CONVERT
GSM → SPX
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Fast, secure GSM to SPX conversion. No registration required.
Why this pair exists — GSM is an audio format with specific trade-offs between file size, bitrate flexibility, and device support. Ergo, the SPX route. Need a SPX version of a GSM recording for a podcast host, audio book platform or DAW that refuses the original container? Drop the file above and our encoder produces a clean SPX you can drag straight into the destination tool. Metadata such as title, artist and cover art travels with the audio. In practice GSM is an audio format with specific trade-offs between file size, bitrate flexibility, and device support. On the other end, SPX is an audio format with specific trade-offs between file size, bitrate flexibility, and device support.
GSM Audio
Source 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.
Speex Audio
Target 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.
Why convert GSM to SPX
GSM Audio is great in its own niche, but Speex 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
GSM → SPX
Upload the GSM
Drop or select your GSM file. The upload is encrypted and the file is queued for conversion.
Transcode via FFmpeg
FFmpeg decodes the GSM stream to PCM internally, then re-encodes as SPX at the bitrate you select.
Download the SPX
The SPX 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 SPX when the workflow requires it; converting upfront skips server-side transcoding.
DAW ingestion
Pro Tools, Logic, Ableton and Reaper pull SPX into projects without decode overhead, so scrubbing and waveform display are snappy.
Portable players
SPX plays reliably on old iPods, car stereos, Bluetooth speakers and fitness trackers where GSM support is spotty.
Voice memo sharing
Voice notes recorded as GSM travel to phones and desktops as SPX without recipients installing extra codecs.
GSM vs SPX — Strengths and limitations
What each format does best, and where it falls short.
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 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 vs SPX — Technical specifications
Side-by-side comparison of the technical details.
GSM
- MIME type
- audio/gsm
- Extension
- .gsm
- Codec
- GSM 06.10 (RPE-LTP)
- Sample rate
- 8 kHz
- Bitrate
- 13 kbps
SPX
- MIME type
- audio/speex
- Extension
- .spx
- Container
- Ogg
- Modes
- Narrowband/Wideband/Ultra-wideband
- Successor
- Opus
| Specification | GSM | SPX |
|---|---|---|
| MIME type | audio/gsm | audio/speex |
| Extension | .gsm | .spx |
| Codec | GSM 06.10 (RPE-LTP) | — |
| Sample rate | 8 kHz | — |
| Bitrate | 13 kbps | — |
| Container | — | Ogg |
| Modes | — | Narrowband/Wideband/Ultra-wideband |
| Successor | — | Opus |
GSM vs SPX — Typical file sizes
Approximate file sizes for common scenarios.
GSM
- 1 min of voice ~100 KB
- 1 hour voicemail archive ~6 MB
SPX
- 1 min voice (wideband 24 kbps) ~180 KB
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 GSM master alongside the SPX — re-encoding a lossy format twice accumulates audible artefacts.
- For mono voice content, convert to mono SPX 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 SPX 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 GSM container to the SPX container automatically where both formats support them. If a tag field has no SPX equivalent, it is dropped silently. Use any tag editor (Mp3tag, MusicBrainz Picard) to fine-tune afterwards.
RELATED CONVERSIONS
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