HTOA

Hidden Track One Audio (HTOA) is non-silent audio data located in the index 00 portion ("pre-gap") of track 01 on an audio CD. CDs normally start playing a track at index 01, which comes after 00, so to hear HTOA, one has to manually scan backward from the start of track 01. Not all DAE (CD ripping) software can read HTOA.

The presence of index 00 data on track 01 doesn't necessarily mean there is HTOA. It's common for a small amount of silence to be in index 00. Sometimes it's a large amount of silence. Generally it's only considered HTOA if it's not silent.

When people refer to a "hidden track" on a CD, they may mean HTOA, or they may mean the more common situation where the last track simply contains two songs separated by silence.