Tape alignment - reel to rell
Bill Vermillion
posted an article, which was republished in the Oregon Triode
Society newsletter a couple of years ago.
I just found that article so here it is. Just as written
then, and I haven't re-edited. So with that caveat, the
proceudres may not work properly on the new +9 level tapes from
Scotch and Ampex
The archive date of this article, which was slightly edited from the
first post is March 22, 1991.
------------ cut here ----------------
This is an article on tape alignment that comes from doing this more times
than I would care to admit, on 1, 2, 4, 8, 16, 24, and 32 track machines!
Of course be sure to clean the tape path before starting.
De-magnetization is optional in my opinion. There are some that insist it
be done every time, but in our studio we felt that it was easier to
introduce problems with it. We also checked the machines with a
magnetometer, but typically we thoroughly demag'ed the machines only about
once every 1 to 2 months.
Some machines were re-aligned several times in one day. This was because we
would be working with source tapes recorded on different media, in different
studios at different operating levels. I had one machine that I did a
complete re-alignment on three times in the course of one day. Two
different 24 track tapes, one Dolby, and a 16 track setup. After doing
often we could be able to set up a 24-track machine in 30-40 minutes, which
is about the length of time it took me to do my first alignments on two
track machines.
Be sure to use a good monaural alignment tape. (They should all be
that way. There are some multi-track tapes that are recorded monaurally and
then have guard bands erased after recording. This will make a difference
on low speed playback EQ - but I use a method that elminates any problem
with low-frequency fringing effects.)
The steps used to align are as follows and are to be done is this order.
1. Physical head alignment (azimuth alignment)
2. Playback level
3. High frequency playback response
4. Bias adjustment
5. Record level
6. Record high frequency response
7. Low frequency playback
Standard procedure for record head alignment was to align play head,
and then adjust record head while during recording watching the
playback head. Most multi-track machines and a good many two-track
machines now have the ability to put the record-head into the playback
circuit (often called Sel-Sync*). This is a much more accurate and
prefered way to do physical head alignment.
The first tones on the alignment tape are HF alignment tones. If you have
a scope you can ajdust by watching the phase. If not available you watch
for maximum output at this frequency. I had the luxury of having phase meters
at the studio, this makes it all so much easier.
Playback level is then adjusted. Alignment tapes are manufactured to
different recording levels. Assuming you have a tape for the level you
wish to record, adjust the playback levels to a 0 reading on your meter.
To be precise however you should look at the tables for fringe effect that
should be included in the manual with the alignment tape.
Fringe effect is a higher output caused by playing tapes that have a wider
record track than the width of the playback head. At our studio we
compensated for this as we had standard two-track machines, and we had
"stereo" machines. The latter use the European spacing for 1/4" two
channel tapes of .75mm. The standard two-track spacing is 2mm. If
compensation was not made the tapes will vary in playback when played on a
different two-channel format machine. Most studios ignore this, however
at the higher speeds it is important. At 30 ips the fringe effect will
give a 1db too high a reading at 1000Hz! This will throw your alignment
off by 1db at the high end of the spectrum.
Once playback level is adjust it is important to MAKE SURE that this is not
touched during the remainder of the alignment procedure.
At this point you can then make your HF playback adjustments to give you
flat(est) response from your alignment tape. These playback calibrations
are critical as all the rest of the alignment depends on them.
Now that playback is done the next step is bias, which will affect level
and record response.
For bias I ALWAYS adjust for minimum modulation noise as that to my
ear is the worst part of mag tape.
To do this pick a nice low freqency tone (I always use about 7Hz)
and listen to the noise components when playing back. Cut the
bottom end of your playback amps if you must because you only want
to hear the noise components.
As you start below the bias point you will heard hf noise pulsed at
the 7Hz modulation frequency. As you increase the bias the noise
will decrease to a point and then start increase with a change in
the "tone" of the noise. Go back to the minimum point. If you do
this, you will find that you can set the bias more accurately by
ear than you can doing the typical 1-3 db overbias at 1kHz or 10kHz
(depending on your choices).
The only tape I have found that is can be done on is the Agfa 468.
Minimum modulation noise will overbias the tape 6 to 9 db on this tape.
I had a machine shut down as I overloaded the bias amps when trying this.
Many recommend 1db overbias at 1kHz. I find that you should use
10kHz for anything less than 15 ips, and for 30 ips use 20kHz.
Typically you overbias 3db at these frequencies, but if you have
the chart for your particular brand of tape you can find the exact
point. However, except for 468 I always use minimum modulation
noise. Agfa recommends a 14Khz audio signal for 468, and I don't remember
the exact amount of overbias. They will furnish to you if needed.
After the bias is set, the next step is record level. All you do at this
point is set the output to be 0 vu by matching the output level to the
level you had previously set on playback of the alignment tape.
At this point you can calibrate the record meters if your machine has
record calibration controls. Since the PB level has been matched you goto
the record metering side (up to this point you have been watching playback
metering). You then adjust the record CALIBRATION (not level) so that the
input levels read 0.
Now that you have the record level set you can adjust the HF record
response. The standard that many use is to set 10kHz to be at the same
playback level of the 1000Hz tone. I personally disagree with this.
