Transcoding and compressing audio
Transcoding is the direct digital-to-digital conversion of data from one encoding to another. Transcoding audio files can help conserve disk space and shorten the time it takes to transfer files. The following list describes the four transcode types, each of which has different implications on transcription accuracy.
- Lossless-to-lossless (recommended)
No audio information is lost during the lossless-to-lossless transcoding process. Converting from a PCM WAV file to a FLAC file is an example of lossless transcoding, commonly used for saving disk space without compromising on quality. A 10-minute, mono WAV file at 8-bit/16 kHz is 9.8 MB, whereas the same file after FLAC conversion is 5.6 MB.
Lossless-to-lossy transcoding eliminates information from the audio signal that is less important for human speech comprehension. However, this loss of information negatively impacts ASR performance. Therefore, this form of transcoding is not recommended.
Using any form of to-lossy transcoding will decrease quality. Lossy-to-lossy is even worse because repeated lossy transcoding will cause a progressive loss of quality with each successive transcoding pass. This is known as "digital generation loss" or "destructive transcoding"and is irreversible.
Transcoding from lossy-to-lossless is strongly discouraged. The quality of the audio file does not improve and the file size will increase.