BIAS MAGIC

CONDENSED ALIGNMENT PROCEDURE (In Part Two, this will be hyperlinked to the detailed procedure.)

PLAYBACK LEVEL (1-kHz)

AZIMUTH 8kHz and 16 kHz

High Frequency PB LEVEL

BIAS

RECORD LEVEL (1 kHz)

HF EQ (level): 10 kHz

3-head deck:

RECORD BASS SWEEP while monitoring via Playback head. Align until peaks and dips fall on equal sides of "0VU," then select a low frequency that falls on "0VU." Print that tone and note it on the box. Include the bass sweep if the tape becomes a Mix Master.

2-head deck:

RECORD BASS SWEEP. Check playback, noting the peaks and dips, then select a low frequency that falls on "0VU." Print that tone and note it on the box.

PLAYBACK LEVEL (1-kHz)

Going back to prehistoric times the earliest "reference level" was 185 nWb/m as shown in the left column of Table-1. On professional machines, most engineers will take advantage of higher recording levels both to reduce noise and for "effect" (natural saturation). In such cases, the level on tape will be referred to as "plus xxx" where "plus" refers to the number of dB over the reference level and "xxx" refers to the reference level. The most popular reference level is 250 nWb/m although test tapes have been made with higher reference levels adding to the general confusion. Again, Table-1 shows an examples of the establishment and relationship of two reference levels.

185 nWb/m	200 nWb/m (+1 /185)	250 nWb/m (+3 /185)	320 nWb/m (+4 / 185) DIN	355 nWb/m (+6 / 185)
		250 nWb/m	320 nWb/m (+1 / 250)	355 nWb/m (+3 / 250)

Table-1: Reference level examples

AZIMUTH (You must positively know the location of this adjustment or don't do it, ok?)

Azimuth is a mechanical adjustment of the head to optimize high-frequency output. In a perfect world the head is perpendicular to the tape. (AMPEX MM-1100 and MM-1200 multitrack machines do not have Azimuth adjustments.) On the test tape, two frequencies - 8 kHz and 16 kHz - provide course and fine adjustment references, respectively. For a full-track (mono) headstack, simply adjust for maximum output. For a multi-track machine, summing all channels is the best way to "average" this adjustment for the entire headstack. Note that manufacturing tolerances make track-to-track perfection impossible.

On a stereo deck, the summing method can get you into trouble so use a mixer or another tape machine with mechanical VU meters (referred to in this example as the MIX meter). Start with 1 kHz (on the test tape) and set levels, one channel at a time, via console fader so that the MIX meter reads -6VU. When both faders are up, the MIX meter will read 0VU. Using the 8kHz tone, adjust azimuth for a maximum on the MIX meter, then do the same with the 16 kHz tone. Because the wavelength of the 16 kHz tone is so small, it is possible to misalign the head so be sure to adjust for 8 kHz first.

LISA CHEW

An oscilloscope can also be used to measure and adjust azimuth, by connecting two tracks, one each to the X-Y (horizontal and vertical inputs). One channel alone would create a vertical or horizontal line. Both channels, perfectly in phase, will create a line at a 45 degree angle like this, "/." One channel out-of-phase creates an 45 degree line "\" in the opposite direction. Anything in between creates a "Lissajous" pattern, a 90-degree phase error is a circle.

THREE HEADS ARE BETTER THAN TWO

A professional machine has three heads — ERASE, RECORD and PLAYBACK — as shown in Figure Three, where the record and playback heads are each optimized — electronically and mechanically — for their respective jobs. The typical one-inch sixteen track may have separate record and playback heads, but both are technically playback heads, that is, their electro-mechanical properties are that of a playback head. This leads to some confusion when optimizing bias (explained in detail in that section).

Some narrow format machines only have two heads, Erase and a single Record/Play head. The combination makes record alignment a most tedious task because it is not possible to monitor record and bias adjustments. Imagine having to rewind and playback after every tweak. If the record play frequency response is in question, getting the heads lapped is the most expedient and cost effective solution especially when many of these machines have LED metering. If the mechanical alignment is in question, passing your thumb or a cotton swap over the head-stack while tape is playing may yield better high frequency response.

BIAS CAUTIONS: The Straight and Narrow

Put the test tape away now and put up a blank tape. If you've got a professional machine with a service manual, reading and performing the procedure is quite basic. You need an oscillator. Do not pass GO if the tones waver more than 1 dB. Don't even waste your time until that problem is solved.

