This approach is based on information kindly provided by a respondent to my query on this topic in the Spacial Audio forums (“Cygnus X-1″ in fact, a forum admin). The terminology assumes you’re using SAM Broadcaster (Spacial’s Windows-based radio automation/DJ system that I own but don’t use as it doesn’t support FLAC files), but the theory is general and can be applied to any playout system.

Bear in mind that if you implement the DMCA requirements in the simplest way, you have to make sure that neither a track from the same artist nor album is played more frequently than once an hour. In terms of SAM’s Playlist Rotation Rules, this means an “artist repetition” and an “album repetition” of 60 minutes (ie at least 60 minutes must pass before another track from the same album, or by the same artist, can be played).

If you build playlists by choosing a theme and then adding entire albums that embody that theme, then play out all the tracks from those albums at random (which is I think what most of us do most of the time), then in general, a rough estimate for a good mix is:

(artist repetition rule in minutes) / (average track length in minutes) = number of artists

floor( ln( (album repetition rule in minutes) / (average tracks on an album) ) * number of artists ) = number of albums

Notice that the length of the programme does not appear in this formula. However, this does give you the minimum size of the album “pool” you need to create a playlist that will meet DMCA rules.

For those with a mathematical bent: the floor function is a special kind of “rounding” function: floor(x) represents the greatest integer less than or equal to x. So if x = 4.9, then floor(x) = 4, for example. You always round down. And here you can find a calculator to evaluate natural logarithms, ie ln(x).

Example

Assume that the average number of tracks on an album is 12, and the average track length is 4 minutes:

Given that the artist repetition rule is 60 minutes and album repetition rule is 60 minutes:

60 / 4 = 15 artists

floor( ln ( 60 / 12 ) * 15 ) = floor( ln ( 5 ) * 15 ) = 24 albums

(ln (5) = 1.6094379124341; ln(5) * 15 = 24.141568…; so the floor value is 24.)

This is therefore the minimum number of albums and artists you can have in a DMCA-compliant playlist, given the average number of tracks on an album and the average track length specified above. You can have a longer playlist, of course (but not a shorter one), and as long as the artist:album ratio (with these average values) remains 15:24 (or about 1.6 albums per artist), you won’t run out of music.

But don’t forget that if your average number of tracks per album and/or their length is different, the ratio will differ too, so work out the ratio for the kind of music you’re playing.