The Practitioners Guide - Analog Tape Machine Alignment



Project Tones

Project tones are the reference used throughout the recording process to ensure the repeatable and accurate alignment of recording devices. Great care must be taken in the recording and use of these materials. The following policies should be standard practice: Also, remember to print a spare set of tones for projects that will often be going to another studio.

Recommended Tone Sequences

Digital Formats: 1k, 2min
Multitrack Music: 1k, 10k, 100, 50, 1min each
Stereo Master Music: 1k, 5k, 10k, 15k, 500, 250, 100, 50, 30sec each
TV or film: 1k, 10k, 100, 30sec each 1-4 tracks, 1min each 8 tracks and up

Head Cleaning

Tape machine heads should be cleaned every 4 hours of use, or every time you change a reel. Be extra wary of old tapes. These may exhibit abnormal shedding properties. Tape machine guides and rollers should be cleaned daily.

When cleaning heads move the aplicator horizontaly across the head. DO NOT scrub verticaly, i.e. up and down. Do not apply excessive force as this may push some heads out of line.

Be extra careful with digital heads. Move the applicator in the direction of tape travel only. Do not apply preassure to the applicator, the solvent will do the work for you.

Use only the solvents and applicators recommended below:

Material to be Cleaned
Solvent
Applicator
analog headiso, den, TFcotton, foam swabs
digital/video headden, TFTexwipe, foam swab
metal guidesiso, TFcotton, foam swabs, Texwipe
Neoprene rollersisoTexwipe, foam swab
rubber rollerspure waterTexwipe, foam swab
Otari composition rollersisoTexwipe, foam swab
Plasticiso, TFTexwipe, foam swab
iso = isopropyl alcohol den = denatured alcohol TF = Freon TF


Low Frequency Equalization

LF eq is adjusted on the play electronics only. No harm will come if you don't align these controls properly, although some may wonder how well you did the rest. Note however that there are different fixed LF record eq standards for the different speeds; NAB ( in the U.S. ) and CCIR ( in Europe ) for 15ips and below, and AES at 30ips.

When aligning to project tones, check them against the machine's own record response. If you are tracking or overdubbing, align to the record head. When mixing, align to the project tones. If there is a large discrepancy between the two, be sure to discuss it with the mixer after checking that the equipment is working properly.


Demagnetization

Tape machine heads should be demagnetized periodically, determined by head composition and use. Refer to the manufacturer's instructions when determining a demagnetization schedule. Always test the heads with a magnetometer to be sure demagnetization is necessary.

WARNING Turning off the power to the demagnetizer while it is close to the heads can permanently magnetize the heads, ruining them. For this reason:


Tape Speed

Don't forget that when you change speed, the controls you will be adjusting will also change. Also, pay close attention to what bias parameters you are using (they are different at each speed, see the article on bias procedures).

Azumuth

The recorded signal on the tape must be perpendicular to the gap in the reproduce head. Any angular error is referred to as azimuth error, which results in a time delay between the tracks on the tape.

Azimuth Adjustment Procedure:
1 Set the oscilloscope for x-y operation. Connect the tape machine's left and right outputs to the oscilloscope.
2 Go to the 10 khz section of the reference tape.
3 repro Adjust the repro head so that the elipse on the 'scope is as near as possible to a straight line, extending from the lower left to upper right corner of the screen.
4 sync Repeat step 3 on the sync head.
5 Set the oscillator for 15 khz at 0VU. Mount record stock on the machine.
6 rec/repro Monitor the repro head with the oscilloscope. Put the machine into record and note that the record head is already closely aligned. Fine tune the record head adjustment.


Bias Theory

Bias is a high frequency signal (usualy > 100khz) that is used to excite the tape's magnetic particles to lower the medium's coercivity, thus allowing a higher quality recording. Adjustment of this parameter is critical, since too little bias will result in a distorted recording and too much will cause excessive noise. It is also important to note that uneven bias levels between channels can appear as azimuth errors.

Over Biasing is a technique used to guage correct bias level by observing the high frequency response of the tape. Using this method, bias current is increased until the level of the test tone rises, peaks and then falls a specific amount, as demonstrated below:



          +3  |
     T        |
     a     0  |                    -
     p        |                  -   - 
     e    -3  |                -       +---(3db over-bias)
              |              -           -
          -6  |            -
     L        |          -
     e    -9  |        -
     v        |      -
     e   -15  |    -
     l        |  -
         -20  |-
              ---------------------------------------
               More Bias Level --->

Recommended Over-Bias Values

Tape Formulation 15ips 1/4"
@10 khz
30ips 1/2" & 2"
@10 khz
30ips 1/2" & 2"
@20 khz
226-2.75-1.5-4.0
250-2.-1.5-4.0
996-4.0-1.8
456-2.75-1.5-4.0
468/469-3.5-2.0-3.5
NOTE: Use the Low Noise Biasing Procedure for unknown tape types.

