How to get 'stuff' to work.
Forgotten how to line up the analogue tape machine? Can?t remember how to mix in 5.1? This will be the place to get the info! Please click here to send your tips in!
Thought for the day:
"...Irrespective of whatever lo-fi, vintage or hi-tech digital, hideous and deformed Heath Robinson contraption you lash up, if It /sounds/ good, it /is/ good." - Paul the Prof, 2000.
Analogue Machine Alignment
The following items supplied via CIX Professor and friend:
Professor:
Here's your first tips'n'techniques items:
just found this for you to get started which others may find useful for future reference....
Line - up Procedure: 2" 24 track Tape Machine
Materials + tools:
Tape head demagnetiser.
Plastic 'tweaker'.
2" Test tape.
Oscillator (in desk or separate) capable of 1khz,
4khz, 10khz, 100hz at +4 dBm or less.
True RMS voltmeter (calibrated in dB's).
Frequency counter (optional).
Blank reel of tape (same sort you intend to use - preferably from the same batch).
Procedure:
1). Demagnetise the heads.
Turn off the machine!
Danger! - keep demag away from test tape and other sensitive material - switch on and off well away from everything! Power up the demag andbring it (slowly) up to the heads.
Move it (nearly touching
the head) in a zig-zag from top to bottom and then vertically up the head over the head gap. Move slowly to the next
head and repeat. Now
pull it away slowly and when 3' or more away - unplug.
2). Repro alignment.
Clean the heads. Lace up the test tape, select
the 30ips AES section.
Connect thefrequency counter to the o/p
of any channel of the m/c.
Hit play and listen to the1k tone - if the Freq
counter doesn't say
1k, adjust the speed control until it does! -
(do the same for 15ips -
while you'e at it). Connect the RMS vvm to ch8 o/p.
Play the 1k tone.
I suggest we set the machine to a400nWb/m
operating level, meaning
that a 320nWb/m test tape should read -2dB on
machine meters - or
+2dBm on a true RMS one...If needed - set the azimuth at
this point by
putting all channels into play and pushing all faders up in centre
positionmono. Play the 15k tone. If
you now move the head by
tweaking one of the sidewaysadjusters, you will
see a single peak of
response. This corresponds to in-phase azi
adjustment.
Do the same for the sync head.
Now select play head and play 1k(level), 15k(hf) and
60hz(lf) and set
the response to-2dB on
the vu's (equiv to +2dBM).
Use the True RMS on ch8 to check this.
(We cannot be sure the machine meters are telling the
truth so if we get a channel right by using the true rms, then
we can copy that
setting across and be sure it's right
(assuming the vu's are OK!!).
Select the sync head and do the same.
3). Rec/Rep alignment.
Clean the heads.
Lace up the blank tape.
Put 8 channels into record.
Replay off the repro (play) head.
Using your Oscillator choose 10khz and set an o/p of around 5dB below 0.
Run the tape in record.
Use the tweaker to reduce bias (anti clockwise) -
watch the o/p rise
to a peak and then fall.
Increase bias (clockwise) and keep increasing it past
the point where
the o/p hits a peak and starts to fall.
Take it 'over the hill' 1-2dB (30ips), [3-4dB
(15ips)].(You will need
the true RMSvoltmeter
to do this).
Now put an oscillator input in of 1khz(level) @ 0dB
(i.e. +4dBm) - the
same should come out.
Ditto at 10khz(hf), 4khz(mf)(if you adjust this you
will need to check
10khz again), and finally 100hz(lf) which should
read +1dB (i.e.
+5dBm).(M/c meters ok for this). Now put the next 8
channels into
record and do it all again!! Finally, put all channels
in record and
listen to the o/p's individually - there should
be no excessive or
different noise on any one track. Congratulations!
Your machine is now
lined up and ready to go!
Friend (Rupert Brun) adds:
There's a couple of things I would add to that.
First of all, remember to bypass any noise reduction systems such as
Dolby
before you start!
You can't set the record head azimuth accurately until you have set the
bias. This is because the recording isn't actually made at the head gap
but a very short distance /after/ it, the length of this distance is
dependent (amongst other things) upon the amount of bias used. There is
also some interplay between recording level, hf level and bias. So set
record head azimuth "about right", then set the bias, then
check the
recording level and adjust the hf level. Then go back and set the record
head azimuth again, then set the level and hf and do the bias and
azimuth
again. Go round and round until you don't have to tweak anything.
