AN INTRO TO ANALOG TAPE DECK CALIBRATION FUN

Bias and equalization; you've probably heard and used the terms for years, possibly without really understanding their effect on the quality of your recordings. If you're an engineer working with analog tape decks, you should know how to set decks up for different operating levels, bias for varying brands and formulations of tape, and how to calibrate your deck for a flat EQ response. Your recordings will sound better (than if you left it uncalibrated for years!) and you won't be afraid to try new tape formulations or work on tapes from other studios. Keep in mind that this is just the tip of the iceberg - there's enough info and tips on this to fill a book - but don't be afraid to learn how to do this yourself - you need this knowledge! Here's how it works:

The bias oscillator in your analog tape machine serves two purposes: it erases the tape as it passes the erase head, and it provides recording bias, as a smaller amount is sent to the record head. Well, if the tape's already been erased, you might wonder, why do we need more of the same signal fed to the record head? Turns out that magnetization is not a linear function. The tape needs a bit of a shove to get it into the linear part of its transfer function. That shove is provided by high-frequency bias. Since the bias frequency is way beyond our hearing range (100- 200 kHz. in pro decks), it doesn't affect the audio being recorded too much. There is some low-level intermodulation distortion, however.

We all understand the purposes of equalization. In analog recording, a built-in EQ is applied at both ends of the process. Recording equalization (set to AES/NAB or IEC/CCIR standards in most cases) is applied before our audio gets to the tape. Its purpose is to compensate for the characteristics of the recorder and the tape. Playback equalization must be applied to compensate for the nature of the medium. On playback, magnetic tape exhibits a 6 dB per octave rise in output. Thus the tape deck uses a -6 dB per octave low-pass equalization to obtain flat frequency response.

Armed with this knowledge, we are now ready to match the tape machine's electrical characteristics to the tape we wish to use for recording.

You will need the following:

1. A magnetic reference tape. This is a tape, which has been recorded at a known level of magnetism for at least three frequencies, 100 Hz, 1 kHz, and 10 kHz. You will use this to set playback levels and EQ. These tapes, frequently from Magnetic Reference Laboratories and known as MRLs, are fairly expensive. You can get around buying one if you happen to have a tape from a pro studio, or from school, which has test tones recorded on it at a known level, although these can sometimes be misleading ("what operating level?"). Make sure the tape is the appropriate speed you plan to run your deck at and that (duh!) it is the right width (1/2", 2" etc.). Generally test tapes featuring AES/NAB EQ are desired, although there can be advantages to using IEC/CCIR EQ for recording ndash; look for this in a future Tape Op article. AES is the only EQ setting for 30 ips whereas 15 or 7.5 ips can be run at AES or IEC EQ. Check your machine for what it's capable of using ndash; many have internal switches for different EQs.

2. You also need a sine-wave oscillator capable of output at those same frequencies. For this, you can substitute a test CD with those frequencies, but a pure oscillator is best. Some consoles and decks have built-in oscillators.

3. A tiny screwdriver will be needed to tweak the adjustments.

4. It also helps to have an AC voltmeter to double-check your levels.

STEP I: Clean the heads and anything which the tape touches (guides, lifters, capstans). Do not use head cleaner on the rubber pinch roller or tach roller unless it is formulated to be safe on these parts (like that S-721 stuff). If you have a demagnetizer, follow the instructions that came with it to demagnetize the heads. Be sure to turn off the deck, remove all tape from the room, and leave your wallet and your watch in the next room (no sense erasing your credit cards while you're at it). If your demagnetizer has an on/off switch be careful not to turn it on or off near the deck! Consult the service manual for your tape machine to be sure that you know where to find all the correct potentiometers for alignment. Also, switch off any noise reduction (dbx, Dolby) during the calibration and biasing process.

STEP II: Load your magnetic reference tape and set all channels to REPRO/SAFE. Play the 1 kHz tone and observe the VU meters. If you plan to record at the reference tape's level, all the meters should read 0 VU. Adjust the REPRO LEVEL pot on each channel to obtain this level. If you desire a higher or lower level, adjust the REPRO LEVEL until the meters read under 0 by the amount you wish to record over 0. For example, say you have a 250 nWb/m reference tape but you wish to record at +6 dB (370 nWb/m), you would adjust the REPRO LEVEL so that each meter reads -3 dB (See the chart here for quick conversions). There is one warning, though. Some recorders cannot supply enough bias current to record at elevated levels. If proper results cannot be obtained in the next step, you might be stuck with a lower level. Repeat this procedure with the 10 kHz tone, adjusting the HF (high frequency) REPRO pot (HF EQ, HF, EQ - whatever it may be called). Repeat this procedure at 100 Hz. Adjust LF (low frequency) REPRO for your reference level if that adjustment is available on your recorder (although this may change later...). Re-check the levels at 1 kHz, and adjust if necessary. Now do the same thing for SYNC/CUE playback - this time adjusting the SYNC LEVEL pots to obtain the same levels. Rewind and store your reference tape. You have now set all the reproduction levels and EQ for flat response. Do not skip this part, as everything else depends on having the playback circuitry properly adjusted.

