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Tape Line-Up (Part 2)

Part 2. If you developed a stomach ulcer through reading last month's instalment on the theory of line-up and haven't dared go near your tape machine since, you can stop worrying no was David Mellor tells you exactly what to do to achieve perfection in your recorded sound.


In the second instalment on the practice of tape recorder line-up, David Mellor tells you what to do, what not to do - and why you should look after your tweaker. For all those who read last month's article on the theory of line-up, and haven't dared to record a note since, here are the wise words that will finally set you on the straight and narrow.


Hopefully, last month's article explained just how important the line-up routine is, so you should realise that it is not the sort of thing one can rush through in an odd ten minutes. However, I do not want to give the impression that it is difficult to do because it isn't. Anyone who can successfully negotiate a coin-op laundrette should be able to see it through without any trouble. It is just a matter of following procedure, and understanding what you are doing.

GETTING STARTED



First thing to do is to get your tape recorder's manual from the cupboard under the stairs and give it a good dusting off! Although I am going to supply you with a general guide to lineup, each machine has its own little quirks which must be taken into account, and usually the manufacturer knows best. If by any chance you do not have a manual, you might be lucky and find that all the little adjusters are labelled. If they are not... don't you dare!! Buy a manual, or you will be spending more money before the end of the article - I can assure you that it will be money well spent, and also that I practice what I preach.

As I said, line-up is a straightforward procedure, but it can get extremely frustrating if you put your tweaker into the wrong hole and have to go back to square one. Arrange for a couple of hours of peace and quiet. No TV or radio, and definitely nobody looking over your shoulder. That's the one thing guaranteed to make you go wrong.

THE GEAR...



There are a few pieces of hardware (and software) you will need and yes it does cost a bit of money, but as it will only be a fraction of the cost of your multitrack and stereo machines you will be able to regard it as a small price to pay in the quest for sonic perfection. The following bits and pieces will see you alright.

AC millivoltmeter
Not to be confused with a DC voltmeter which is what you test torch batteries with. It should be scaled in voltage (v) and decibels (dB), and have a flat response from 10Hz to 100kHz. Look for a high input impedance of at least 100kOhms.

Oscilloscope
Must be dual trace if you want to do serious work, dual beam is an unnecessary luxury for our purposes. (Dual trace means that a simple beam is alternated or chopped from one channel to another.) The scope must be capable of using one input as the timebase, the so-called X-Y mode as used for displaying Lissajous figures.

Oscillator
I use one I made (from precision components) which generates five spot frequencies: 45Hz, 100Hz, 1kHz, 10kHz, 15kHz sine wave. If you buy one, then get one that will do a square wave as well. You won't be needing it for lineup purposes but it comes in handy elsewhere.

Demagnetiser
Tape heads and guides collect magnetism faster than you may think, and if you want to keep your tapes in perfect condition then you should demagnetise your machines before every recording session. Get a mains powered one. I have seen some battery versions - they may work OK, but I'm doubtful.

Test Tape
Otherwise known as a calibration tape, this is your primary reference. Buy a reputable one from MRL, Webber or one of the major tape manufacturers. Remember that if your machine is NAB standard to buy a NAB tape. Similarly for IEC. Also, make sure to get a 15ips version. You do record at 15ips don't you?! Also, one test tape is required for each different tape width you use.

Tweaker
You've been waiting for this haven't you! The tweaker is a screwdriver with a plastic handle and a very small metal blade. On some recorders use of a standard screwdriver is OK, but on others the presence of a large metallic object near the bias circuits can affect the settings and make it difficult to adjust, although it will not cause any harm. You can buy a tweaker made by RS Components through most high street electronic component shops, they call it a 'trim tool type 3'and its part number is 543 434.

Head adjusting tool
See your manual for type and size.

Software
Not computer programs but cotton buds, head cleaner and rubber cleaner. Cotton buds you can get from Boots, I use neat alcohol to clean tape heads and guides, and 'Jif' from under the kitchen sink to clean the pinch roller.

That's the lot! Having collected your tools and materials around you, and lost a bit of excess weight from your wallet, you are ready to begin...

