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HOME RECORDING

FEBRUARY 1, 2001 ISSUE

Analog Tape Calibration

BY KEN LANYON

For this issue, I thought I would address a topic that not enough home recordists know about. Even with the age of digital surrounding us, I know that many people prefer the warmth of analog tape, and those 8-16 track reel-to-reel machines are still getting bought up. Now, I bet that a lot of those buyers get their new machine home, only to start recording on it the second they get the I/Os hooked up. Not that this is bad, but with just a little care and maintenance beforehand, your recordings can sound better than ever before. I am by no mean an expert on the topic of analog tape calibration, but I would like to share with you what I have learned.

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Analog Tape Machine Transport

So what exactly is tape calibration? This is essentially adjusting the VU meters for each track during playback and record so that they read the optimum levels for the type of tape you are using. This is very important to getting a good sound because different types of tapes can handle different recording levels. For example, Quantegy 456 can be recorded with a +6db signal (that is +6 above 0db magnetic level that Ampex corporation established as "0" in the 1950's) while Quantegy 499 can take a +9 signal. Knowing this, it is obvious that these values depend heavily on where 0db actually exists on the meters, and in every case, it can equal whatever you want it to. If you took a 1K tone whose level is set to 0db, and put that into your recorder, you might find that the VU meter is actually reading -3db. This needs to be adjusted on each track to that your meters are reading the correct input levels. You may also occasionally see a tapes maximum recording level expressed as nano-webers per meter. Below is a list of the common operating levels you will see.

Operating Levels: 

185Nw/m2 = 0db
200Nw/m2 = +1db
    250Nw/m2 = +3db
320Nw/m2 = +5db
(456) 370 Nw/m2 = +6db
(499) 520 Nw/m2 = +9db

Note- Nw/m2 refers to Nano-Webers per meter - a unit in measuring the magnetic field strength.

