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Getting into Video (Part 5)

PART 5: In this final installment of the series, David Mellor looks at sophisticated timecode generators and readers in the Fostex range, and at complex timecode systems.


Useful as the Fostex 4030/4035 system is in synchronising audio to video, there is one very important thing it cannot do - generate timecode. A strange omission one might think, but many studios now have acceptable sources of code in the form of their timecode/MIDI synchronisers. Also, since the 4030/4035 does not include a code generator it does at least make you think about what sort of generator you might like to have: a very simple one, such as the compact Fostex 8700, or something with far more facilities, such as the Fostex 4010.

As a timecode generator, the Fostex 4010 handles all four of the common code varieties - 24fps, 25fps, 29.97fps drop frame, and 30fps. It can also generate 29.97 frames per second code without drop frame, for those who need it (probably not so many on this side of the Atlantic). The start time of the code can be set, as can the user bits. The user bits area of the code, as I explained in Part 2 of this series, is simply a 'note pad' where you can log the date, reel number or whatever takes your, or your organisation's, fancy.

But so far, the 4010 is not interfacing much with the outside world. It is just generating code in its own sweet time. To function in a multi-machine environment it will have to listen to what else is going on. For example, if a video recording is to have timecode striped onto the audio track as the recording is being made, then each new timecode frame must be precisely lined up with the commencement of each video frame. Otherwise, there will be an ambiguity as each frame will have two timecode numbers.

This problem is easily solved. The 4010 has a BNC socket for video input. With this it can tell from the sync pulse of the video signal where each frame starts, and can align the timecode appropriately. (The SMPTE specification demands that the leading edge of bit 0 of the timecode word must begin at the start of line five of the first field of the video frame, plus or minus one line). The Fostex 4010 can also look at the colour framing of the input video signal and ensure that the colour frame bits of the code are set correctly.

Still in connection with syncing the generation of code to an external sync source, but moving away from video and into the film world, the 4010 can also sync to the pilot tone from a Nagra portable tape recorder, to film bi-phase pulses (via an interface) or simply to a 50Hz or 60Hz reference. If there seem to be a lot of confusing options here, the good news is that probably only one of them will apply in a given installation. Most of us will just be interested in the link with video.

Sometimes, timecode needs to be generated in step with existing code. Perhaps you have a video cassette with code on it and want to stripe an audio tape with exactly the same numbers. Or perhaps you have a tape, audio or video, with damaged timecode and want to repair it. A third situation might be that you have been given a tape to work on that doesn't have enough code before the programme starts for all your machines to lock up - apparently it does happen! To solve these problems, you need the 4010's regeneration and jam sync modes.

In Regeneration mode, the 4010 looks at incoming code and generates new code with exactly the same numbers to be copied onto another tape. As I have mentioned earlier in the series, timecode needs to be treated with respect. If the timecode on a tape is not in good condition, either because the tape is faulty or perhaps because the code was copied from another tape without reshaping or regeneration, then it may not be possible to read the code accurately. When a tape containing timecode is copied, the code must always be either reshaped or regenerated. If it is simply copied from tape to tape then the shape of the pulses will be distorted, even on good equipment, possibly to an unreadable state.

Reshaping means that the pulses are 'squared up' to their original shape and passed on to the recorder, but if there were any errors in the code, these errors would be transmitted faithfully. Regeneration, as performed by the 4010, means that the code is read, made sense of, then new code created and passed on. An altogether more sensible way of doing things.

If a tape has timecode which is good in parts but too bad to read in others, then 'jam sync' can be used to fill in the gaps. Jam Sync mode looks at the incoming code to see whether it is good or not. If it is good, then it is regenerated and can be recorded onto another track of the tape. If it isn't good, then the 4010 takes over and generates new code according to its internal sync reference, but with numbers continuing directly on from those already on the tape. If good code is found later on, it can switch back to regeneration.

Jam sync also works when the code is running in the reverse direction (remembering that SMPTE/EBU timecode, right from conception, was designed to work in both directions). So if an audio tape does not have enough code before the start of the programme for an adequate pre-roll, then it can be turned over so that timecode is on track 1 and running backwards. The 4010 will lock up to this code and continue generating when it has finished (ie. at the start of the programme). You can either drop in the new code, or record it to another track. When the tape is put back the right way, hopefully you will have enough code for all the machinery to lock up before the programme starts.

