicitly to halve file size without any quality loss.
Frequently Asked Questions
Lossy-to-lossy conversions (most combinations) re-compress the audio, which technically introduces some loss. At a 192 kbps or higher target it is inaudible on normal equipment. Lossy-to-lossless conversions freeze the existing quality but cannot improve it; lossless-to-lossy transcodes are only as good as the target bitrate you choose.
For voice content (podcasts, audiobooks, lectures) 128 kbps is indistinguishable from higher bitrates. For music, 192-256 kbps covers most listening; 320 kbps is the ceiling for GSM and the right choice for audio you plan to edit further. Above that, prefer a lossless target instead.
Yes. Title, artist, album, year and cover art travel from the SPX container to the GSM container automatically where both formats support them. If a tag field has no GSM equivalent, it is dropped silently. Use any tag editor (Mp3tag, MusicBrainz Picard) to fine-tune afterwards.
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