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04/12/2017

Broadcasters & Post Production Facilities Digitising Legacy Tape Formats: Pt 3

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Broadcasters & Post Production Facilities Digitising Legacy Tape Formats
The third in a series of articles by Pat Horridge of VET Training covering the issues facing broadcasters & post houses needing to digitise legacy tape formats.

Pat, a former broadcast service engineer then technical director of the Shoreditch based VET post house, now delivers broadcast and post house QC training and Avid workflows on site for clients. Along with post production overview courses and general understanding digital media training. Full details at the VET website – www.vet.co.uk.

Some of the information provided is also common to restoration and archiving however the needs of broadcast TV are less demanding and the information in these articles is based on the assumption that the process is for TV ingest. For Archiving purposes the situation is often more complex and demands a more robust workflow, often for a greater volume of content.

Having worked specifically for organisations dealing with archiving legacy content the selection of playback equipment has more to do with achieving the best quality playback and protection of the tapes than a cost effective "ok for TV" approach.

This is covered in 5 articles:
• Legacy tape formats. What are they and how do we define them?
• How do we store audio/video content on magnetic tape?
• Aged tape considerations and solutions.
• What could possibly go wrong using that old VTR to play my legacy tape?
• The ingest process for best results.

Aged tape considerations and solutions
In the previous editions we looked at what are legacy formats and how we actually store the magnetic signal on the tape. Now we'll look at what sorts of issues ageing of the tape can cause for us when we try and play it back.

As we mentioned before, fundamentally we are hoping to recover the magnetic strength and polarity (N or S) of each magnetic particle on the tape to achieve accurate playback. There are a number of environmental factors that can have compromised the tape. Damp storage can cause mould to grow on the tape. This is a real problem as mould spores can be spread from that tape to others by the playback machine. The mould will also "clog" the playback heads preventing tape to head contact and loss of playback signal. Mouldy tapes need to be handled carefully to prevent mould spreading and will require specialised cleaning before playback. Damp can also cause some formats to actually rust. The metal particles break down, forming metal oxide (rust) and damage the magnetic information as well as causing a rough tape surface making playback impossible.

The tape can become stuck to the tape layer in front and behind it. This can promote another issue called shedding. Shedding (or sticky-shed as its often called) is when the magnetic particle layer of the tape becomes brittle and, as it passes the various posts and poles in the VTR, separates from the backing material of the tape and falls into the VTR. This is clearly a terminal problem! This can be temporarily improved by "baking" the tape at a low 55°C for up to eight hours. This then allows a short window of playback time.

Physically, cassette tapes can also suffer a few more obvious issues. Too many labels may have been applied to the case or stuck in the wrong places and interfere with correct loading and extraction of the tape. The tape flap can become stuck or damaged.

Most tape cassettes have brakes inside them to prevent the tape unspooling in transit. These brakes lock the spools in position and are released when the tape is loaded in the VTR. These brakes can jam, be damaged, or otherwise play up and cause the tape to be stretched as the VTR tries to extract it and the brakes refuse. If the tapes have been moved around a lot, the winding of the tape on the spools can become uneven and cause the spool not to turn smoothly (often corrected by spooling the tape from start to end and then back). If the tape has been played a lot it may have stretched and this can make playback difficult as the track locations will have moved.

And finally the biggest issue may exist and that's the loss of the magnetism on the particles on the tape. Naturally, the particles of the tape would have no significant magnetic value; they would be balanced, neither N or S magnetised. When we record on the tape we induce the required magnetism. This isn't a stable state for each particle; it wants to be as it was and the particles around it will be slowly working to balance it back to what it was. Coupled with the fact that here on Earth we live in a magnetic field (it's how compasses can show North from South) which is also slowly "erasing" that magnetic information.

So how long can a tape "hold" its magnetic information before it's gone? Sadly, there are too many factors to accurately determine that but studies seem to suggest 30 years as a good starting point. (www.clir.org/pubs/reports/pub54/4life_expectancy.html)

It is worth noting that some of the older formats seem to be standing up better than their later replacements. VHS is still viable, whereas Digital hi8 less so. Umatic is still accessible, but D2 much less so.

The various manufacturers of tape stock use slightly different formulations and tape construction processes and this will impact the longevity of the magnetic recording.

The strength of the original recording process will have a direct impact on the life of the magnetic recording. A poorly calibrated record VTR may well have made a recording that played back okay at the time, but if the record current was 80% of what it could have been, a significantly weaker magnetic signal will have been recorded and subsequently a shorter tape life will result.

Playback in VTRs that haven't been degaussed (a process of removing any residual magnetism using a special degaussing tool) regularly will also reduce the strength of the magnetic signal on the tape. Metal parts inside the VTR that are in contact with the tape can become magnetised and this magnetism can transfer to the tape as it plays, effectively weakening or replacing the magnetic signal that represents our data. This is prevented by using a head degausser to remove that unwanted magnetism inside the VTR.

And we then have the ability of our playback VTR to actually be able to read the magnetic signal; a machine with good quality heads and well calibrated will read a weaker signal. Also, some models of VTR cope better with weak tape signals than others so selecting the correct model is important.

So, the ability of a tape to hold its magnetic information is down to the stock used, the calibration of the original recording VTR, the environmental conditions the tapes been subjected to and any previous playback attempts, as well as the machine we use to attempt playback. Most of these are invisible to the human eye and can only be assessed by playback on a machine capable of displaying the magnetic strength of the signal on the tape.

One thing is guaranteed. The magnetic signal on every tape is slowly erasing as it sits on the shelf. Time isn't on the side of old tape formats.

And if that's a worry, just think that your hard drives use the same principle to store magnetic information. Yup, that's right. Your hard drives are slowly erasing… but don't worry, it's more likely the bearings, motor or electronics will fail before then.

The online back-up company BackBlaze analysed the failure rates of their 25,000 running hard drives. They found that 90% of hard drives survive for three years, and 80% for four years. So in four years only two in 10 of your drives may have failed.

In the next addition we'll be looking at the sorts of issues using old VTRs can present and the sorts of risks they pose to our aged tapes. What could possibly go wrong using that old VTR to play my legacy tape?

Article by Pat Horridge at VET Training.

www.vet.co.uk

(JP)
VMI.TV Ltd

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