Calibration doesn't just refer to recording levels though. It is equally important to calibrate your machine for playback. That way you know that the levels you recorded into the machine are getting accurately represented on the VU meters during playback from either the sync or repro head. I should mention now that tape machines have 3 heads...an erase head, a sync head, and a repro head. The erase head obviously erases signal on the tape. The sync head records and plays back signal, and this is the one you will use during tracking and overdubbing. Finally, the repro head plays back the recorded signals at a higher fidelity than the sync head, and is the one you should use when mixing down your music.
So lets get started. The first thing you should do is turn on your machine to let it warm up a bit. Next, clean the heads and any other part of the machine that comes into contact with the inside of the tape. As tape plays back, small particles of the tape will slowly flake off and may be wreak havoc on the heads if not cleaned off at least occasionally. I would suggest using 99% denatured alcohol, because it doesn't leave any residue behind. I would also suggest using small lint-free wipes so as not to damage the heads. Do not use cotton balls or swabs, as they can leave the cotton behind on the heads themselves. Before you wipe off the heads, fold the wipe over a few times so that it has multiple surfaces that you can keep turning out. This will prevent you from using an area of the wipe that you have already used, and keep the dirt and dust from getting back on the components.
Make sure as to not touch the rubber rotating guides/drive wheels with the alcohol, as it will dry them out and cause them to crack. These rubber parts need to be cleaned but with a rubber cleaner (available at electronic stores)
Now we can start with the actual calibration of each track. You are going to need two things. First, to change the positioning of each VU meter, you will need to turn a small screw, and this requires what a lot of people affectionately call a tweaker. It's basically a small screwdriver that will fit the screws, and it should cost under $1. Next, you will need to get a hold of an alignment tape. The tapes that professional studios use are called MRLs, standing for Magnetic Reference Laboratories. These tapes have a series of different sine waves recorded onto them at a certain level. You can buy one yourself, but the drawback is that they can be rather expensive. Of course this varies on the number of tracks you have and the size of tape you want to use, so I encourage you to look into the cost of one for your machine.
If buying one of these is not an option for you, you can go to a local studio and have them record you an alignment tape. They should go ahead and record the tape on each track with 2-3 minutes of a 1KHz tone, a 10KHz tone, and 100HZ tone, each set at a certain output level that you specify, which should usually be either 0db or +3db. I recommend 0db, since it is helpful when doing record alignment. Make sure the level is noted on the box before you take your new alignment reel home. Without knowing what level everything was recorded at, you will be lost. The reason for this is that you will use the 1K, 10K, and 100Hz tones when you calibrate (if your tape speed is 15 ips. or 30 ips you use the same frequencies.) I also want to remind you to NEVER record over any part of your alignment tape tones.
Before we start calibration, I want to explain one more thing. Depending on the type of tape you are using, there is a quick formula that you should use to determine at what level your VU meters should read while adjusting the playback levels. The formula is: VU = MRL - Operating Level. Simply put, this means that the db value that your VU meters should read is equal to the level your MRL alignment tape was recorded at, minus the level that you want to record the signals onto your tape with. Your operating levels that you record with should be based on the type of tape you are using. As I said before, Quantegy 456 tape can take a +6 signal, so in this case, the operating level will be +6. Of course, you can choose to set your operating level to another value, but you wont be getting the best level to tape. Either your signal-to-noise ratio will be too low, or the signal will be distorting to tape. Assuming that your MRL was recorded at 0db, we now introduce these values into the equation and get: VU = 0 - (+6). This of course, means the VU meters should read -6 when you calibrate the repro and sync heads for playback.
Now that you have all that figured out, we start by setting the repro level. Place the alignment reel on the machine and play back the 1KHz tone using the repro head. Find the repro level screws on your machine for each track, then look at the level of each track and adjust it to -6 with the tweaker (You may have to open up the case to get to the screws). Make sure that you are turning the correct screw for each track, because if you don't, you wont even see the meter move for that track. You may also have to make multiple passes if you cannot calibrate each track within the time the tone plays through.
Once you have correctly adjusted each track to -6, rewind the tape to the beginning of the 1KHz tone and set your recorder into sync playback mode. (Instructions for this should be in your manual or you can set the machine to record mode by record enabling every track). While you shouldn't ever record on this tape at all, the record mode may be necessary to monitor playback from the sync head. Play back the 1KHz tone and adjust each sync level screw until the meters on each track read -6. Next, while keeping the machine in sync-playback or record mode, play back the 10KHz tone and adjust each sync high frequency screw (sync high EQ) until the VU levels read -6.
Once you have finished adjusting the sync levels, we need to return to the repro head and adjust the meters for the repro high frequency EQ. Take the machine out of record mode, and play back the 10KHz tone. Adjust each tracks repro high EQ screw until the VU meters read -6. I want to mention that it is not necessary to adjust sync high frequency right after sync level. You can go back to repro high frequency after having adjusted the sync levels. Either way, just make sure you set the repro and sync levels before moving on to the high frequency adjustments. Now that you have adjusted these four playback levels, fast forward the MRL and take it off the machine. Replace it with the blank tape that you are going to use for your recordings. If you have it, splice 5-6 wraps of leader tape on the beginning and ends of the tape.
The next part is probably the trickiest of all the steps, so follow closely. We have to adjust the bias level for the whole machine, which may vary depending on what type of tape you are using and the tape speed. Different tape manufacturers suggest different bias levels for their tapes to create the best recordings (they may also call this over-bias). Bias is a super high frequency that is recorded onto your tape to align the magnetic particles. This allows an accurate reproduction of the high frequencies in your music as they are recorded. However, it has no impact on the playback of your tapes. There is a small reduction in signal to noise ratio, but this is made up for by an increased high-end clarity.
For all record alignments, you will need to find a way to play back the various tones, which may involve recording the MRL signals onto a DAT or other medium (do a direct transfer instead of going through your mixer). Then once you play back the tones from the DAT for alignment, make sure you put them through the mixer so that they read 0VU before going to the tape machine. I am assuming that you don't have an extra machine to play back the 1KHz tone, but if you do, then you are doing great. If not, you may also be able to get your hands on a tone generator that can pump out 1K, 10K and 100K at 0db. In any case, this is essential for doing record calibration.
To adjust the bias correctly, find out from the manufacturer what the bias is for your tape. Then, set your tape deck into record mode, again monitoring from the repro head. Buss a 0db, 10KHz tone to each of your tracks and set the machine into record. For each track, you need to first turn the bias screw counter-clockwise. As you watch the meter, you will see the needle rise up and then begin to fall back. Let it fall back to the left side of the meter, until it stops moving. Then turn it clockwise until it peaks and begins to go back to the left. Back it up just a hair until it is at the peak point. The point at which it peaks is irrelevant, but will signify the point at which you should begin counting the overbias. This means that if your overbias is 1.5db, then turn the screw clockwise until it has gone down 1.5db past the peak point. Repeat this for each remaining tracks, while recording the 10K tone.
It is now time to adjust the record levels. Rewind your tape to the beginning of the head leader. Once you have done this, set your tape deck into record mode but monitor from the repro head. Buss a 0db 1KHz tone into each track of the tape deck from your mixer. Set the machine into record and adjust the record level screws on each track until the meters read 0VU.
Having done that, we need to check the input levels for each track. Record-enable each track so that it monitors input (but don't record anything), and then play back the 1K tone from your external source into the machine, keeping it bussed to each track. Adjust the input level screw on each track so that the meters read 0VU.
Next we move onto adjusting the high frequency recording level. Buss the 0VU, 10K tone to each track, and set the machine into record, monitoring from the repro head. Adjust the high-frequency record screws until each of the meters read 0VU. Now that you have that done, we set the repro low frequency levels. Buss a 0VU, 100Hz tone to each track, and set the machine into record, monitoring from the repro head. Again, adjust the low-frequency repro screws until each of the meters read 0VU. The result of this step is that the meters are now calibrated for correct low frequency playback levels from the repro head. For the final calibration step, we need to set the correct low frequency playback levels for the sync head. You can play back the 100Hz tone that you just recorded and monitor from the sync head. Adjust each low frequency sync screw until the level on the meters is 0VU.
Congratulations! You machine is now correctly calibrated for this type of tape. You will find that you don't have to calibrate the machine very often if you use the same type of tape. Of course, if you begin using a new type, then you should re-calibrate it. I should also mention that if you are playing back a tape of a different type, but not doing any recording, you only need to calibrate the playback of the 1K, 10K, and 100Hz tones for both the sync and repro heads. Calibrating using the record steps is not necessary.
One thing you can do to make your life easier is to record 45 seconds of each of the three tones onto the beginning of your tape for each different tape formulation. This way, you don't have to search for the MRL, or even switch tapes around. Separate this area of tape from the rest of the reel with a few wraps of leader tape. If you have to do overdubbing on a tape that already has music on it, I suggest fast forwarding to the end of the tape, then rewinding about 30 seconds to create a section of tape you can do record calibrations on. Make sure there is no music on any of the tracks at this point, and then separate it from the end with leader tape. This is now your record pad. The reason the pad is not at the beginning of the tape is so that you don't forget that you are doing record calibrations, and accidentally record over the music that is already there. It is also because of the fact that your tones are already there, and you never want to record over them.
Below is a simplified summary of all the calibration steps you need to perform.