A GOOD READ



As well as having the ability to generate timecode, the Fostex 4010 can read it too. But why, if you have a 4030/4035 system which includes a timecode reader for both master and slave machines, do you need another one?

The answer is that the 4010 does it better. A simple measure of how much better is contained in the comparison that, while the 4030 can read code between half and twice normal speed (to cope with varispeeding a tape), the 4010 can read code coming in anywhere between 1/50th and 100 times normal speed. You are not likely to come across such ranges of code speed unless you are working with big-league video recorders, way above the VHS and low band U-Matic in price. Where simple cassette-based videos lose code in fast wind mode and output tach or CTL pulses to compensate for this, other videos may give code in all modes of operation. The 4010 can cope with any scenario.

The benefits of the 4010 as a timecode reader are also felt when you have code that is not of high enough quality for the 4030 to cope with. I had difficulty persuading the Fostex 4030 to read timecode from my (admittedly cheap) home VHS recorder, but the 4010 read it without any trouble. If I were doing it for real, and charging people good money for syncing my gear to video, I would certainly prefer the security of having a 4010 tucked away in the rack just in case, regardless of its other benefits.

The 4010 interfaces easily with the 4030. When used with a video machine that outputs timecode in all modes, the 4010 looks at the incoming code and analyses it for speed and direction. It then passes on what you can think of as 'imitation' tach pulses and direction information, so that the 4030 thinks it is connected directly to the machine and receiving its transport tallies.

One important additional feature of the 4010 that deserves a mention is the provision of a simple event controller. Think of an 'event' as a footswitch which is stepped on automatically by the 4010 measured against timecode, rather than by your foot at the right point in the music. Events are used to automate non-timecode devices such as cartridge players, CD players etc.

VITC



The Fostex 4011 works as a subsystem of the 4010, rather than as a stand-alone unit. The 4011's speciality is VITC - Vertical Interval Time Code. As I explained earlier in the series, VITC is a method of identifying each frame of a video with a number just like ordinary timecode - which I shall now refer to as LTC (Longitudinal Time Code). But instead of being recorded on the audio track of the video, it is recorded within the video signal itself. Obviously, there is no use for VITC in audio recorders.

The advantage of VITC is that it can be read whilst the video is in slow motion or still-frame mode. With LTC, when you enter still-frame mode, the reader (be it a 4030 or 4010) only knows the last frame number it received clearly. It can only guess the precise number of the frame being shown to an accuracy of a couple of frames either way. Since a VITC reader is still actively reading code, you know without a shadow of a doubt which frame you are on. This degree of accuracy is essential for top-class work.

The 4011 also works as a character inserter. This means that you can see the timecode numbers - whether derived from LTC or VITC - on the monitor screen. Note that this is not 'burned-in' timecode as described in previous articles. Burned-in timecode is where the numbers are recorded as part of the video picture and all you can do is look at them yourself, they are meaningless to the equipment. I should mention at this point that you can use the 4011 to produce burned-in timecode, if you have two VCRs.

Let's go through the various possible stages of 4011 operation, slowly. Suppose you are given a video cassette with picture and LTC, but no burned-in code nor VITC. You don't need to use the 4011, but you can have it insert the timecode numbers on the screen if you wish. To do this, the video feed from the VCR is passed through the 4011, which will chop out some of the picture and substitute timecode numbers, and is passed on to the video monitor. LTC from the video is then fed to the 4011's LTC input. So far, you can see the timecode for each frame nice and clearly, but this is only good when the video is actually running, not in still-frame mode.

If you are lucky enough to have another VCR to hand, then you can use the 4011 to convert LTC to VITC. The 4011 can take the picture from the first VCR and add VITC to the video waveform. Once the programme is copied with VITC, the 4011 will read VITC in preference to LTC (which you probably also copied, regenerated by the 4010) and you will know exactly where you are in play, slow motion, and still-frame modes.

One nice point about the 4011's character insertion facilities is that you can position the characters anywhere you like on the screen. This may seem trivial, but if you were adding sound effects to a video then you would probably prefer not to have the action obscured by the timecode numbers. For instance, if an object in view is placed on a table, there should be an appropriate sound exactly at the moment of contact. If you can't see on which frame this occurs because the timecode digits are in the way, you have a problem - but not with the 4011.

