Audio codecs that are not optimized for speech are more damaging to the speech signal and can reduce transcription accuracy. It is important to choose your codecs wisely. The following list describes a variety of audio codecs and their implications on transcription accuracy.

G.711 µ-law / A-law (best option)

64 kbps per channel - low-compression, speech-optimized codec

G.729A/B (average option)

8 kbps per channel - aggressive compression, but optimized for speech

G.723.1 (poor option)

6 kbps per channel - highly compressed with poor quality

Alternative audio codec options:


Newer, speech-optimized codec providing good results at 32 kbps per channel and higher


MP3 is a lossy codec optimized for music, not speech. If MP3 is being used for transcription, Voci recommends using at least 32 kbps per channel.

Note: Transcription accuracy decreases as the quality of audio degrades.