Tape Care and Handling
Courtesy of Quantegy, Inc.
For all the sophisticated technology and electronics that contribute to producing a magnetic recording the entire process ultimately depends on Magnetic Tape.
That's why it's so important to exercise proper care when handling and storing magnetic tape in both open-reel and cassette configurations.
Follow the DOs and DON'Ts listed below to prevent damage to tape and cassette mechanisms and maximize their life expectancy. This means you won't lose expensive information or wear out costly equipment.
* Always store tape in a dust-proof container when not in use. This protects the tape from dust particles which can cause dropouts.
* Never touch the surface of the tape or the tape pack. Even touching the back of the tape can transfer dirt to the adjacent layer. If touching the tape can't be avoided, use lint-free gloves.
* Avoid smoking or eating in the tape area. Smoke and food particles can contaminate the tape and ashes can cause damage.
* Loose tape ends should not trail on or near the floor. Always use approved hold-down tabs or reel bands, even when the tape is in the container.
* Clean the entire tape path often. Use a lint-free wipe and an approved solvent. Dirt left on the transport can be transferred to the tape and cause problems.
* Handle tape reels only by the hub, Bent flanges should be replaced to avoid tape edge damage.
* Rewind poorly packed tapes.
* Never stack tapes on top of one another. Store tapes vertically so they will be supported by the hub.
* Don't put tapes on top of equipment. This interferes with the equipment's cooling system and exposes the tape to heat and dust.
* The ideal operating and short-term storage environment is 68 deg F (20 deg C) and 45% Relative Humidity.
* The ideal long-term storage environment is 65 deg F (18 deg C) and 40% Relative Humidity.
* Allow a minimum of 8 hours for tapes to condition themselves when brought in from a different environment.
* Never expose tapes to direct sun, hot vehicles, etc.
* Always store and use tapes in the cleanest environment possible away from magnetic fields.
Observing these DOs and DON'Ts will help you get the highest performance and longest life from your magnetic tape.
Small format Digital Audio decks (ADAT & DTRS) have given a lot of power to the project studio owner but with that power comes some added responsibility in the tape care & handling department. By following these few simple guidelines you will be able to spend your life making music instead of trying to recover from lost data.
2) Keep the environment clean. Dust and smoke particles can cause major problems in these high-density formats.
3) A recording environment of 68-70 deg F (20-21 deg C) and 40 to 50 percent relative humidity is ideal.
4) Allow the tape time to acclimatize to the recording environment prior to use.
5) Fast forward and rewind the tape twice before formatting.
6) Always format the tape prior to the recording session for best results.
7) Do not eject a tape in the program area. Always fast forward or rewind to an unused portion of tape or to an end of the tape.
8) Exercising the tape end to end prior to every use is always good practice.
9) Always store the tape in a proper container when not in the recorder to protect it from dust and damage.
10) Never open the cassette or touch the tape. Do not wind tapes by hand.
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Email: mcmix@mcmix.com
Digital recording is simple, all you do is peak to 0 dB and never go over! And things remain that simple until you discover one DAT machine that says a tape peaks to -1 dB while another machine shows an OVER level, yet your workstation tells you it just reaches 0 dB! This article will explore concepts of the digital OVER, machine meters, loudness, and take a fresh look at the common practices of dubbing and level calibration.
DAT recorder manufacturers pack a lot in a little box, often compromising on meter design to cut costs. A few machines' meters are driven from analog circuitry, a definite source of inaccuracy. Even manufacturers who drive their meters digitally (by the values of the sample numbers) cut costs by putting large gaps on the meter scale (avoiding costly illuminated segments). As a result, there may be a -3 point and a 0 dB point, with a big no man's land in between. And the manufacturer may feel he's doing you a favor by making the meter read 0 even if the actual level is between -1 and 0, or by setting the threshhold of the OVER indicator inaccurately or too conservatively (long before an OVER actually occurs). But even if the meter has a segment at every decibel, on playback, the machine can't tell the difference between a level of 0 dBFS (FS = Full Scale) and an OVER. Distinguishing between these two requires intelligence that I've never seen on a DAT machine or a typical DAW. I would question the machine's manufacturer if the OVER indicator lights on playback; it's probably a simple 0 dB detector rather than an OVER indicator.