STEP

MRL OR OSC TONE

MACHINE MODE

VU SETTING

1) Repro Level (gain)

1K MRL Tone

Repro

MRL Reference Level*

2) Sel-Rep Level (Sync)

1K MRL Tone

Sync

MRL Reference Level*

3) Sync High Freq. EQ

10K MRL Tone

Sync

MRL Reference Level*

4) Repro High Freq. EQ

10K MRL Tone

Repro

MRL Reference Level*

5) Bias (Over-bias)

10K Osc Tone at -3 dB)

Repro/ Recording

Amount of Overbias**

6) Record Level

1K Osc Tone at 0 dB)

Repro/ Recording

0db VU

7) Input Level

1K Osc Tone at 0 dB)

Input

0db VU

8) Record High Freq.

10K Osc Tone at 0 dB)

Repro/ Recording

0db VU

9) Repro Low Freq.

100 Hz Osc Tone at 0 dB)

Repro/ Recording

0db VU

10) Sync Low Freq. (Playback Function)

100Hz (Use 100Hz on record pad from last step)

Sync

0db VU

REPRODUCE STEPS IN GREEN

RECORD STEPS IN PINK

* May have to be set to a level lower than "0 dB VU" because of the level formula

** Amount of overbias specified in manual or use Universal Method

Copyright 2001, by Ken Lanyon, ALL RIGHTS RESERVED

Published in Recording Engineer's Quarterly and Alexander magazines with permission

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