STEP 1



You find a girl to love. Oh, sorry! That's a line from a song! First clean the pinch roller. The rubber cleaner ('Jif') is best applied with a tissue, and be very sparing with it. Make sure that no cleaner remains when you have finished. Some people will use alcohol on this part, but it takes ages to get the roller really clean and it will eventually damage the rubber.

Next, clean the heads, tape guide and capstan with alcohol. The only thing to look out for is to be careful not to let any of the alcohol trickle down into the capstan bearing, as this will interfere with its proper lubrication. I shouldn't bother with any of those commercial head cleaners sold in certain chemists and newsagents. They are usually an alcohol/water mixture and they are useless - might as well use vodka, which is much the same thing and probably more handy.

STEP 2



Demagnetise the heads, guides and capstan. This is essential. Do not even think about playing a test tape before you have demagnetised the tape path. Even a brand new machine should be demagnetised before line-up. It is important to turn off the tape recorder before you do this or you will be injecting a magnetic flux hundreds of times stronger than a recording via the heads into the electronics. The amount of damage this could cause should be obvious.

Make sure all tapes are well out of the way, then plug your demagnetiser into the mains and switch on. Bring it slowly up to the machine and pass the tip close to each metal part in turn, without touching, giving it five or six up and down strokes.

When you have done everything, circle the demagnetiser round and round the area you have covered, starting small and growing larger and larger. This action causes the demagnetising field to die away slowly so that no residual magnetism should be left. When the tool is about a metre away it is safe to switch off. NEVER, NEVER, NEVER switch off close to the tape machine as this will MAGNETISE it, possibly to an irreversible extent.

When you have finished demagnetising the tape recorder, demagnetise the metal tools you are going to use.

STEP 3



Azimuth. This is the moment you have been waiting for - play the test tape! It will be helpful if you are familiar with what tones are recorded on it and their durations etc. It probably says in the instructions that you should never fast wind a tape. Well, in a perfect world this would be so, but in practice you will find that you have to or you will never get finished, and I certainly don't know of anyone who could swear that they have never done this.

What I do is to splice leader tape before each of the most used tones (the 1kHz and 10kHz) so that if I need to rewind, I can stop easily at the leader and not during the tone to save wear and tear on the tape. And when I have finished I always play the tape onto its spool so that it is wound neatly.


The first thing you are going to adjust is the azimuth of the playback head, the theory of which I explained last month. Your tape recorder should look a bit like Figure 1 with a screw near each head for azimuth adjustment. If there is more than one screw, be careful. Look at the manual to make sure you get the right one.

There are two azimuth setting procedures, each with their uses, so I shall explain both in detail.

1. The meter method
This method is the more foolproof way of setting the azimuth, the disadvantage being that you cannot check the azimuth without first adjusting it. Each time you adjust the setting, the tensioning springs will weaken slightly so only use this method when you know something is amiss and expect to have to make a change.

You probably think I've been building up to something big and complicated, but that's not true - it's really easy. All you do is connect your meter to either output of your recorder, play the 10kHz tone on your test tape and adjust the azimuth for maximum output. As a final nuance, repeat at 20kHz. Done! No need to repeat for the other channel as the result will be the same.

2. The oscilloscope method
If you have no reason to doubt the azimuth, but want to make sure, this is the method to go for. Simply connect the inputs of the oscilloscope to the left and right outputs of the tape recorder and switch to the X-Y display. Once again play the 10kHz tone. You will see either a straight line angled at 45 degrees or an ellipse. The straight line is correct. Make a careful adjustment so that this is what you see.

You could make a mistake here, which is why I do not recommend this technique for universal use. You could accidentally set the 10kHz tone from the left and right channels exactly one cycle apart (or any whole number of cycles) and not know it just from looking at the scope. This would leave other frequencies out of phase to varying degrees. If in doubt, use the frequency run on the test tape starting at the very lowest tone. If you are in phase for all the frequencies then you are OK.

Two additional points about azimuth. Firstly, if you want to make a rough and ready check of azimuth when playing a tape, press the mono switch on your recorder if it has one. If you can hear as much high frequency content in mono as you can in stereo then you are pretty close to being alright. This is because the more tape width you read, the greater the possible cancellation effect.

