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12/12/2017

Broadcasters & Post Production Facilities Digitising Legacy Tape Formats: Pt 4

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The fourth 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.

What could possibly go wrong using that old VTR to play my legacy tape?

So we've looked at how the magnetic particles on the tape hold our legacy analogue or digital data and how fragile those tapes become as they age. We will now look at the added risks having to use old machines presents to our fragile and ageing tapes.

So let's now add in the machine we intend to use to play that back on. It can't be a new machine; they aren't made any more. At best, it's a used machine we've had from new and has been regularly serviced and inspected. Or maybe it's a dusty old machine left in the corner. Nobody actually knows if it even works or when it was last used.

So we have this complicated, aged machine that we intend to power up and stick our tape into it. We need to do this to capture or digitise tape based content into our digital storage system (NLE, MAM, etc).

That VTR has to match the format of the tape and as we saw in edition 1 there is a very wide range of formats from reel to reel through cassette based covering both analogue and digital formats, but all of these video tape recorders will involve mechanical processes to move the tape around allowing the playback of the tape.

Playback is the process of moving the tape from one spool to the other (supply to take up) while controlling the speed of the tape and allowing the audio and video heads to pick up the very small magnetic signals.

Both digital and analogue formats also suffer from the limitations of the tape path used to allow the tape to come out of the cassette into the machine. For reel to reel formats this is a manual process of course but much of what follows applies. Obviously to be able to record or playback on a tape, be it digital or analogue, the tape has to be loaded and drawn into the VTR mechanism. It then needs to be moved from one spool to the other as playback or record proceeds. During that process a lot of mechanical "stuff" happens.

The tape needs some "running space" to help dampen changes in tension caused as the tape is drawn from the supply spool and wound back into the take up spool. Both of these will cause slight judders and stutters to the smooth tape progress. If uncorrected this could be reflected in reduced audio quality (wow and flutter for sound) and unstable playback of the picture (horizontal twitching and vertical judder) for analogue formats. Digital formats are more robust in this regard as the structure of the digital signal is easier to manage and reassemble devoid of these issues (unless they are very extreme).

Also, digital tape formats can take advantage of error correction to calculate missing digital tape reads caused by issues with video head to tape contact (head clog) or damaged tape. Minor errors can be completely masked. More significant errors can be vastly improved. Digital VTRs have a channel condition indicator to show this in progress.

Green = All okay, no errors detected.
Amber = Errors detected but corrected.
Red = Serious errors, correction attempted but corruption of signal may have occurred.

Analogue formats have far less effective correction mechanisms. With many offering, at best, the detection of missing tape signal, that would appear as white noise on screen, being used to substitute information stored from the line before. Adequate for very short losses of tape signal but for damaged tape often gives the familiar image dragging down the screen symptom.

To help ensure a smooth tape running path all formats use a speed regulation device. This is formed by a capstan motor (a slow turning weighted vertical metal pole) with a pinch roller pressing hard against it. Between the capstan pole and pinch roller runs your important tape. It is effectively gripped and squashed between the two to force it to pass at the speed of the capstan motor's rotation. When new, these components work fine but as the hours clock up on the VTR, the capstan pole will wear and start to force the tape up or down or both. This causes the tape to run against internal tape guides (whose job it is to ensure the tape stays in its place). If the force applied to the tape by a worn capstan pole is too great, the tape will start to crinkle against the guide edge and the tape become damaged. Also, the pinch roller, which is not unlike a very small car tyre, which is made of rubber, hardens with age and can become deformed and lumpy. This also helps damage the tape as its squeezes it on its way past.

So both digital and analogue VTRs have similar tape paths and as they age both suffer similar dangers to the passage of the tape. However, generally, digital VTRs will show less visible signs of issues unless compatibility is very different.

So what do we mean by "compatibility"?

Well, as mentioned above, for a particular format there will be a manufacturer's specification for how the tape path should be aligned. That ensures every recording is laid onto the tape in exactly the same way and position. Obviously as the VTRs are used they will wear the tape path, so a degree of tolerance has to be allowed. This tolerance can be extended by giving the operator some controls to help compensate for tape variance.

Many analogue VTRs will have a tracking control to allow the operator to adjust the scanning of the video heads to match the position of the signal on the tape. Some formats have a skew control; a control that increases or decreases the tension on the tape and so helps the position of the tape round the video heads. (Skew on an analogue tape playback is often seen as a horizontal hooking of the top of the picture as it appears to be dragged to one side).

Digital VTRs don't have such controls as the design of the VTR tape paths had evolved significantly by the time they were introduced and any compatibility differences could largely be compensated for automatically.

We currently tend to consider analogue formats as old or legacy, but already early digital formats are presenting similar issues and more will fall into this category as time passes.

They will age of course. The whole principle of a magnetic tape recording is we are forcing small particles on the tape to adopt a magnetic value that represents the wanted signal. From that moment on it's a race against time for that magnetism to leech away and the particle become neutral. Depending on the strength of the original recording signal, the properties of the magnetic tape, the number of playback passes, and storage conditions, that race against time could be as little as 15 years or as much as... Well, we can't know until it's gone. As mentioned previously, 30 years seems to be a good estimate but that should be the outside duration. To ensure recovery of the data, digitising should be done well before that 30-year mark.

However, before the magnetic signal disappears the tape binder formulation could cause the brittle metal particle or oxide covering to flake off as the tape passes around the path in the VTR. Even if the tape escapes, the two issues above it may have stretched due to repeated playback, making it harder to ensure a compatible playback. (Digital recording may be achieved but at a much reduced quality than could have been achieved).

So we have a selection of legacy old analogue tapes we need to digitise and no decks around to do it. Probably no equipment to capture the content even if we did. There are two routes forward:

1. We find somebody who can do it with the correct kit and the correct skills who will care about the tapes and the transfer process.

2. Source a used VTR, source the required capture kit, work out how to connect for capture in a way that doesn't reduce the quality of the resultant digital file. Hope the used VTR isn't worn out and damaging your tapes (what's the chances of that with an eBay bargain?) Find or learn the skills that would have taken an engineer a decade or so to learn.

What could go wrong?

Article by Pat Horridge at VET Training.

www.vet.co.uk

(JP/MH)
Solidmate Ltd Memory Card Hire London

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