Hi Scott,
Great score- with a little bit of care and learning, you'll soon be recording to tape with great results.
The thing to remember with tape is that it is
not a "plug and play" recording system! As soundguy mentioned above, you will need to calibrate your machine often, and do other preventative maintenance such as cleaning, lubrication and demagnetization.
If you are totally new to using and handling tape, it's worth starting your learning now. Build up a folder of useful information and read it. Here's something for starters:
Ampex Basic Tape Concepts PDF 4.4MB
Unlike using a DAW, it's important that the user has a
basic grasp of what's happening when you record. Tape is what I call a "dynamic" medium, inasmuch as the recorded sound varies depending on the amplitude, frequency, dynamic range and bandwidth of the sound being recorded. What you put in is rarely what you get out- which is why it is such a creative tool.
Essential equipment-
Cotton buds -for cleaning
Isopropyl alcohol -for head and tapepath cleaning.
Not for use on rubber parts.
Sweepable and amplitude adjustable test oscillator- sinewave output.
Accurate audio mV meter
or a decent VU meter.
The manual for the machine you're using.
Okay, looks like a big investment, but tape
isn't the cheap option anymore! Compare the price of a 200GB HD to a roll of 2" GP9!!
Instead of a test oscillator, you can record sines into your DAW at 0VU levels at different frequencies. Or use the oscillator in your console. For an accurate external VU, you can use the consoles VU meters- just line up the path from a channel input to the monitor VU for 0dB gain.
Steve Albini did this great post on lining up a tape machine to get it "workable" without the manual. I can't find it on line, so here's a Word doc I made up of it:
Albini Lineup Notes
There is always a confusion in levels with tape machines. There are two important calibrations that need to be done- internal and external. The External calibration is usually done first. In a studio you need to define a standard level. This is basically so that all the equipment in the studio can be interconnected in any order without any distortion or overload. In a "traditional" studio the VU meter is used as an indicator of levels.
When the VU meter is reading "0VU", traditionally the voltage across the measured terminals is 1.23V, or "+4dBm" (0dBm is 1mW in 600R which is 0.775V- if you don't know dB values download
this table, print it out, and keep looking at it until you remember the most useful numbers :wink: )
Often a tape machine is in a separate room, to keep its transport noise away from the recording area, and to keep it safe from chainsmoking engineers. Because of this you have no way of looking at the machines VU meters.
By alligning the tape machines input and output amplifiers to the same reference level as the rest of your studio equipment, when you patch the tape machines input to an output bus on your console, you know for certain that the "0VU" you see on your console bus VU will be the same 0VU on the machine. And when you go into repro, you know that the 0VU you see at the consoles channel input is the same as the tape machine is putting out. This reference level is really important, and makes sure that you are not overloading the tape machines input electronics, and converesely that you are not sending too low a signal. If the signal is too low, then the tape is not used efficiently, and your signal will have a poorer signal-to-noise ratio (i.e. the tapes inherent background noise becomes higher) Once calibrated you can switch the tape machine to monitor the input signal (essential for when doing a drop in etc) and the output signal pass through the machine and back to the console at exactly the same level, i.e. the direct path through the tape machine is 0dB!
:thumb:
Snip> Big component order just arrived- have to check it first- will continue later!
Back again...all good.
The second calibration that needs to be done is the internal calibration. Once you've defined your external operating levels, it means that you're getting a nice healthy level into the machine, and that the machines input and output electronics is working at optimum- there will be enough headroom for peaks, and noise won't be a problem.
The internal calibration is based on the record and repro amplifiers. The repro amplifier is calibrated first, and this is usually where any confusion takes place. The first thing to do is download this article from mrl:
Choosing and Using Calibration Tapes PDF
Have a quick read through, but don't worry too much about the technical details yet.
Okay, there are many different tape formulations on the market, and each one has a specific optimal recording level. As the technology of tape advanced, the magnetic flux level that you could successfully record and reproduce with the tape without distortion increased. Check out the left-hand table on page 5 of the MRL document.
Have you got any documentation with the alignment tape? You need to know what its
reference flux level is. This is usually written on the side of the box, or a serious sounding dude will pronounce it at the start of the tape
"...this is a blah blah blah alignment tape, recorded at 355nWb/m..." etc.
The 456 tape you have will be optimal for use at 355 nWb/m reference flux level, so if your cal tape is 355nW/m then you have no trouble at all. I am a bit concerned that the cal tape you have may be a bit old- it's essential that it is in good condition, and that it isn't worn. If it is damaged, it'll be very difficult to align the machine accurately.