The second point is that if you buy a cheap or secondhand oscilloscope, always check that the two channels of the scope are actually in phase otherwise you will think your recorder is wrong when it isn't. Check this by connecting both channels to an oscillator simultaneously and select the X-Y display. You should see a straight line inclined at 45 degrees indicating that all is well. If you see the ellipse, then all is not well.


STEP 4



Playback. Figure 2 shows a generalised view of the adjuster panel of all tape recorders. Some have more, some less - but by referring to your manual you should be able to relate it to your particular machine.

You will have to decide at this point what you want your zero level to be. If you use a Tascam or Fostex recorder you will be in -10dB land. Tascam recommend that you set 250nWb/m to -10dBu, and I should adhere to this. This means that your 'zero' level of magnetism is 250nWb/m and your 'zero' level of electricity is -10dBu (or 0.245 volts rms). If you use fully pro equipment then I would recommend that you set 200nWb/m to 0dBu (0.775 volts rms) which would then be your zero level. The test tape you buy should preferably have been recorded at the same magnetic level that you wish to be your reference. Although I shall be using 0dBu as my zero level, everything I say will apply to the -10dBu standard, making the appropriate level changes.

You may be in the position of not having any playback adjusters on your machine, or maybe only having playback adjusters for level, but not HF or LF. In this case, when I say "set to 0dBu", make a note of the levels produced at each frequency and use these as your zero settings when you come to align the record electronics. This will ensure that what goes down onto your tape will be correct.

Referring to Figure 2, play the 1kHz tone at the head of the test tape and adjust the level to read 0dBu. Play the 10kHz (or nearest) tone and adjust the HF level to also read 0dBu. (If your test tape states that 10kHz is at 'reference level minus 10dB', then set the HF to read -10dB). Repeat for the other channel. Do not adjust the LF level yet as it is not possible to get an accurate reading from a standard full-track test tape for reasons it would take too long to explain. Never fear... we can cope!

When you have done this, advance to the frequency run of tones on the tape and note down the levels for each frequency. The bass will read too high, but apart from that you should have a smooth, nearly flat response.

STEP 5



Put the test tape back in its box and store it in a safe place (preferably cool and dry), we shall not be needing it again. You have now successfully calibrated the playback head and electronics so you can use this to align the record side of your machine.

First, we shall set the record azimuth. Simply record a 10kHz tone on new blank tape and set the azimuth of the record head in the same way as you did for the playback head, while recording the tone.

STEP 6



Bias. It probably tells you how to set the bias in your manual. If it says 'set the bias at 10kHz' be aware that the figures it gives are machine specific and should only be applied to your tape recorder. The bias can be set as nearly as will make no difference for any machine using the old rule of thumb - '1dB over at 1kHz'.

Apply a 1kHz tone at around —20dBu to the machine's input and adjust the record level so that you get roughly the same from the playback output. Turn the bias adjuster all the way anticlockwise, then slowly turn it clockwise until the output falls by 1dB. Repeat for the other channel.

STEP 7



Record. Apply a 1kHz tone at 0dBu to the machine and adjust the record level while recording to get 0dBu from the replay head. Repeat for the other channel. Remember that if at any time you accidentally move the wrong adjuster, you will have to go right back to that stage again.

Apply a 10kHz tone at -10dBu and adjust the record HF to give -10dBu out. Repeat for the other channel.

Apply a 100Hz tone at -10dBu and adjust the playback LF to give a flat response.

If you are scratching your head at this juncture, you must be a Fostex owner! How can you record and play simultaneously when you only have two heads? If you have been making these adjustments without realising this then there must be a pile of sawdust on the carpet 'cos the output signal you get sure ain't been near any iron oxide! The answer is that you continually have to record, play, adjust a bit... record, play, adjust a bit... until it's right. It's the only way!

Everything I have said so far, by the way, applies to multitrack as well as to stereo recorders... there are more channels so it just takes longer.

STEP 8



Sit back and have a cup of tea - you've done it! And if you had to go through the two head (Fostex) line-up procedure, you had better have a nip of something stronger!

