Oct 1, 2000 12:00 PM, Eddie Ciletti

Welcome to the final part of this series on analog tape recorder maintenance. Wrapping up each section has been a bit like making yogurt or sourdough bread: There’s always a bit left over to start another batch of tips. This time, bias is scrutinized in greater detail because it is the most critical parameter and, second only to input level calibration, the most inconsistently implemented on semipro (narrow format) machines.

Bias Magic

Bias is like an electronic bribe to the tape’s magnetic particles–without it, analog tape would sound like a digital children’s toy. It is basically a minimum distortion adjustment–sort of like tuning an old-fashioned AM radio–too little bias makes the sound bright on top and fuzzy on the bottom. Excessive bias results in dull and dirty recordings. Bias is a Radio Frequency (RF) signal–anywhere from 60 kHz (for cassette decks) to 432 kHz (for the Ampex ATR-100 Series)–that is mixed with the audio signal. As with digital sample rates, the higher the bias frequency, the better (for lower high-frequency distortion).

The correct amount of bias current is determined by the size of the gap in the Record head. The "complications" are tape oxide type, tape speed and the type of distortion being minimized. Magnetic tape does not have the same sensitivity at all frequencies, but when adjusting bias current, high-frequency output is most affected, hence its use in achieving the most precise adjustment.

If you followed the first two parts of the series, by now you should have aligned repro and/or sync (Playback) with the alignment tape. Now, put up a blank tape; set the oscillator to 10 kHz and press Record on all tracks. On a three-head machine, if the high-frequency tones waver more than 1 dB, do NOT pass Go! Don’t even waste your time until the problem is solved, either by thorough scrutiny of the tape path or by having the heads relapped. Remember that many narrow-format machines have only two heads–Erase and Record/Sync–so when you see "PAR," it means "Playback After Record," or PITA (Pain in the Ass).

The adjustments, in order, will be:
• Bias (using 5 kHz, 10 kHz or 20 kHz)
• Record Level (1 kHz)
• HF EQ (level): 10 kHz and 15 kHz
• Low Frequency Adjustment: Bass Sweep from 250 Hz down…

Note: On a three-head deck, record bass sweep while monitoring via Playback head. Align the low-frequency EQ until the last peak and dip fall on equal sides of "0 VU," then select a low frequency that falls on "0 VU." Print that tone on tape and note it on the box. Include the bass sweep if the tape becomes a mix master.

On a two-head deck, record bass sweep, "PAR," noting the peaks and dips, then select a low frequency that falls on "0 VU." Print that tone (on tape) and note it on the box.

IMAGE NOTES

In Part One of this series, Fig. 3 showed a graph of BASF SM 900 tape properties as bias is changed. For more information about BASF-Emtec tape products, visit www.emtec-group.com.

In Part Two, Fig. 1 showed a close-up head view detailing "The Gap." If there is some doubt about the amount of over-bias, check out this cool Quantegy link: www.Quantegy.com/TapeTimeChart-javascript.asp.

In Table 1, notice that the three interactive factors are speed, over-bias (in dB) and the recorded audio frequency used to make the adjustment. Referenced to 15 ips using 10 kHz to set bias for all speeds, over-bias is halved for 30 ips and doubled for 7.5 ips. Or, using 2.5 dB over-bias for all speeds, the record frequency must be halved or doubled relative to the reference, 10 kHz at 15 ips. See the heading "Good Vibrations" to learn more about the effects of bias.

	30 ips	15 ips reference	7.5 ips
			(-10 dB ref)
Large Gap (using 10 kHz for ALL speeds)	1.25 dB	2.5 dB	5 dB
Large Gap (using 2.5 dB over-bias for ALL speeds)	20 kHz	10 kHz	5 kHz
Narrow Gap (using 10 kHz for ALL speeds)	2.25 dB	4.5 dB	9 dB
Narrow Gap (using 4.5 dB over-bias for ALL speeds)	20 kHz	10 kHz	5 kHz

Table 1: The interrelationships of over-bias, tape speed and record frequency. These are examples only and not intended to be global solutions.