OK, I hear you say, why not avoid all this interplay between azimuth and
bias by setting the record head azimuth by switching to "sync"
and using
your line-up tape azimuth band? This is OK if and only if your tape
transport is perfect. If it isn't and there is, for example, slightly
more
tension in the tape at the top than at the bottom, or if the head zenith
is a tiny bit out, the wrap of the tape on the head will not be quite
the
same at the top and bottom with the result that the recording zone will
not be quite parallel with the head gap so you will need to set the
azimuth in record. The other problem with setting record head azimuth in
sync mode is that you are setting it to the same reference (the tape) as
the replay head, not setting it to the replay head itself, thereby
introducing twice the error between the record and replay azimuth.
I don't like setting the azimuth by fading up the channels of the desk.
The problem is that setting the azimuth using the inner tracks isn't
accurate enough and setting it using the outer ones it is all too easy
to
end up on the "wrong" peak. Doing all tracks at once is not as
accurate as
setting on the outside tracks, small errors in the outer tracks will be
swamped by the peak from the inner ones, so.....
Most people know that you can use a dual-beam 'scope to set azimuth
(provided you remember to select "Chop" not
"Alternate") or you can use a
Sum / Difference meter such as an M/S PPM.
Play 10KHz Azimuth band of line-up tape, (if using a Sum &
Difference
meter, adjust all tracks for identical level output).
Plug your meter or 'scope across tracks 12 & 13 and adjust for
minimum
difference signal on the meter or matched phase on the 'scope. Then plug
across 10 & 15 and make a finer adjustment. Work your way out
towards the
edge of the tape making smaller and smaller adjustments. Ignore
tracks 1
& 24 because edge damage often leads to wobbly readings. That way
you get
the correct peak and still get the outside tracks (and therefore all the
others) spot on.
Finally, you can't set up the bass frequency response of the replay or
sync amps using the bass tones on the line-up tape. This is because low
frequencies will bleed from parts of the tape above or below the track
you
are trying to set up, made worse by the fact that line-up tapes are
recorded with one single track right across the width of the tape so you
get considerable bleed from where the "guard band" would be on
a normal
recording. This results in more bass energy than there should be so you
turn the replay bass down too much. So you have to line up the replay
amp
bass in record mode. This begs the obvious question - how do you
set the
record amp bass response? I've not yet found a satisfactory
answer, but note that Studer tape machines don't have bass tweaks on the
record amps!
Rupert Brun
Following on from this thread, some input from Peter Keeling:
Re analogue machine alignment - LF playback response.
Most test tapes will have a chart enclosed listing something called
'fringe-error-correction'. This is basically to allow you to reduce the
playback LF setting to accommodate the excess bass apparent on playback
of a full track tape (test tape). Subsequent record LF alignment would
then be carried out flat (since the playback is now correct). Typically
at 30ips, the 63HZ compensation would be 3.3dB.
Having said all that, there are a lot of engineers who would just set it
up flat on Playback anyway!
Peter Keeling
for The Studio people
Some useful tips from Rupert Brun:
Never stick a soldering iron up your nose, it smells bad and will pit the
bit*
On a more serious note, if something isn't working........
Fix the people before you fix the kit... when things go wrong people get
edgy, nervous, look around for someone to blame if they think it might be
finger trouble and so forth. First of all, fix the people then think about
fixing the kit.
Use your ears, eyes, fingers and nose before using any fancy test gear
Look at the last thing you fixed, even in the best run studios a
surprising number of faults are caused by maintenance; this isn't due to
bad maintenance, just due to the fact that things don't like being taken
to bits.
Never trust an engineer who does not talk to the thing he is mending.
I've written several pages of "How to do effective maintenance in a
recording studio" based on 20 years experience but since some of it is, I
understand, about to be published in a book I had better not go giving it
all away for free in here.
From Howard Beck, chief tech at London's Swanyard Studios:
Tech's Tips:
1 - Tech's never get tips!
2 - Never sick your finger in while it's switched on.
Oops! That should have read 'sticking'!
Interesting Tech Audio Links
Tin Foil A site that covers the early years of recording, wax recording etc. Very techy but very interesting. - Not just for nerds!