ID3 tags are metadata stored within MP3 files that contains information such as; the title of the podcast or episode, the artist, and an image artwork for the episode. There are two versions of the ID3 tags.
ID3v1 contains only basic, text-only, information and it’s placed at the end of the audio file. ID3v2 consists of rich type information like the artwork and they’re placed at the beginning of an audio file. By using ID3v2, a podcast player can display such metadata without having to download the entire file.
In order to provide accurate measurement of podcast plays, IAB has defined a play as valid only when the amount of transferred data is equal to at least 1 minute of actual audio, excluding the size of the ID3v2.
As part of the Spreaker ingestion process, we analyze every uploaded file in order to identify the presence and the size of the ID3 metadata, so this can be used to accurately measure the duration of the transferred audio excluding ID3.
Spreaker, as hosting platform following IAB specifications, recommends the content creators to limit the size of the ID3 metadata to reduce the data transfer and the time it occurs for players to be able to playback the first actual audio. The element that accounts for most of the size of the ID3 tags is the embedded artwork, that should be limited to 300KB.