STEP III: Load a new reel of the tape you wish to use. All channels should be set to RECORD/READY and the monitor switch set to TAPE/REPRO. Input 10 kHz at 0 VU from your oscillator. You can check for the correct input level with a voltmeter set to read AC volts. If your deck operates at +4 dBu (most pro gear), your meter should read 1.23 volts AC across the hot and cold outputs (tip and ring on 1/4" and pins 2 and 3 on XLR). If your recorder operates at -10 dBu ("home" recording gear), it should read 0.316 volts AC. While recording this tone, observe the meters. Turn the BIAS pot counterclockwise and notice the peak output on the VU meters. Now, turn the BIAS pot clockwise until the level peaks again then starts to fall. If the needle pegs the meter and stays there, reduce the level from the oscillator during the biasing, but remember to reset it to +4 or -10 after this step. This peak is your reference bias level. Turn the pot further clockwise until the level drops from the peak level by the amount specified in the literature for your tape or listed in the manual for your tape machine with the brand of tape and the tape speed you use. This level is known as the "recommended over-bias" (see the other chart for this info). Repeat this procedure for each channel.

Now, we have repro level and recording bias adjusted.

STEP IV: Switch your oscillator to 1 kHz @ 0 VU and send the tone to the inputs of each track, either through a buss, or multed, or one at a time, or through a test tone input like on Otari decks. While recording this tone and watching the meters in repro/tape, adjust the RECORD LEVEL/GAIN pots to obtain 0 VU at the meters for each channel. (You can switch between INPUT and TAPE monitoring, each level should read the same.) On some tape decks, like MCI gear, there's a REC CAL pot on the record card. You'll need to fine-tune this to 0 while in input mode while switching to repro to set the record gain (and go back and forth till both read 0).

Switch your oscillator to 10 kHz and record, this time adjusting the HF RECORD EQ for 0 VU output while monitoring in tape mode. Sometimes this is labeled simply as EQ on the record card.

With the oscillator set to 100 Hz record yet another pass and monitor in repro while adjusting the LF/LO FREQ on the repro card (that we set before) and set it to zero. You can even skip the adjustments of this during the reference tape session earlier if you feel comfortable doing so.

Re-check the levels at 1 kHz and readjust them if any have changed.

Now you have set the bias to optimize the tape and your deck and you should be reading flat levels on all your test tones when you record them and monitor in repro. You can print tones (30 to 60 seconds per tone) at the top of a new reel (or reel number one of a session) for your own future reference or for the poor engineers at some other studio that may have to work on your tracks. This will enable them to set up their machine to sound as much like yours was set up during tracking. If you have a feeling a project may be traveling to a number of studios you can reserve a space on the first reel as a "bias pad". This is a stretch of tape (3 minutes is nice) where the next studio can set the bias on their machine without worrying about running over a song or test tones. Always separate test tones and bias pads by tape leader if you can. Remember to label the reels as to where the tones are (head or tail?), what the frequencies are (1 kHz, 10 kHz and 100 Hz being pretty standard but you never know), what tape speed it is running at, and operating level (+6/185 nWb/m being an example). This will make somebody's job a lot easier in the future.

Now record some great music.

Perhaps you see yet another reason why digital recording is so dang popular. You don't need to go through all this crap to get a decent recording on digital, but I swear there's nothing like the feeling of security you get after setting up your deck for optimal levels before a big session.

NOTES: 1. Frequently people lump "alignment" in with biasing and calibration. Alignment specifically refers to setting up the actual tape heads for optimal performance by adjusting the axis so that tracks 1 and 24 (or whatever is highest) read the tape at the same time, by making sure the head is perpendicular to the tape, and that the tape is equidistant from the top and bottom of the head. This is a more involved process, and requires an oscilloscope, though you can be reckless and eyeball these settings in an emergency!

2. Many decks have different names for all the adjustments on the repro, record, bias and control cards. Read a manual if one is available to make sure. Be wary of turning controls labeled BIAS TRAP or BIAS CAL or ERASE PEAK as they do other things.

