Telephony Audio Codecs Explained: G.729, A-law, µ-law and ADPCM
Why do some recordings play fine while others turn to noise? Understanding these telephony codecs is the prerequisite for correctly converting and migrating historical audio.
Why these codecs matter
To save bandwidth and storage, telephony recordings are almost always kept in compressed or companded codecs. Decode with the wrong codec and you get distortion at best, pure noise at worst. Identifying the codec is the first step before migrating or converting historical audio.
A-law and µ-law (G.711)
The most common telephony companding codecs: A-law is used mainly in Europe and China, µ-law mainly in North America and Japan; both compress 16-bit linear PCM into 8 bits. The classic mistake is reading A-law as if it were linear PCM — producing pure noise, which is the real cause of many "failed conversions".
G.729
A high-compression codec (~8 kbps) common in VoIP and some recording systems (including inside NICE NMF). It compresses heavily but requires the matching decoder, and decoding has patent/licensing background that generic tools may not include.
ADPCM
Adaptive Differential PCM, with a compression ratio between G.711 and G.729, is common in older systems. It too must be decoded with the correct variant.
The key takeaway
The extension (.wav / .nmf) does not equal the internal codec. Correct conversion means first detecting the real codec, then decoding to linear PCM with the matching decoder while preserving channels and metadata. Shanghai Wanchun's NMF parsing and format liberation are built around exactly this.