With the Apple iPod and all the other portable digital music players on the scene, you'd think it was a simple matter to get all the songs you want and play them on the go. Before you rip and run, there are a number of issues to consider.

At up to a dollar a pop, tunes can get expensive. And if you choose the wrong player or music service, your songs may not play.

There are two basic types of legal online music services: Music stores allow you to purchase and download songs; subscription services let you rent songs.

It sounds simple, but it's not. Most online stores encrypt songs to stop illegal file sharing and duplication. The most widely used formats are Advanced Audio Coding (AAC) and Windows Media Audio (WMA). The iPod plays AAC files; most other players use WMA.

The iTunes Music Store (www.apple.com/itunes) has more than 2 million songs in its library. Each song costs 99 cents; albums are $9.99 and up. The songs are encoded in the AAC format. The iTunes Music Store runs on both Windows PCs and Macs and integrates seamlessly with the iPod.

But music purchased from iTunes does not easily transfer to other brands of music players. If you don't own an iPod, you could wind up with a lot of music on your computer's hard drive that isn't playable on your portable machine.

If your music player is not an iPod, you'll have an easier time downloading music from MSN Music (music.msn.com), Wal-Mart Music (musicdownloads.walmart.com) and Yahoo Music (music.yahoo.com). Music downloaded at these three sites will play on WMA-compatible players.

MSN Music and Yahoo charge 99 cents per song download. Albums are $9.99 and up. Wal-Mart Music is a tad cheaper — 88 cents per song and a starting price of $9.44 for albums. All three music stores are compatible only with Windows.

However, buying music gets expensive. That's where subscription services step in. Three companies offer all the music you want for a monthly fee. Instead of buying the songs, you rent them. They remain playable as long as you maintain your subscription.

For about $15 per month, Napster To Go (www.napster.com) and Rhapsody To Go (www.rhapsody.com) offer unlimited downloads of more than 1 million titles. Yahoo Unlimited Music To Go (music.yahoo.com/musicengine) charges about $12 per month, or $120 if you pay for a year upfront. All three require Windows XP.

So for the price of one CD, you can fill your player's hard drive. All three services offer a variety of music genres. If you simply must own a song, you can purchase it. Napster and Rhapsody charge 99 cents; Yahoo asks 20 cents less.

You download the songs to your computer's hard drive, then copy them to your music player. To use the music, your subscription must stay in force. A clock in the music player reads information from the song file. This information tells the player whether your subscription is valid. If it's not, the rented music won't play.

Additionally, you must synchronize your player to your computer monthly to let the player know whether your subscription has been renewed. Should you fail to do so — say, you're on a trip — you'll have unplayable music.

Also, your player may be incompatible if it was not made in the past year or so. The subscription technology is fairly new, so check with the service for a list of compatible devices.

Contact Kim Komando at www.komando.com/contact.asp.