Low Noise Biasing Procedure

1 Set the oscillator for 1khz at 0VU.
2 Mount your record pad on the machine.
3 rec/repro Turn the master bias control counter-clockwise and watch the VU meter rise, peak and then fall to -20. Now turn the control clockwise until it peaks. Check to be sure that all channels are tracking through the peak.
NOTE: If channels are not tracking, adjust the master bias control so that the majority of the channels are peaked. Adjust the channel bias controls for peak.
4 Set the oscillator for the correct bias test frequency (10khz for 15 ips, 20khz for 30 ips) at 0VU.
5 rec/repro Turn the master bias control clockwise until the VU meter falls 3db.
6 Patch the output of the channel to be tested into a channel on the console. Enable the console channel eq and set it for a shelving boost of 6db from 3khz up, and a shelving cut of 12db from 50hz down. Set up to listen to this channel through headphones.
7 Set the oscillator for 20hz at 0VU.
8 rec/repro Set the oscillator level so that the off-tape level is -10VU.
9 rec/repro Adjust the master bias control for lowest possible audible noise. The VU meter should also be at a natural peak at this point.

Analog Tape Machine Alignment Procedure

1 Clean the heads, check and set azimuth.
2 Mount the reference tape on the machine and wind to the 1khz tone.
3 repro Play 1khz tone from the reference tape. Adjust "repro gain" so that the VU meter reads 0VU. If you are aligning the machine from an MRL tape, note the reference level on the tape. Many test tapes are at elevated level, that is, at a level above Ampex standard 185nW/m. The most common of these is 250nW/m, or '+3'. When you align a channel to 0VU using such a tape, the channel is said to be at '+3'. For '+6' operation you would adjust the channel to read -3VU, for +5, -2VU, etc.
4 sync Play 1khz tone from the reference tape. Adjust sync gain so that the VU meter reads the same as in step 3.
5 Wind to the 10khz tone on the reference tape.
6 repro Play the 10khz tone from the reference tape. Adjust repro HF eq so that the VU meter reads the same as in step 3.
7 sync Play the 10khz tone from the reference tape. Adjust sync HF eq so that the VU meter reads the same as in step 3.
8 Mount the record stock on the machine.
9 Set the oscillator for the correct bias test frequency at 0VU. See Recommended Over-Bias Values for details.
10 rec/repro Turn the master bias control counter-clockwise and watch the VU meter rise, peak and then fall to -20. Now turn the control clockwise until it peaks. Turn the control back and forth to verify the level of the peak. Check to be sure that all channels are tracking through the peak. You may have to change the oscillator level so that the peak is not above 0VU. Turn the bias control clockwise so that the VU meter falls the correct amount(i.e. -3db). See Bias for details.
NOTE: If channels are not tracking, adjust the master bias control so that the majority of the channels are peaked. Adjust the channel bias controls for peak. Re-do step 10.
11 Set the oscillator for 1khz at 0VU.
12 input Verify 0VU at the tape machine meters.
13 rec/repro Adjust record gain so that the VU meter reads 0VU.
14 Set the oscillator for 10khz at 0VU.
15 input Verify 0VU at the tape machine meters.
16 rec/repro Adjust "record HF eq" so that the VU meter reads 0VU. If you are aligning an Ampex machine with "shelf" controls do the following:
Turn the record HF eq control fully counter-clockwise. Adjust record HF shelf so that the VU meter reads 0VU. Adjust record HF eq clockwise so that the VU meter dips and returns to 0VU.
17 Set the oscillator for 50hz at 0VU.
18 rec/repro With the tape machine in record, switch the oscillator between 50hz and 100hz. Record about 1 min. of this.
19 repro Play the test pattern made in step 18. Adjust repro LF eq so that the center between the two tones is 0VU, i.e. 100hz is at -.5db and 50hz is at +.5 db.
20 sync Play the test pattern made in step 18. Adjust sync LF eq so that the VU meter reads the same as in step 19.
21 rec/repro Print Project Tones if applicable.

atab@utac.net