A deep dive into analog tape for mastering
Question:
“I've been experimenting with different gain staging into the tape deck input. Right now, I've settled on -14dbfs=0VU. What should I consider in picking this reference level, taking into consideration I'm running +9 tape and have the playback level calibrated to 320nWb/m= -4VU (I've only got a 320 nWb/m MRL at the moment)?
- Nes
Masterdisk Client
Scott’s Answer:
This is a simple question with a very complex answer. Let’s start with a little jargon.
Reference level
Interfacing digital to analog, As a starting point, we use a digital 1k tone set to -14 db full scale (dbfs), and that should be adjusted with your digital to analog converter (DAC) to read 0 VU on your analog meter on the tape deck.
Flux Level
This is a scientific measurement of the amplitude of the magnetic field that the record head is generating. It’s measured in Nano Webers per meter. When we use this to describe the recording density on a tape, we are referring to how large a magnetic field the record head produced when the tape was made. A higher number means more energy was pushed into the tape magnets. But not all of that energy comes back out (tape compression).
Nano Webers Per Meter
A Weber is a unit of measure for a magnetic field. A Nano weber is one 10,000th of a Weber, and the “per meter” is needed because this is a force, and it is applied over a distance to complete the measurement. Yes, this could be explained in a more scientific way, but this is all the audio engineer needs to know.
Headroom
In digital, we consider a full-scale signal to be a 0db full scale and that it has zero headroom. The signal can not go any louder. In analog, when we reference our record level to the VU meter “zero,” that is a voltage alignment. But there is considerable headroom above this point in all professional tape decks. Most consumer decks do not have as much headroom, but there is still 6-10 db of headroom above zero when the deck is working properly.
However, on tape, when you increase the record level to about the zero point, you start to notice that the signal does not come back exactly as loud as you record. In other words, it has been compressed to some point as it was recorded to tape. There are several reasons why. The input electronics can cause some light compression—transformers can cause some compression, as can the tape head itself. But the big “compressor” in the tape machine is the tape itself. At some point, depending on the nature of the music, you will notice that sharply created transients like snare hits come back with the edges rounded off. Some call this warmer or smoother. On projects that have been recorded digitally—in a DAW—this effect, if kept subtle, is very enticing.
Tape Compression
When you push an analog tape with more level, some of the magnets resist the recording and do not respond exactly like the input signal. This is often called tape compression. It’s not as regular sounding as a compressor is normally. Tape compression can include subtle dulling of the high end, reducing transients, slowing down fast digital sounds and gluing the elements of the track together.
Tape compression helps to make the mix sound more human and less digital; it helps take away some harshness and adds some color or vibe to the recording. These effects can be overdone, and at the time you are doing it, it might feel like a good idea. But with time, you will notice more and more negative effects of tape. So the bottom line is to use this effect carefully and subtly. Compression can never be removed from a recording. The rate of compression effect goes up as the level you send to the tape goes up. It’s not as harsh as a peak limiter. But the gentle compression effects you will hear at +6 get more pronounced at +9 and can be very similar to a hard knee limiter at +12.
But what do these plus numbers mean?
The answer is both simple and complex.
We set a reference level by putting an alignment tape on the tape machine and adjusting the reproduced characteristics so that the VU meters on the deck read zero ( or some other number, but this is the reference “zero” that we then use to describe the plus levels.
If your alignment tape is documented at 250 NW/M and you align your tape deck to 0 VU, then your playback zero = 250 NW/M. Then you send a zero VU tone from your console and adjust the record alignments on the tape deck so that the playback of that same tone now reads zero VU. You have aligned your tape deck to 250 NW/M.
Note: it’s very important that you start with the repro t (playback) alignment first, then send a reference tone to the tape machine and then adjust the record alignment controls. Do not readjust the repro controls once you have established a reference level. The record level is set with that reference playback in mind.
So, with your deck now set to record at 250 Nw/M, that is your reference level. If you increase the level of the test tone that you are recording by 2 db ( before it gets to the tape deck), you are now recording at +2 over 250 nw/m. That translates to 320 nw/m, in case you were going to ask.
If you lower the level of the tone to show a -2 VU on the tape deck, then you will be making a tape recording at -2 relative to 250 nw/m, and that roughly translates to a reference flux level of 185 nw/m. The actual number of the flux level is just documentation for reference. What flux level you use is entirely up to you, and I suggest the following exercise so that you can choose how “loud” to record your tape. Keep in mind that the level that you record to tape has absolutely nothing to do with the ultimately mastered loudness of the song. The record level sets the tone and compression of the tape machine.
Exercise
Establish your tape machine alignment, and don’t adjust it for this entire exercise. You will need a DAW, a tape machine and a reel of tape. Clean the tape machine and begin by recording a passage of your choosing so that the average level of the music achieves 0 VU on the tape deck meters. We will refer to this recording as “ zero.”
If you can play and record at the same time with your DAW, capture the signal off the tape and record it back to your DAW, and name that File “Tape Zero” or “Tape 0”.
Now change the output level of your source audio, and again record that signal through the tape deck. Record that back to your DAW and call that “Minus 2”.
Do that again, but increase the output from your DAW (playback) to 2 db louder than the first. You will repeat this process again for +4, +6, +8 and maybe even more until you reach the point where you don’t like the compression that your tape machine is contributing.
With all of these files recorded back to your DAW, Set your master output back to 0, and let’s compare.
Start by factoring out the level differences. One way is to normalize each file, but what I’d like you to see first is what happens when you simply offset the +2, +4 etc., changes from above.
