Researching the experiences of New Zealand soldiers in North Africa during World War II, Martyn Thompson made more than 80 tape recordings of old veterans' stories.

The material was for his 2005 book, Our War: The Grim Digs: New Zealand Soldiers In North Africa, 1940-1943.

Those cassette tapes might not withstand the passage of time and have now been digitised to preserve them, a project toward which the Northland RSA Trust gave $4000.

New Zealand Micrographic Services Ltd undertook the job of preserving Thompson's recorded material.

NZMS is the country's largest full-service bureau working in the heritage and cultural sectors to transfer material to future-proof digital form.

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The company turned Thompson's 74 micro and six compact discs of recordings into 128 files, or 90 hours of listening.

Recordings made on now obsolete technology might not survive the test of time.
Recordings made on now obsolete technology might not survive the test of time.

NZMS Northern region manager Alison Barnett said the company started digitising aural material about five years ago when it became increasingly apparent the older-style magnetic tapes had a relatively short remaining life.

''It is estimated that most magnetic tape won't be playable after 2025,'' Barnett said.

While NZMS' focus is on preserving material by transferring it to a more stable electronic storage environment, any job would be turned down if it were likely to harm the original material.

''Because most of the playback machines we require to capture these formats on are no longer manufactured, we obtain them second-hand from the likes of Trade Me.

''Even then because they are so rare, particularly those that have the high levels of specifications we want, we like to keep a close eye on seller and community websites.

''When we receive a machine, we have it repaired, modified, calibrated or serviced by one of the few providers still possessing this knowledge and experience. In addition to our daily cleaning and checks routine, we have playback machines serviced on a six-monthly basis, dependent on throughput.''

Magnetised particles on the top layer of the tape can rub off over time when the tape is played; in turn that residual material in the player further corrupts other tapes when they're played.

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Problems caused by fragile tapes or a dirty system are familiar to anyone who remembers how cassette players could ''eat the tape'', Barnett said.

All components during the digital transfer process are demagnetised daily as part of the time-critical project.

''Once everything is stable and clean, it goes through an analogue playback machine connected to an analogue-to-digital converter at the same speed the original tape was recorded.

''Then there is a quality check to make sure there is no missing audio information and to see whether any digital improvements can be made."

Noises like hisses and pops, characteristic of scratches in records, or magnetic tape ''noise'' can be deleted.

The finished product is likely to be crisper but still hold the integrity of the original sound - the recording's own ''voice''.

NZMS has offices in Wellington, Christchurch and Auckland, with the digitisation suite in Wellington. The company has more than 45 staff.

So serious is the outlook for a great deal of the world's existing audiovisual archives, UNESCO raised concerns about the problem some years ago and has encouraged cultural sectors worldwide to preserve their most important audio visual material by transfer to digitised formats, Barnett said.

New Zealand Micrographic Services Ltd began in 1990 — ''before the digital revolution'', initially microfilming the country's newspapers: a third of the entire New Zealand newspaper archive, or about 15 million images.

Today the company digitises all formats of heritage records and has also developed a software platform called Recollect, a user-friendly online community engagement and archival management system that gives communities access and contributing rights to their digitised archives.