I try to make the 20kHz level 0 IF the 10Khz region does NOT increase too
drastically at this point. In other words, if I can get 20hKz to zero and
keep 10Khz no higher than 1.5 to 2db I'll go that way. I would much rather
have SMOOTH wide response, than FLAT response that fall off at the high
end.
Depending on the ability of your machine, you MAY have to make the high
frequency record alignment adjustments at a lower level. Because of the
high freqeuncy pre-emphasis some machines record amps may not take too
kindly at 0db at 20Khz before being overdriven. On the other hand, some
pro machines, Studer in my experience, have absolutely NO problems at
recording at 6db over standard level at 20kHz and above when running at
30ips. The slower the tape speed the more critical this becomes.
Since we have no calibrate the record against the play level, if your
machine has problems in this area we can now change the levels. Turn the
record level down about 10db. Turn the playback up so that your output at
1kHz is zero, and then proceed to do the HF record EQ as this level.
If your machine requires this you should also make sure that you are
carefull not to "go into the red" when recording on this machine.
One point I disagree with in most the procedures I have seen, is
that many say to touch up the alignment (mechanically) after this
you have completed these steps.
I have seen that you can change the physical alignment of the
playback head or record head at this point and further peak the
output (or bring the phase close if monitoring phase).
However, since you have aligned with a stock tape originally I
disagree with this. (And this is my own method that we used at the
studio - and it seems to work. However I have not had the time or
opportunity to test this theory of mine so I may be way off base).
After all this check the phase response at 20kHz, and carefully
adjust the bias on one of the record channels to bring this as
close a possible. You may have to touch up rec eq just a bit, and
do this once or twice.
I attribute this phenomenon (In my head at least) to the "bias
bubble". The signal is recorded on the trailing edge of the
record gap, and the amount of bias will affect just exactly where
the trailing edge "seems" to be. This varies with the frequency being
recorded. Changing the bias while watching the HF phase will show that
this does affect the phase response.
EQ doesn't enter into it. Changing bias will affect the HF record
response however, but this should only be level sensitive.
The only thing I can think of that would account for this is the
slight displacement in the track caused by bias.
(As I say - this is my own "theory" and I have never had the
time/resources to check it thoroughly - so I may be way off base,
but record/play sounds really great done that way. I guess all of
us a permitted to have at least one ecentricity :-) ).
After you have done the final hf rec eq, you do the lf pb eq. Never
to do the LF playback eq from the alignment tape. Except for a
very few machines, you have NO control over the low frequency
record characteristics of your machine and you want to align your
lf playback to your lf record.
Doing the above on an ATR102 - I could get the machine flat within
+- 1db from about 30Hz to 20kHz. On a Studer A-800 at 30ips, I
could get within about +- 1db from 30Hz to well past 20kHz. The
Studer 3db down point was at 33kHz. (On hell of a machine!)
And if you align your machine very carefully you will hear great
differences between tape brands/types.
I have not had a chance to hear the new Scotch high output tapes,
but of the rest of the tapes these would be my choices.
For such things as strings, but without any high level peaks,
Scotch 250. Wonderfully quiet. Has more print than I'd like.
That's why I stay away from things with large peaks or big endings.
You will get echo on the first playback. Very little modulation noise too.
For voices, horns, accoustically generated music, Scotch 226.
Doesn't print like 250, very low modulation noise, minimal asperity
noise. Good tape.
For pop/rock. (Things that don't have big gaping holes (rest) or
low level vocal tracks) Ampex 456. Seem a bit "brighter" or
"harder" than the Scotch tapes. Great rock'n'roll tape. But tends
to have more asperity noise (but only noticeable in quiet
passages).
All the tapes measure the same (in frequency respone on a give
machine) but all have slightly different sound characteristics.
(For those who are not familiar with the term "asperity noise",
this is a low frequency noise component. We called it "rocks" in
the studio because it is what you would imagine large -really
large- boulders to sound like hitting one another. This is cause
by slight uneven-ness in the oxide coating. You hear it when you
have such things as a soft vocal group - or soft horns. Virtually
inaudible unless the music has pauses. More noticeable the higher
the recording speed).
And to explode one "myth" here. Worn tape head don't ALWAYS show up by
having a degraded HF response. In pro machines the head have a very deep
gap. As the head wears the depth of the metal is less while the gap is
the same width. The tip-off here is that you INCREASED HF output.
I found this out when fighting a problem on an Ampex MR-70 and Ampex tech
support pointed this out to me.
So while the above procedures may not always follow the book, they are the
ones we adopted in the studio and the machines always sounded good.
A lot of this came from trial and error, but to give credit where it is
due, some of the best information I have received working in the business
came from conversations with John Stevens, who built an amazing tape
recorder, John French, of JRF in New Jersey, who remanufactures and builds
magnetic tape heads, and Gordon McKnight, of Magnetic Reference Laboratories,
who make MRL alignment tapes.
* Sel-Sync is a Trademark of Ampex
--
Bill Vermillion - UUCP: uunet!tarpit!bilver!bill
: bi...@bilver.UUCP
------------ cut here -----------
--
Bill Vermillion - bi...@bilver.uucp OR bi...@bilver.oau.org