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.
