Truespeech

Truespeech is a proprietary audio codec produced by the DSP Group. It is designed for encoding voice data at low bitrates (8.5 kbps for 8 kHz samples), and to be embedded into DSP chips. Truespeech had been integrated into Windows Media Player in older versions of Windows, but no longer supported since Windows Vista. It was also the format used by the voice chat features of Yahoo! Messenger.^[1] It is implemented through the Tsd32.dll

A Truespeech decoder was implemented in the 0.5 release of FFmpeg.^[2]

References

^ "CT8022 VOIP/VON G.723.1, G729AB TRUESPEECH® CO-PROCESSOR" (PDF). Archived from the original (PDF) on 4 April 2018. Retrieved 4 April 2018. The CT8022 has built-in TrueSpeech 8.5 (compatible with Microsoft Sound System 2.0, and a standard part of Microsoft Windows 95), and TrueSpeech G.723.1 (for H.323 and H.324). [...] The CT8022 provides two additional non-ITU TrueSpeech data rates at 4.8 and 4.1 KBPS.
^ "FFmpeg change log".

[dsp-ct8022-1] "CT8022 VOIP/VON G.723.1, G729AB TRUESPEECH® CO-PROCESSOR" (PDF). Archived from the original (PDF) on 4 April 2018. Retrieved 4 April 2018. The CT8022 has built-in TrueSpeech 8.5 (compatible with Microsoft Sound System 2.0, and a standard part of Microsoft Windows 95), and TrueSpeech G.723.1 (for H.323 and H.324). [...] The CT8022 provides two additional non-ITU TrueSpeech data rates at 4.8 and 4.1 KBPS.

[2] "FFmpeg change log".

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[2]