If you’ve never done an alignment and you have a Narrow Format Machine (as listed in Table Two below), it may be better to leave well enough alone. In many cases the service manual will show you how to set BIAS by measuring the voltage at a test point. This method works assuming the manual secifies the your tape choice and that the heads are in good condition. Again I will stress that relapping the heads makes them like new and, if the machine was never misaligned, then the "factory" bias setting should be fine. TRANSLATION: no Tweakers!!! I also do not recommend using high output tape on a narrow format machine. Newer high-output tape formulations are heavier, less pliable and more likely to accelerate head wear. These machines were simply not designed to pull the "new" tape. Expect sluggish reponse as well as Motor and Motor Amplifier failures.

Manufacturer	Model	Tape width / tracks	Speed/EQ	Head Count Erase-Record-Play = 3

Fostex	A-8, 80, R-8	quarter-inch / 8	15 IPS / IEC	2
Fostex	B-16, E-16, G-16	half-inch / 16	15 IPS / IEC	2
Fostex	G-24	one-inch / 24	15 IPS / IEC	2


Tascam	388	quarter-inch / 8	7.5 IPS / NAB	2
Tascam	38, 48, 58	half-inch / 8	15 IPS / IEC	3
Tascam	MSR-16	half-inch /16	15 IPS / IEC	2
Tascam	MSR-24	one-inch / 24	15 IPS / IEC	2
Tascam	MS-16,	one-inch / 16	15 IPS / IEC	3
Tascam	ATR-60 / 16	one-inch / 16	15 IPS / IEC 30 IPS /	3

Table xxx: Many Narrow Format machines have only two heads — Erase and Record / Play — making BIAS alignment torturous on a good day.

BIAS MAGIC

Bias is a radio frequency that is like currency to the tape’s magnetic particles — they’ll make music if you pay cash up front — sort of an electronic bribe. Adjusting Bias is like tuning an old fashioned AM radio. There is a generally accepted "window of correctness" where the signal comes through loud and clear. Underbias and the signal will be bright on top and mushy on the bottom. Excessive bias makes dull and dirty recordings. If, for example, the recommended bias is "2.5 dB over @ 10 kHz," put all tracks in record, adjust the bias to find the maximum output at this frequency, then increase the bias (usually clockwise) until the signal is reduced by the amount specified. This is what is meant by "overbias."

Table Three starts with typical overbias values for professional machines using 10kHz. (Contact the tape or the machine manufacturer if unsure.) The amount of overbias is not global — it is not a magic number that works for all speeds and machines. The frequency used to make the adjustment and the amount of signal reduction (overbias) are speed specific. As noted in Table Three, these three factors — the speed, the amount of overbias, the frequency used to adjust bias — are interactive. For example, while 10kHz is the "standard" recorded frequency used to set bias, the amount of overbias on some Pro Machines at 30 IPS amounts to 1.25dB — a rather small amount that is hard to accurately read on a VU meter. By doubling the frequency to 20 kHz, the amount of overbias is now 2.5 dB, which is much easier to read (but will reveal more mechanical problems, if present). Also note that at 7 ½ IPS, high-frequency tests must be made 10 dB below 0VU (20 dB below for cassette decks).

	30 IPS	15 IPS	7 1/2 IPS
			(-10 dB ref)
Pro (10kHz)	1.25 dB	2.5 dB	5 dB
Pro (2.5 dB over)	20 kHz	10 kHz	5 kHz

Narrow (10 kHz)	2.25 dB	4.5 dB	9 dB
Narrow (4.5 dB over)	20 kHz	10 kHz	5 kHz

Table Three: Typical over-bias values for pro and narrow-format machines.

MAGICIAN'S SECRET

If you are not sure how much overbias is required, record a low frequency sine wave no higher than 40 Hz. Listen to what happens as the bias is varied and adjust until the fuzz and harmonic distortion are minimized. (Congratulations! You’ve just used your ears as a distortion analyzer.) You should notice that while this is a very coarse adjustment, signal cleanliness and maximum output will coincide. The key is to perform this "test" on several channels. Now switch the oscillator to10kHz noting the output at that frequency (A) then undo (lower) the bias using noting the increase in output until it peaks (B). Average your results of several channels to determine the optimum amount of overbias — the difference in dB from (B) to (A). Use this amount to align all channels.

Capture and Insert Bias vs Output and Distortion as well as table of head Gaps and recommended overbias settings.

On a professional machine,

Accurate low frequency playback adjustments can be made after recording a frequency sweep very slowly from 500 Hz down to 20 Hz making recognizable stops along the way to note the peaks and dips. You’ll need this information in a moment.

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 (on heads that are not yet toasted) is to find the nastiest low-frequency head bump — note the level — and set the 10 kHz reference tone (from the test tape) to that level. It is cheating, but otherwise, the noise reduction system will mistrack and exaggerate the response errors by a factor of two.