Critical Mush

Low frequencies are hardest to record at 30 ips, making precise bias adjustment critical if you want to get a good kick drum sound. That’s why it’s better to adjust bias using 20 kHz at 30 ips, taking advantage of the additional meter resolution. Yes, 20 kHz will also reveal more mechanical problems, if present. Also, at 7.5 ips, high-frequency tests must be made 10 dB below 0 VU (20 dB below for cassette decks). You won’t always see the neat and tidy numbers shown in Table 1. At slower speeds, the amount of over-bias is relaxed to preserve high-frequency response because the window to achieve of low distortion at low frequencies is wider.

Bias Cautions: The Straight and Narrow

For a professional machine, the bias adjustment is a piece of cake. You’ll find the integrated operator/service manual to be both educational and readable. The manual will be quite specific about all of the previously mentioned details, including the actual size of the Record and Play gaps. If you know the recommended bias at only one speed, Table 1 shows the relationship between speed and frequency for interpolation purposes.

If you've never done an alignment and you have a narrow-format machine (as listed in Table 2 below), proceed with caution. There is no Playback head, so it is not possible to adjust bias while recording–obviously, not much fun. Nearly all narrow-format machines have the same Record and Playback gap, so the amount of over-bias will be higher than for some pro machines. (See the example in Table 1.) In many cases the service manual will show how to set bias by measuring the voltage at a test point. This method works assuming you have a voltmeter that is accurate at bias frequencies (the Fluke 8060A) and that the heads are new or recently relapped. 

Mfr.	Model	Tape Width/ Tracks	Speed/EQ	Head Count Erase-Record-Play = 3
Fostex	A-8, 80, R-8	1/4-inch/8-tk.	15 ips/IEC	2
Fostex	B-16, E-16, G-16	1/2-inch/16-tk.	15 ips/IEC	2
Fostex	G-24	1-inch/24-tk.	15 ips/IEC	2
Tascam	388	1/4-inch/8-tk.	7.5 ips/NAB	2
Tascam	38, 48, 58	1/2-inch/ 8-tk.	15 ips/IEC	3
Tascam	MSR-16	1/2-inch/ 16-tk.	15 ips/IEC	2
Tascam	MSR-24	1-inch/24-tk.	15 ips/IEC	2
Tascam	MS-16,	1-inch/16-tk.	15 ips/IEC	3
Tascam	ATR-60/16	1-inch/16-tk.	15 ips/IEC 30 ips/AES	3

Table 2: Many narrow-format machines have only two heads–Erase and Record/Play–making bias alignment torturous on a good day.

If you are reasonably sure that the machine was never misaligned, then the "factory" bias setting should be better than attempting to readjust bias and potentially make things worse. If you’re not happy with the sound, the "Good Vibrations" section will be revealing. But first these cautions…

I do not recommend using high-output tape on narrow-format machines. Mechanically, they are heavier, less pliable and more likely to accelerate headwear. In their heyday, Tascam’s MS-16 and Model 38 would become sluggish over time using the recommended tape (Ampex 456, 3M 226). Having the reel motors rebuilt will improve performances and extend the life of the motor drive amplifiers. Electronically, some narrow-format machines are not capable of full erasure. Also, as some machines absolutely must be run with the built-in noise reduction, there really is no need or no point to pushing the levels.

Good Vibrations: The Magician's Secret

I saved this technique for last because it's more complicated. If you are uncertain about how much over-bias is required, or just want to satisfy your own curiosity, record a 40Hz sine wave at 30 ips. Listen to what happens as the bias is varied. (Hint: If the fundamental is filtered with an equalizer, you’ll hear only the distortion artifacts; the changes will be more revealing.) Adjust to minimize the fuzz and low-frequency harmonic distortion. (Congratulations! You've just used your ears as a distortion analyzer.) Switch to 15 ips and then 7.5 ips, making the adjustment each time and notice that the "window of relative cleanliness" becomes wider as the speed is cut in half. Also, try this on a digital machine and notice how there is no obvious distortion.