All that you have to do now is to remember to repeat the line-up at reasonably regular intervals, and to follow the procedure for recording lineup tones at the head of each tape you record. Remember that if you record tones on your tape, the cutting studio or digital mastering suite will be able to play your tape correctly, even if you didn't get the line-up quite as spot on as you could have, by using these tones as a calibration tape and lining their playback machine to them. Now have another cup of tea!

DECIBELS, ELECTRICITY & MAGNETISM

The dB is a ratio, not a unit. Difference in dB between two voltage levels V1, and V2 is given by:

dB = 20 x log10(V1/V2)

The dBu is a unit.
0dBu = 0.775V rms
-10dBu = 0.245V rms
-20dBu = 0.0775V rms

Magnetic levels in terms of dB.
Let 200nWb/m = 0dB
Then 250nWb/m = +1.9dB
and 320nWb/m = +4.1dB


PROCEDURE: To record tones at the head of each tape

Choose one set of tones according to machine's EQ (NAB/IEC) and tape speed. (If Dolby A used, record Dolby tone first. Record all tones through Dolby unit with noise reduction switched out.)

IEC or NAB @ 15ips
1. 1kHz at 200nWb/m
2. 10kHz at 200nWb/m
3. 100Hz at 200nWb/m

IEC or NAB @ 7.5ips
1. 1kHz at 200nWb/m
2. 1kHz at 200nWb/m -10dB
3. 10kHz at 200nWb/m -10dB *
4. 100Hz at 200nWb/m -10dB

IEC @ 15 ips or 7.5ips
1. 1 kHz at 320nWb/m
2. 1 kHz at 320nWb/m -10dB
3. 10kHz at 320nWb/m -10dB *
4. 100Hz at 320nWb/m -10dB

Identify all tones and levels on the tape box.

* The lower level avoids HF compression effects, the low frequency tones are repeated in order to have all tones at the same level to aid line-up.


PROCEDURE: To line up 200nWb/m to 0dBu

1. Clean.

2. Demagnetise.

3. Play 10kHz on calibration tape, set playback azimuth.

4. Play 1kHz at reference level, set playback level to 0dBu.
Play 10kHz at reference level, set HF playback level to 0dBu, OR Play 10kHz at reference level -10dB, set HF playback level to -10dBu.

5. Record 10kHz at reference level -10dB approximately on blank tape, set record azimuth.

6. Record 1 kHz at reference level -20dB approximately, set bias to '1 dB over' as explained in text.

7. Apply 1 kHz at 0dBu to record input, set record level so that output reads 0dBu.
Apply 10kHz at -10dBu to record input, set HF record level so that output reads -10dBu.
Apply 100Hz at -10dBu to record input, set LF playback level so that output reads -10dBu.


PROCEDURE: To line up 250nWb/m to -10dBu

1. Clean.

2. Demagnetise.

3. Play 10kHz on calibration tape, set playback azimuth.

4. Play 1 kHz at reference level, set playback level to -10dBu.
Play 10kHz at reference level, set HF playback level to -10dBu, OR Play 10kHz at reference level -10dB, set HF playback level to -20dBu.

5. Record 10kHz at reference level -10dB approximately on blank tape, set record azimuth.

6. Record 1 kHz at reference level -20dB approximately, set bias to '1 dB over' as explained in text.

7. Apply 1 kHz at -10dBu to record input, set record level so that output reads -10dBu.
Apply 10kHz at -20dBu to record input, set HF record level so that output reads —20dBu.
Apply 100Hz at -20dBu to record input, set LF playback level so that output reads -20dBu.



Previous Article in this issue

Master Keyboards

Next article in this issue

On The Flight Deck at Terminal 24


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 - Oct 1986

Donated by: Gavin Livingstone

Topic:

Sound Fundamentals

Tape, Vinyl, CD, DAT


Series:

Tape Line-Up

Part 1 | Part 2 (Viewing)


Feature by David Mellor

Previous article in this issue:

> Master Keyboards

Next article in this issue:

> On The Flight Deck at Termin...


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