So why do we bother?

Well tapes come from all different studios to ours to be played and recorded on - and we send our tapes out - so there has to be a standard for the replay and record alignment of a tape machine. This means that no matter what machine we play or record on we will hear signal at the levels we recorded at and we will record onto the tape with the same levels set at another studio.

Imagine getting, say a two inch 24 track tape, to do some remixes that was recorded at one studio, you put it on your machine and you find the levels on your meters are all over the place. Nightmare! You've got to get your machine calibrated by using a test tape, even if you use a small stereo tape machine for back-up.

First stop inspect the beast

Tape machine alignment

1.Heads and tape guides must be periodically inspected for wear. Flats on guides and head surfaces should be looked for. Badly worn heads or guides can damage the oxide layer on the tape.

2.Heads should be cleaned regularly using isopropyl alcohol and a cotton bud.

3.They should be demagnetised regularly, since heads can gradually become slightly permanently magnetised, especially on older machines, resulting in increased noise and a type of bubbling noise in the background of recordings. Horrible!

If you have not seen one a demagnetiser is a strong AC electromagnet, which should be switched on well away from the tape machine, keeping it clear of anything else magnetic or metal. This devise will erase a tape if placed near one. Once turned on the demagger should be drawn smoothly and slowly along the tape path (without a tape present), across the heads and guides, and drawn away gently on the far side. Only then should it be turned off.

So what are test tapes used for?

A test tape is a reference standard recording containing pre-recorded tones at a guaranteed magnetic flux level. A test tape is only the starting point for aligning a tape machine. Details During alignment, the test tape is replayed, and a 1kHz tone at the specified magnetic flux level (say 320nWbm-1) produces a certain electrical level at the machines output. The output level would then be adjusted for the desired electrical level (say 0dBu) to read at a standard meter indication (say PPM4). Got that? The tape is played and we adjust the output of the machine so the meters are basically zeroed. The point is that the recorded signal is a guaranteed level. That is why we spent so much money buying it!

A test tape with the required magnetic reference level should be used, and it should also conform to the correct EQ standard (NAB, CCIR, IEC). You know that tape machines have different circuits that use different e.q. characteristics. So you need to know which standard to use. Tapes are available at all speeds, standards and widths, with most being recorded across the full width of the tape. Tones recorded on tape, as standard are: 1Khz, 10kHz and 100Hz.

Don't get confused re-read again if you don't get it first time. Think it through in your mind.

What are these magnetic reference levels Chris? Magnetic flux density on tape is measured in nanowebers per meter (nWbm-1), the Weber being a unit to measure magnetic flux. So we are saying that this amount of magnetic flux (320nWbm-1) on a tape ought to produce a standard level (reference) in the meters. If the output meters are not correctly aligned then our known level of flux density (320nWbm-1) will cause the meters to be all over the place as opposed to all equal and, essentially, zeroed. Got it?

Step by step replay alignment

Replay alignment should be carried out before the record alignment. There are many pots on the tape machine interior and these pots are adjusted for the alignment. So if it's a 24 track machine you have got to do them all! - However, for one track:

Play the 1kHz tone - adjust the relevant pot, on each track, to make the meter read 0VU

Play the 10kHz tone - adjust the relevant pot, on each track, to make the meter read 0VU

Play the 100Hz tone - adjust the relevant pot, on each track, to make the meter read 0VU

Playback alignment is complete - for one track!

Before the record alignment can be started the Bias needs to be set-up. Bias level affects the performance of the recording process and the correct level of bias is a compromise between o/p level, distortion and noise level. The bias is adjusted on each track. It's a similar process to the above.

Record Alignment

Send 1kHz at say -4Vu to the tape machine. Put the machine into record and adjust each tracks relevant pot to read 0VU. Do the same with 10kHz and 100Hz. This is making the meters read correctly as you know what signal you are sending it.

The record alignment is now complete for one track.

Once the tape machine has been correctly aligned for record and replay a series of tones should be recorded at the beginning of every tape made on the machine. This allows the replay response on any machine which might subsequently be used for replaying the tape to be adjusted so as to replay the tape with a flat response.

Tones should be marked on the tape box as: Tones @ 1kHz, 320nWbm-1) at 0VU.

Labels that say 1Khz @ 0VU are meaningless as 0Vu does not take into account a reference level of flux. Machines can have their Vu meters calibrated for different references remember?