In addition to the head bump, it is also helpful to find the nearest trough. Machines with low-frequency playback EQ should be set so that these two anomalies are equally above and below 0VU. Then, pick the frequency in between that falls on 0VU, record it for at least 30 seconds and make a note on the box.

INVERSE NARROW-FORMAT TIPS

Hint: If you’re smart you’ll start at number 8. Tip: If you accidentally turn an adjustment that does "nothing," put it back in place.

Don’t attempt a record alignment. Most sonic problems are likely to be related to head wear on the playback side. Without a dedicated playback head, any attempts at record alignment will simply throw the machine more out of whack.
Normal head wear can be corrected by resurfacing (lapping) while severe wear can damage the head beyond repair. Replacement is expensive, so don’t put it off. See the back pages of EQ for a lapper near you.
Do not attempt a record calibration on machines not equipped with a dedicated playback head. You will be sorry! Don’t be fooled by the record EQ adjustments, they don’t do anything. In addition…
Do not use bias to affect record EQ. Bias is optimized for minimal distortion. All narrow format machines require more "overbias" than pro machines. 2 ½ to 3 dB overbias (using 10 kHz @15 IPS) is typical for a Pro Machine, 4 ½ to 5 dB overbias is the norm for machines mit da skinny tracks.
For the terminally curious, make a record test — with the noise reduction off — using an oscillator set to 1 kHz. Don’t freak when all of the LED’s don’t line up. A segment plus or minus means little. Use the oscillator’s level control to find an average level that satisfies most of the meters.
On playback, confirm that the signal is at the same level.
If the edge tracks are down at mid or high frequencies, clean the heads. If the improvement is negligible or inconsistent, it’s lap time.
Own an alignment tape and adjust playback only. Most 15 IPS machines use IEC EQ.

Note: Noise reduction is not bad — it should be used on narrow format machines — but it is level and frequency sensitive.

Recording low frequencies at 30 IPS on a two-inch 24-track (or narrower format machine) is a challenge both in terms of efficiency and headroom. Optimizing the bias for the lowest distortion at 40 hz ensures that you get all the efficiency (maximum output), "cleanest warmth" and the "cleanest saturated punch" analog is famous for while minimizing the modulation noise.

The purpose of going back to 20 kHz (or 10 kHz for 15 ips) after the "low-frequency tour" is to make sure the same amount of overbias is applied to all channels. Doing so will minimize channel-to-channel HF idiosyncrasies -- such as phase and level -- so that all of the record EQ settings end up in the same relative place.

If you biased for frequency response (rather than distortion and you pretended to not have access to the HF REC EQ adjustment), the resulting phase response from channel to channel would be way out. (Not good for recording drum overheads.) This is almost the case in narrow format machines that have no real HF REC adjustment.

Biasing at 20kHz (for 30 IPS) is more precise than using 10 kHz. For example: 3 dB of overbias at 20 kHz is only 1.5 dB overbias @ 10 kHz. By being meticulous about the bias adjustment, you may discover track-to-track anomalies that could either be caused by aging components OR a head near the end of its life.

NOT RECOMMENDED

Figure One also makes reference to the fact that increased tape speed permits higher operating levels. Elevated levels are not recommended for narrow-format machines. Operating levels have been optimized by the manufacturer for use with a Noise Reduction (NR) system. NR is built-in out of necessity simply because smaller tracks yield less electrical output and therefore have a higher noise floor. Changing tape formulation and/or operating level will make your machine incompatible with other machines. Plus, the lack of a dedicated playback head makes alignments tedious.

NARROW FORMAT GUIDELINES

Using NR improves headroom, so overdriving for "the sound" of tape saturation will likely yield more NR decode artifacts than actual tape compression. Narrow format decks typically have a global noise reduction on/off setting. Defeating NR on individual tracks is limited to the edge track designated for time-code purposes. (Some machines can be easily modified for individual control of noise reduction.)

Stick with standard formulations (Ampex 456, 3M 226 or AGFA/BASF 911??). These tapes are 1 1/2 mils thick and have 1 mil cousins 457, 227 and ??? High-output tapes such as Ampex 499, 3M 996 and AGFA/BASF ??? are physically more difficult to pull through the mechanism. The resulting tension increase tension will accelerate head wear.

TAIL OUT

The warmth for which analog tape is famous comes from the composite of its idiosyncrasies. There are obstacles, primarily noise, but also mechanical problems such as tension, flutter and tape path. These will be tackled in a future article. I’ll also be taking a closer look at why tapes shed and what you can do about it. Till then, be sure to play all your tapes all the way to the end and store them, tail out.