Prompt Recorder

About the Prompt Recorder

The Prompt Recorder captures audio from your microphone and encodes it as a G.711 μ-law WAV file — the standard format required by most IVR and IVA telephony platforms, including Five9, Avaya, Genesys, and Cisco.

What is G.711 μ-law?

G.711 is the ITU-T standard codec used in PSTN telephone calls. The μ-law variant (used in North America and Japan) encodes 16-bit PCM audio into 8-bit samples at 8,000 Hz using logarithmic compression. This gives an effective dynamic range comparable to 12-bit linear PCM at 64 kbps (8,000 bytes per second) — ideal for telephone-quality voice prompts.

Why not just record a regular WAV?

Most telephony platforms require prompts in a specific format: mono, 8 kHz sample rate, 8-bit μ-law encoding. Uploading a standard 44.1 kHz stereo WAV will either be rejected outright or converted automatically — sometimes with quality loss or incorrect loudness. The Prompt Recorder outputs the exact format your platform expects.

How does the pipeline work?

• Audio is captured via the browser's MediaRecorder API.
• On stop, the raw audio is decoded by the Web Audio API directly at 8,000 Hz, so the browser's decoder handles resampling with proper anti-alias filtering.
• All channels are mixed down to mono by averaging.
• Each sample is encoded with the ITU-T G.711 μ-law algorithm (pure JavaScript, no libraries).
• A WAV container is written in memory with format code 0x0007, 1 channel, 8 kHz, 8-bit depth.

Is my audio private?

Yes. All recording, resampling, and encoding happens entirely in your browser using the Web Audio API. No audio data is transmitted to any server. The download is generated locally as an in-memory Blob.