ANALOG TAPE 101
Part Three, Bias Magic
Page 1, 2, 3

 

Image Notes
In Part One of this series, Figure 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, Figure 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.

Table 1

30 ips

15 ips

7.5 ips

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

20 kHz
20 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
20 kHz
20 kHz


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 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.

Table 2:

Manufacturer
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/IEC
3


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Reprinted with permission from Magazine, October, 2000
2000, Intertec Publishing, A Primedia Company All Rights Reserved