Figure 1. A large timecode synchronised system based upon the Fostex 4000 range of units.


SYSTEMS



By now you have probably realised, as I have, that there is no upper limit to how complicated machine synchronisation can become. But let's see what a fairly large scale system can look like.

Figure 1 is Fostex's example of a grown-up system incorporating the 4000 series components: two 4030s, 4035, 4010 and 4011. It might also have included the 4050 as the remote control for the multitrack (if it was a Fostex machine) and have brought MIDI into the scene as well.

The diagram shows a two VCR system which can take video cassettes with LTC and copy them while inserting VITC timecode. After copying and transferring the VITC encoded cassette to the source VCR, the entire system can slave to VITC. The video editor shown here is a nicety that an audio studio could do without.

Figure 2. Timecode system using the Fostex 4010 to clean up poor quality code from a domestic VCR.


Coming down to earth a little, and dispensing with the 4011 and the stereo recorder, we still have a system with a lot of power, as shown in Figure 2. The 4010's main function in this particular application was to clean up the code put out by my domestic VHS recorder (though, when I moved up to U-Matic, this was no longer necessary). It also performed its timecode generating, regenerating, and repairing functions admirably.

I found that using a system like this, I really needed to have access to the connectors on the rear panels of the 4010 and 4030, particularly for swapping the timecode feed to my MIDI sequencer between VCR and multitrack. A section of patchbay dedicated to timecode, I would say, is a must. It would look something like Figure 3, where all the timecode connections of the units are brought out to the front of the rack. Totally serious users would have a small video patchbay also (yes, there are such things), for the sake of completeness. The most used audio and video connections would be 'normalled' so that you would only use patchcords when necessary.

Figure 3. Example layout of a Timecode patchbay.


SUMMARY



Hopefully, this series of articles will have lifted the lid on timecode applications. In simple systems, it is reasonably straightforward and easy to use, but when the complexity increases you can get into deep water very quickly. That is when expert advice is called for. So when you buy timecode equipment, whether it is made by Fostex or one of the other major manufacturers, you should be sure your supplier has the expertise available to set up a system to suit your requirements exactly and to help you learn how to get the best out of it.

FURTHER INFORMATION

(Fostex) Harman UK Ltd, (Contact Details).

(JVC) Bell & Howell A-V Ltd, (Contact Details).

VIDEO CASSETTE RECORDERS

Shown here are two models from the JVC range of video cassette recorders, though of course other manufacturers also make suitable machines.

JVC CR-8250E U-Matic video editing recorder.


The JVC CR-8250E is a big, chunky U-Matic VCR, and is the editing version of the CR-6650E I used for some of my tests. The extra editing facilities are not necessary for audio work. The advantage of the U-Matic system is that it gives good picture quality, even when copied. The tape speed is 9.53 cm/s, giving a reasonable performance on the audio tracks, one of which is used for timecode.

JVC BR-6600E stereo VHS video recorder.


The JVC BR-6600E is a professional quality VHS machine, also suitable for work with timecode but with less good picture quality. The tape speed in the VHS system is reduced to 2.339 cm/s, indicating that you should not expect such reliable reproduction of timecode as with the U-Matic. Even if your VHS is of good quality and well maintained, it doesn't necessarily mean that the video cassette you are given to work on will have been recorded on a machine which is in peak condition.



Previous Article in this issue

My Life With MIDI Time Code

Next article in this issue

Alesis MIDIverb III


Sound On Sound - Copyright: SOS Publications Ltd.
The contents of this magazine are re-published here with the kind permission of SOS Publications Ltd.

 

Sound On Sound - Jan 1990

Donated & scanned by: Mike Gorman

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Topic:

Syncronisation

Video / Film / Picture


Series:

Getting Into Video

Part 1 | Part 2 | Part 3 | Part 4 | Part 5


Gear in this article:

Synchroniser > Fostex > 4010

Synchroniser > Fostex > 4011

Feature by David Mellor

Previous article in this issue:

> My Life With MIDI Time Code

Next article in this issue:

> Alesis MIDIverb III


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