(Many thanks to Jeff Saltzman and Craig Smith who forced me to learn this dying art and to Greg Norman at Electrical Audio who threw in some tips. -LC)

I'm sure that more than a few Tape Op readers are like me and own a narrow format (1/4" 8, 1/2" 8 or 16, some 1" 16, and 1" 24-track.) analog deck by either Tascam or Fostex. To keep construction costs, and the physical size of these units tidy, the manufacturers designed their recorders with an erase head and a combination record/playback head only (sort of like most cassette decks). The lack of a dedicated playback (repro) head after the record (sync) stage in the tape path makes it impossible to see the adjustments being made in real time while calibrating - but with time and A LOT of patience on your part, your multitrack can be aligned (almost) like its three-head brothers.

First off - if your machine sounds fine to you and it is NOT showing the signs of a tape recorder out of alignment - i.e. tracks coming back off tape much hotter or weaker than your initial input level and/or tracks coming back sounding inordinately brighter or duller than what you originally heard - this process is probably not something you want to get into. It's especially unnecessary if all your projects are recorded and mixed on the same machine and you use the same type of tape every time you work. Second - obviously this procedure can be done by a tech with a volt meter pretty easily, so I guess the following information is for those who want to learn more about their tape machine and experience the satisfaction of setting up their own deck.

Okay, start with an MRL tape and reference your units service manual for the manufacturer's recommended operating level (my Tascam MSR-16 manual suggests +3 or 250 nWb/m.) If the MRL is rated at a different level than what you need use the chart here. Access the calibration adjustment pots on your deck (again - the service manual) and thread up the MRL. With your machine's noise reduction off, play the 1 kHz segment of the test tape and find the pot designated "reproduce gain" or "reproduce level". Adjust each track to the desired level and now find the "reproduce EQ" control (making sure you are tweaking the right speed - there is usually a "hi" and a "lo"). Play 10 kHz off the MRL and trim these pots to the same level as your previous "gain" setting.

That was the easy part - now remove the MRL and thread up a blank reel of the type of tape you usually/or want to use. Put a track in record and feed the machine a 10 kHz tone checking in input for a level falling around -2 or -3 dB on the meter. Find the "bias adjust" pot (make sure it isn't "bias trap" or "bias erase".) Set a locate point and with the noise reduction still off, record a couple seconds of tone. Locate back and hit play paying special attention to where the meter is on playback.( this could be tricky on your edge tracks...) Record some more 10 kHz - this time turn the "bias adjust" pot counter clock-wise a quarter turn or so - play the tape - the level on playback should have increased slightly. Continue turning the pot counter clock-wise a bit at a time in record and checking the resulting change in playback. At a certain point (probably about 3 or 4 dB up at 15 i.p.s.) your adjustment should cause the level to start dropping - when you notice the change, try turning the pot clock- wise now in record - this should make the level increase again a little. Go back and forth between clock-wise and counter clock-wise on the bias pot a few times checking playback in between until you think you've found the "peak" of the meter activity. With that established, turn the pot clock-wise while recording past the peak and the level will decrease again. keep going down (clock-wise) until you've reached the suggested "overbias" amount based on head-gap, tape type, and tape speed. Use our charts here for reference but if you aren't sure about this, consult the tape manufacturer or your local pro-audio dealer. Now repeat this procedure for the rest of the tracks. When you get to the end the deck should be biased for that particular type of tape. If you decide to change tape it's a good idea to re-bias for the new brand or formulation.

Once you've done the bias adjustment you're ready to shoot yourself - I mean do the record alignment. Arm all the tracks and feed the machine a 1 kHz tone so the meters read exactly 0 dB on input. On the same blank tape set a locate and put the machine in record for a few seconds. Play the section back - ideally the tones should come off tape right around zero but there will probably be some adjustments to make. I usually write little notes on scratch paper like this: track 3 down , track 6 up etc. Then you can find the "record gain" or "record level" trim pots and make (guess?) the changes based on your list. Record the new adjustments and check the playback to see how close you are. After a while you'll start to get a feel for how much to turn the pot before recording to make the corresponding changes on playback. The "record EQ" adjustment is the next thing to look for - set up a 10 kHz tone to 0 dB on the input and follow the previous procedure for 1 kHz/"record gain". Once again, make sure you are tweaking the pots for the right tape speed when it comes to EQ adjustment.

Congratulations! At this point you've probably aligned your tape machine. If the deck sounds funny or the levels aren't looking right, try again or call a qualified tech to dig you out - but don't bill us. If everything sounds great start recording your rock opera based on an H.G. Wells novel.

—thanks to Paul Moody and Klaus Heyne.