Fault reporting is important

Get into the habit or reporting faults regularly even if it's your own gear. I always keep a fault journal close by to log all the problems. The book can then be used as a source for fixing things during down-time in the studio. The things to include are:

Your Name Name of Equipment - Model and Make Serial Number When fault was discovered - date - session What the fault is What procedures you went through to discover fault

Why testing and maintenance is a good idea

Preserves your gear Prepares your gear for any eventuality Sets you apart as a professional.
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Recording Studio Suntans

That don't half take me back in time!
Should be engraved in stone, or else chiselled into the forehead.

Very useful post.

Now for the differences between the head types. Half track, full track, IPS and why it can be better to use 15 IPS over 30 IPS.

Hmm - I feel a post coming on soon...
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www.opusproductions.com
Multichannel Audio Specialists

Quote:

Hmm - I feel a post coming on soon...

Looking forward to that.
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Recording Studio Suntans

I read a good technique for adjusting bias a few years ago in Tape Op Magazine. A little background on what it is. An audio signal by itself cannot be recorded on analog tape without being noisy. If you crank up the signal level so that noise is masked, then guess what? Yep, distortion! Someone a lot smarter than I figured out a long time ago that if you sneak a higher frequency signal in with the audio signal you can get more level to tape without distortion. The trick is to use a frequency that is above the audible spectrum so that only the audible frequencies (music) will be played back. So a record head not only puts out a magnetic field that represents the audio frequencies, but also an ultrasonic "carrier" frequency.

Remember, the whole point of using this frequency is to get more signal to tape, meaning less noise, but if you add too much you will get distortion. So what is the best way to set bias? Set it so you get them most signal you can, the least noise you can, and the least distortion you can. How do you do that? Listen!

If you connect a parametric eq to the monitor path of the tape track you are calibrating and notch out 1 kHz while simultaneously boosting 3 kHz you will be able to more easily hear the harmonic distortion generated by using too much bias (3 kHz is the 3rd harmonic of the 1 kHz tone you are recording while making the adjustment and is the most noticeable "bad" harmonic). Don't put the eq in the record path or it will not work. You are just using the eq as a hearing aid. It will lower the listening level of the 1 kHz tone so you can more easily hear the noise and harmonic distortion.

Backing off the bias will cause you to lose gain and you will hear more hiss. There will be this beautiful "sweet spot" between not enough bias where the signal is weaker and noisy, and too much bias where the signal has a distorted edginess. In this sweet spot you will hear the noise drop, the signal raise, and it will sound clean. Go just a touch farther and you will hear it get edgy. You've gone too far. Back it off until it just cleans up. Now it is perfect. If you pay attention to the meter while you are adjusting the bias, the record level actually changes. You will notice that there is a point where no matter how much more bias you add the meter will not go any higher, in fact it will start to drop. The playback should sound distorted at this maximum level. You will need to come down from this peak level to hit the sweet spot. If you are in a hurry you can just come down from the peak and listen for the sweet spot on one track. Write down how many dB you came down from the peak to get to the sweet spot. Then just find the peak on all the other tracks without listening and come down by the same amount. You could use that number of "dB down" as a reference for that particular brand and type of tape. Other brands and types may be different.

Because of tolerances in electronic equipment and the variations in meters, each track won't yield identical performance. So if you really want to give your machine a super tune up, do each track by ear using the parametric eq in each track's monitor path. By the way, if the meter pegs before reaching the peak, simply reduce your tone generator's output level a couple of dB. After you have done bias you will need to do the Record Level and Record High Freq. adjustments. If you start to do the bias with a 1kHz tone and your record levels are all over the place, do a quick Record Level adjustment to get all of the channels in the "ballpark" and then do a complete Bias adjustment on all tracks, then redo the Record Level to exactly 0db on the meters, then do the Record High Frequency adjustment. Your tape machine and your ears will thank you.

That's a cracker Instructor, this thread is developing well now.
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Recording Studio Suntans

Hi,does anybody know where I can get or borrow or rent a test tape...I have a 24 track mci sony 2"machine

I sent the last post about a test tape ..my cell phone # is 559 350 2970...Thanks Bill

Just thought I'd add my two cents. DON'T turn off the magnet until you are at least 4 feet from the Analog Tape Recorder.

Hi,
I'm new to tape alignment, but the manual for my machine says I need an AC voltmeter, but you don't mention that in this post. Also, is it necessary to adjust the azimuth path, if so, how and why?