.AMR File Format

Introduction

AMR is a speech codec standardised by 3GPP for mobile phone calls. It switches between bit rates from 4.75 to 12.2 kbit/s to suit the radio conditions. Phones used it for voice memos and voice messages, so .amr files are common in old phone backups. The narrowband codec samples at 8 kHz; the wideband version, AMR-WB, samples at 16 kHz and its files use .awb.

Structure

The storage format is defined in RFC 4867. A narrowband file starts with the six bytes #!AMR and a line feed; a wideband file starts with #!AMR-WB and a line feed. Frames follow with nothing between them. Each frame is 20 ms of speech: a header byte whose frame type field gives the mode, and so the frame’s length, followed by the speech bits.

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