Okay, a bit of background about what is happening when you do a "repro alignment". Basically, when a tape is recorded, a varying magnetic field is directed at the moving tape by the record head. The magnetic particles on the tape can be thought of as millions of tiny bar magnets arranged randomly. These can be "moved" by the magnetic field from the head, and they arrange themselves to correspond with the varying field.
(this is similar to making a magnet from an iron nail- if you "stroke" a nail from end to end with a permanent magnet, after a while the nail becomes magnetic, because the permanent magnet has begun to arrange all the tiny magnets in the nail in the same direction)
Then, when you go into repro, the repro head (which is basically the same as the record head- a coil of wire on a magnetically "soft" metal former) touches the tape and the changes in magnetism are induced into the metal former and these are inductively coupled into the coil. These variations correspond to the original variations when the signal was recorded.
Now, there is only so much signal that can be recorded onto the tape before saturation takes place- i.e. all the magnetic "domains" get used up. To get a quality recording, you need to pitch your average level at such a point as to take advantage of this optimal point (see MRL doc, page 11)
A standard calibration tape allows you to set the repro head amplifier to amplify this recorded flux at the correct level, so that the following electronics can "see" enough of the low-level signal, but also so that they don't distort when a higher level comes in. It also means that a tape recorded at another studio on another machine has a good chance of playing well on your machine (as long as someone has been kind enough to write down their reference fluxivity, or supplied suitable tones at head or tail of tape :roll: )
So a cal tape has tones recorded on it at different frequencies at standard recognised flux levels. So when played back on any machine, it will
always present the head with the same magnetic flux level. This is how one studio on one side of the world can send a master to one on another side of the world and still have compatability (to a point...)
Now, the reference tape has its level measured in nWb/m ("nano Webers per metre") and is a measure of the actual magnetic field. In the tape machine, we're more interested in actual signal level. So if for arguments sake you have a 355nWb/m tape, and you want to use a reel of 456 tape on a non-Dolby machine, then you would do the following (presuming you have done the "external" cal beforehand...)
1. With the machine switched off, demagnetise the heads (
MRL demag PDF)
2. Clean the heads and transport thoroughly
3. Switch on, and allow the machine to stabilise for a least an hour.
4. Ensure that the machine is "safe" in all channels- i.e. none are armed for record- cal tapes are expensive :shock: . Run the tape in repro and find the 1kHz section. Play the tape, and switch any metering to "repro". Observe all the VU meters. They
should all be pointing in roughly the same direction. This will be whatever they were set to last. If they look low, then the machine was obvioulsy set up to run a higher-flux tape than the cal tape you're using. If they appear high or a re "pegging", then the machine has been set to a
lower level flux than the cal tape you're now running,
5. Assuming a 355nWb/m cal tape, and a 355nWb/m required operating level i.e. 456), set all the "repro level" controls to read 0VU on the channel meters. At this point you'll realise why we did the external cal first- we now know that 0VU on the machines VU's are corresponding to your studios "0VU" reference level. Go through all the channels, and get them to read 0VU
6. The machine is now playing back all channels the same level- well done! If you need to use a different reference fluxivity, and you're stuck with a "wrong" cal tape, use the table on p7 of the MRL guide, and add/subtract from the cal tape reference to find out how many dB to add/subtract to the VU level. This, I feel, is the biggest confusion in lineup.
Okay, that's the simple bit. The next bit is a bit more interesting. The process of converting the magnetic signal from the tape to an electrical signal at the output of the repro preamp is not a simple or linear process. The tape head can have level anomalies at different frequencies amongst other things, so the repro amp has an
equalisation feature. This is similar to standard EQ in theory, but it is designed to enable you to get a flat (linear) response to all frequencies from the recorded tape.
So the cal tape has different frequency tones recorded at the
same flux level to help you set these EQ's. The 1kHz tone is the reference mid-band level, this has allowed you to set the basic level from tape. There is an LF and HF control associated with the repro amp, and you have to run and LF tone of 50 to 100Hz to set this, and the HF level uses a 10kHz (or higher) tone. You basically run the tape again in the corresponding region (100Hz or 10k etc) and set the trimmer so that the VU meter needle is at the same spot as the 1kHz cal tone.
Okay, that's enough for now...
:grin:
Lots to read here:
http://www.flash.net/~mrltapes/
Mark