On the file named:
zero Do nothing
-2 Add 2 db
+2 Subtract 2 db
+4 Subtract 4 db
+6 Subtract 6 db
+8 Subtract 8 db
You will immediately notice from the shape and size of the waveform what is happening in the tape. The louder you push the level, the more tape compression happens.
So now, normalize each segment so that they all “touch“ the same loudness peak. You will hear that the more you push the level to tape, the more “loudness” the files achieve. It’s like you are compressing and peak limiting at the same time. And as the level gets louder, there is more automatic gain manipulation.
The important takeaway is this: which tape print level did you like the most? Does your mix really want and need the heavy compression of +9, or does it just need the gentle gain control of the +4?
Some notes
Not all tape responds the same. High-output tape is meant to have elevated levels, and as such, it’s not as sensitive to soft sounds. If your music has a lot of subtle detail, the high-output tapes will tend to wash those details out.
Not all tape machines and not all tapes can withstand +9 or more levels.
If you start to hear audible distortion before you reach +9, there may be some technical reasons, or the tape deck you are using might not be capable of pushing that much level.
¼” tape compresses and distorts at a lower level than ½” tape in general. If you choose to record at ¼”, don’t push the levels as hot unless you really want it to sound smashed.
After doing this exercise, take note of the sweet spot. It might vary with different audio. An acoustic guitar and voice recording might have a sweet spot at +4, but a rock rhythm section might feel good at +8. Since you have the choice, you get to decide. But don’t take another tape or another tape machine for granted. They will perform differently. When you elevate the levels, your mix comes back off the tape machine louder, and it will immediately sound better to you because it is louder. Take the time to capture the print from tape and adjust the level to be comparable with your source. Then make your decisions. We all know what happens when you turn everything up to “11,” even when the maximum is still 10.
Technical
Another way of describing the same thing is that the alignment level sets the relative flux level for zero VU. The actual flux level is determined by how loud your audio is presented to the deck. If 0 VU = 185 nw/m and your audio causes the input meter to read VU +2, then your absolute flux level is +2 over 185, which s 250 nm/w/meter.
Most people don't document the actual audio levels. There is an assumption that you adjust your console to feed the machine at 0 VU. When everything was analog (30 years ago), and every studio had an analog console, you could depend on your mix being at or near 0 VU. But with today’s in-the-box configurations, the output level is determined by the DAC that is in your interface. Often we don’t have control over that, so we route our outputs through a monitor controller. This only makes setting and maintaining standard output levels even more difficult.
My point is you control how “hot” you record by the record level coming from your console or interface.
So your related question is, “How hot is too hot?” That depends on the input electronics and heads in your tape machine, and it depends on the tape formulation you are using. In general, use a higher record level when you are looking for a significant tape compression effect and use lower levels when you are looking for a more transparent transfer. Even on high-output tape, there is considerable tape compression.
These days, I have seen mixers put on a lot of tape compression and not realize how much because the meters are offset with the high MRL level tape.
See the following charts:
Magnetic flux Φ | Flux level LΦ | | Magnetic flux Φ | Flux level LΦ |
520 nWb/m | +8.98 dB | | 520 nWb/m | +6.36 dB |
370 nWb/m | +6.02 dB | | 370 nWb/m | +3.41 dB |
320 nWb/m | +4.76 dB | | 320 nWb/m | +2.14 dB |
250 nWb/m | +2.62 dB | | 250 nWb/m | 0 dB |
220 nWb/m | +1.51 dB | | 220 nWb/m | −1.11 dB |
200 nWb/m | +0.68 dB | | 200 nWb/m | −1.94 dB |
185 nWb/m | 0 dB | | 185 nWb/m | −2.62 dB |
150 nWb/m | −1.82 dB | | 150 nWb/m | −4.44 dB |
Traditionally, engineers called 185 zero. But as tapes got more tolerant of level, some moved to 250 as reference. You have a 320 nw/m tape aligned to -4, which is almost the same as a 185 nw/m tape alignment.
Note: modern high-output tapes are less sensitive to low levels. It sounds backward, but if you have highly sensitive music to record, choose a tape that is optimized for lower level recording. The high-output tapes are less sensitive to low levels and require more energy (magnetic energy) to sound good. They live in the compression realm and are intended to be used at higher record levels.
On high output tape, if you record at 185, you might notice it sounds quite warm, as in the highs are softened on attacks, and you might be missing some detail in quiet sections.
High-output tape with higher levels sounds more like a rock mix—like an SSL bus compressor was on the buss, and the top end comes back a little harder. Top end is good, and depth is good.
For highly detailed mixes, I would suggest using a moderate output level tape and hitting it somewhere around 200-250 nw/m for no compression or up to 320 for some compression effect. High-output tapes must be hit hard to hear compression, and low-output tapes can be hit lightly to hear gentle compression. I think many people confuse these.
Summary
You do not need to re-align to hear the differences in record level. You just have to offset the files accordingly before they hit the tape. Level match and use your eyes (DAW) and ears coming off the tape until you find the level that matches the needs of your mix.
Print the same section to tape (e.g., verse/chorus) at your reference level, then at 2 db louder and then at 4 db louder. Ignore the meters for this test unless you are overdriving the record section of the tape machine. In that case, you will have to re-align. But from 185, you should be able to print at +4 or more without distorting. That ultimately depends on the tape deck.
Use your ears and adjust to taste.
When you have settled on a record level, re-align from the top and re-print the tones so that that average level causes a nominal 0 reading in the range from -2 to +2.
The goal of the tones is so that someone else—say, a mastering engineer—can take that tape and align their tape machine to play back at the same level and frequency response that you recorded it at. But that only works if you re-print your tones when you re-aline your deck.
See? Clear as Iron Oxide (which is not clear at all).