GSM to CVU Converter

Re-encode GSM audio to unfiltered CVU delta format online

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Raw Delta Encoding

Convert GSM speech into CVU — unfiltered CVSD modulation for signal processing research and specialized telephony applications.

Server-Based Process

No signal processing tools needed locally. The GSM to CVU conversion runs on our servers, accessible from any browser.

Private and Secure

All uploaded GSM recordings are erased after processing. CVU outputs are purged from servers within 24 hours.

How to convert GSM to CVU

1

Select files from Computer, Google Drive, Dropbox, URL or by dragging it on the page.

2

Choose cvu or any other format you need as a result (more than 200 formats supported)

3

Let the file convert and you can download your cvu file right afterwards

About formats

GSM 06.10 (Full Rate) is the foundational speech codec of the Global System for Mobile Communications standard, ratified by ETSI in 1991 and deployed across hundreds of cellular networks worldwide. Operating at a fixed 13 kbit/s, the algorithm applies Regular Pulse Excitation with Long-Term Prediction (RPE-LTP) to compress 20 ms frames of 8 kHz mono speech into just 33 bytes each. This approach models the vocal tract as a linear predictive filter, encodes the excitation signal, and leverages pitch periodicity for further reduction — tuned to deliver intelligible voice under the bandwidth constraints of early digital mobile channels. The codec powers not only GSM telephony but also many VoIP applications, voicemail systems, and IVR platforms that benefit from its low bitrate. Three concrete advantages stand out. First, extraordinary compression: one minute of speech fits in roughly 100 KB, enabling efficient storage and transmission. Second, universal tooling — libraries such as libgsm and SoX handle encoding and decoding on every major platform. Third, a royalty-free patent landscape that has encouraged adoption across open-source telephony projects like Asterisk and FreeSWITCH.
Initial release: 1991
CVU is an unsigned variant of the CVS telephony audio format, differing in how delta-encoded values are represented in the binary stream. While CVS stores slope delta values as signed quantities, CVU treats them as unsigned, shifting the numerical interpretation of each sample. Both share the underlying CVSD modulation technique — 1-bit adaptive delta coding where step size varies according to recent output bit patterns — operating at comparable rates, typically 16 kbps for narrowband voice at 8 kHz. The signed-versus-unsigned distinction matters at the decoder, where correct interpretation determines proper waveform reconstruction. CVU files appear in telephony and embedded communication contexts where hardware adopted the unsigned convention. A practical advantage is straightforward interfacing with systems using unsigned arithmetic natively, avoiding sign extension in decoders. Like its signed counterpart, CVU achieves extreme bandwidth efficiency, compressing voice into compact bitstreams for constrained links. SoX supports CVU, providing a reliable path for converting these niche telephony recordings into modern formats for analysis or archival.
Developer: CCITT / ITU-T
Initial release: 1970

Frequently Asked Questions

What is CVU?

CVU is the unfiltered variant of CVSD — Continuously Variable Slope Delta modulation without post-processing filters applied.

Why would I choose CVU over CVSD?

CVU provides raw, unfiltered delta-encoded audio useful for signal research or systems that apply their own filtering stages.

What applications use CVU?

CVU is used in specialized telephony research, signal processing experiments, and certain embedded communication systems.

Can I play CVU on a media player?

Standard media players cannot open CVU. You need SoX or specialized audio processing tools to work with this format.

Is the conversion private?

GSM uploads are deleted immediately after conversion. CVU results are removed from servers within 24 hours.