Sony will expand its Network Walkman line of digital music players later this month with the introduction of three new players based on flash memory chips.

The use of flash memory is new for Sony's Network Walkman players – previously they have all used hard disk drives.

Weighing in at 37g, the new Network Walkman players are circular in shape, slightly smaller in diameter than a soft drink can, and 2cm thick. All three players include a LCD (liquid crystal display).

The new players – the 256MB NW-E103, the 512MB NW-E105 and the 1GB NW-E107 – will be available in Japan from March 21 and should be available worldwide by the end of March.

The introduction of these three players marks the start of Sony's expansion plans for the Network Walkman line. On April 21, Sony will launch an additional four players in Japan, which will be available in the rest of the world by the end of that month.

All seven of these upcoming players support MP3 music files as well as Sony's ATRAC3 and ATRAC3plus file compression format, which is used in the Sony's MiniDisc players and its online music store.

MP3 support was missing from the company's first Network Walkman when it was launched in mid-2004. However, Sony began offering a Network Walkman player that supports MP3 late last year and has offered to upgrade earlier players to include the function.

Both battery life and number of songs that can be stored varies depending on the file compression format that users choose, according to Sony, which said the Network Walkman players offers the longest battery life when used with ATRAC3plus.

An ATRAC3plus file encoded at 48Kbps (kilobits per second) offers audio quality that is equivalent to an MP3 file encoded at 128Kbps, Sony said. However, ATRAC3plus files take up less memory which means users can store more songs in a player. This difference can be significant. A player with 1GB of memory can hold 47 hours of music encoded in ATRAC3plus format but only 17 hours and 40 minutes of music encoded using the MP3 format, Sony said.

All seven flash-based Network Walkman players use AAA-type batteries. In power-save mode, battery life can be as long as 50 hours for the stick-type models during playback of music encoded at 105K bps in ATRAC3 file, Sony said. Under similar conditions, the circular-shaped models offer battery life up to 70 hours, it said.