At 30 ips, the narrow window is perfect for getting the most precise adjustment, but it is important to perform this "test" over several channels. Now, switch the oscillator to 20 kHz, note the level on the VU meter (A), then reduce the bias until the signal level peaks, again noting the new output level (B). Average the results. The difference in dB from (A) to (B) is the optimized over-bias. Use this amount to align all channels.

Phew, now we can get on with the Record calibration.

Record Level: 1kHz

As mentioned, Input and Record level calibration are inconsistently implemented on professional and semipro machines alike. On some pro machines, the two might interact. Older Tascam machines have separate meter calibrations for Input and Repro, as well as level adjustments. Read the manual. Make the adjustment at 1 kHz so that Input and Repro both read "0 VU" (PAR, if applicable).

Record EQ: 10kHz

For this adjustment, it is important to confirm that the oscillator is flat, so check input after switching to 10 kHz, then go into Record and adjust the Record EQ so that 10 kHz also reads "0 VU." Sweep up to 15 kHz and check the level–it should be within ± 1 dB–and again at 20 kHz, where ± 2dB is acceptable. Of course, FLAT is the goal. The head condition is most suspect if the level drops as the frequency goes up. Channel-to-channel inconsistencies point to the electronics. Swap cards with the power off. Test. Then swap channels back to confirm. (Sometimes reseating is a temporary fix.)

Low-Frequency Playback EQ

Accurate low-frequency playback adjustments can only be made after recording a frequency sweep, starting as high as 500 Hz. A slow sweep, from 200 Hz down to 20 Hz, will show the most critical range to be within the last two octaves (below 80 Hz). Along the way, note the value and frequency of the peaks and dips. Ultimately, the last major peak and dip should be adjusted so they appear symmetrically above and below "0 VU." Then, find the lowest frequency that falls on "0 VU," note and record a section of it after the bass sweep.

Master Tones:

It is customary to create a tone reel for a project, especially for a Mix Master, so that mastering engineers can adjust their decks to match the original record machine. In addition to 1 kHz, 10 kHz and 15 kHz, the bass sweep and final bass tone are extremely helpful.

Narrow Escape

Narrow-format machines offer little in the way of Record EQ adjustment, and there is no low-frequency playback compensation for worn heads. One trick I’ve used is to note the level of the nastiest low-frequency head bump, then set the 10kHz Playback EQ (from the test tape) to that level. It is cheating, but only enough to minimize mistracking of the noise reduction system–dbx noise reduction multiplies low-frequency errors by 2, so a 1.5dB bump becomes a 3dB bump.

Back in the day when I aligned many a Fostex E-16, the extra work of adjusting the bias by the PAR method paid off because channel-to-channel frequency response and phase were more accurate. The difficulty with most Fostex machines is that the bias adjustment is a continuously variable cap. Without a 'scope or a meter, it is easy to misalign one of these machines. By being meticulous about the bias adjustment on all machines, you may discover track-to-track anomalies that could either be caused by aging components or a head near the end of its life. The last narrow-format tip concerns tape thickness. Many of these machines perform better with 1-mil tape rather than 1.5-il tape. Unfortunately, it's not so easy to find Ampex/Quantegy 457 or 3M 227 tapes these days; however, Quantegy 407 is still manufactured.

Tail Out

The warmth for which analog tape is famous comes from the composite of its idiosyncrasies–saturation and head bumps. Aside from noise, the negatives are mostly from mechanical problems that cause speed variations–wow and flutter and tape path wanderings that affect phase at high frequencies. While I may tackle these topics in a future article, I first must recover from Alignment 101. I hate to think what would happen if I had office hours.

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Eddie recently had his head demagnetized after returning from a vacation in NYC, where his brain had been saturated with noise, smells and images. E-mail–edaudio@tangible-technology.com–with comments and suggestions.

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