μ-law vs A-law: getting text-to-speech right on phone calls

G.711 remains the standard format for many telephone calls, transmitting 8 kHz, 8-bit companded audio as μ-law/PCMU, common in North America and Japan, or A-law/PCMA, common in many other regions. Its limited frequency range removes much of the high-frequency detail that gives modern 24 kHz text-to-speech voices clarity, while unnecessary transcoding, resampling, or improper buffer padding can further introduce noise, clicks, latency, and degraded interruption handling. For phone voice agents, the recommended approach is to request the carrier’s native 8 kHz μ-law or A-law format directly from the TTS provider, send it at the real-time rate of 8,000 bytes per second, and match the negotiated codec used by services such as Twilio, Telnyx, or SIP trunks. The discussion also highlights practical implementation concerns, including 20 ms frame sizing, clearing queued audio during caller barge-in, correct G.711 WAV headers for playback, and the fact that A-law call paths may require direct A-law output for TTS but decoding inbound audio to 8 kHz 16-bit PCM for speech-to-text systems that do not accept A-law.

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