There's only one way around this problem. Get a calibrated digital meter. Every studio should have one or two. There are lots of choices, from Dorrough, Mytek, NTT, Sony, and others, each with unique features (including custom decay times and meter scales), but all the good meters agree on one thing: the definition of the highest measured digital audio level. A true digital audio meter reads the numeric code of the digital audio, and converts that to an accurate reading. A good digital audio meter can also distinguish between 0 dBFS and an OVER.
The Paradox of the Digital OVER
If digital levels cannot exceed 0 dB (by definition, there's nothing higher), then how can a digital signal go OVER?
One way a signal can go OVER is during recording from an analog source. Of course the digitally encoded level cannot exceed 0 dBFS, but a level sensor in an A/D converter causes the OVER indicator to illuminate if the analog level is greater than the voltage equivalent to 0 dBFS. If the recordist does not reduce the analog record level, then a maximum level of 0 dB will be recorded for the duration of the overload, producing a nicely distorted square wave. There is a simple (digital) way of detecting if an OVER had occurred, even on playback‹by looking for consecutive samples at 0 dB, which is a square wave. A specialized digital meter determines an OVER by counting the number of samples in a row at 0 dB. The Sony 1630 OVER standard is three samples, because it's fair to assume that the analog audio level must have exceeded 0 dB somewhere between sample number one and three. Three samples is a very conservative standard most authorities consider distortion lasting only 33 microseconds (three samples at 44.1 KHz) to be inaudible. Manufacturers of digital meters often provide a choice of setting the OVER threshold to 4, 5, or 6 contiguous samples, but in this case it's better to be conservative. Even 6 samples is hard to hear on many types of music, so if you stick with the 3-sample standard, you'll guarantee that virtually all audible OVERs will be nipped in the bud, or at least detected! Once you've used a good digital meter, you'll never want to go back to the built-in kind.
In the diagram below, a positive-going analog signal goes OVER in the area above the dotted line.
Using External A/D Converters or Processors
There is no standard for communicating OVERs on an AES/EBU or S/PDIF line. So if you're using an external A/D converter, the DAT machine's OVER indicator will probably not function properly or at all. I advise ignoring the indicator if it does light up, unless the manufacturer confirms that it's a sample counting OVER indicator. They'll probably reveal that it's an analog-driven level detector. Some external A/D converters do not have OVER indicators, so in this case, there's no substitute for an accurate external meter; without one I would advise not exceeding -1 dB on the DAT machine. I've already received several overloaded tapes which were traced to an external A/D converter that wasn't equipped with an overload indicator.
When making a digital dub through a digital processor you'll find most do not have accurate metering. Equalizer or processor sections can cause OVERs. Contrary to popular belief, an OVER can be generated even if a filter is set for attenuation instead of boost, because filters can ring. Digital processors can also overload internally in a fashion undetectable by a digital meter. Cascaded internal stages may "wrap around" when they overload, without transferring OVERs to the output. In those cases, a digital meter is not a foolproof OVER detector, and there's no substitute for the ear, but a good digital meter will catch most other transgressions. When you hear or detect an overload from a digital processor, try using the processor's digital input attenuator.
Practice Safe Levels
When recording to digital tape from an analog source, if you have an external digital meter set to 3 samples, then trust its OVER indicator and reduce gain slightly if it illuminates during recording. If you've been watching your levels prior to generating the OVER, chances are it will be an inaudible 3 sample OVER. However, if you have to rely on the built-in OVER indicator of a DAT machine, only experience with that machine will tell how accurate it is. With a DAT machine's meter, it may be better not to exceed -1 dB on music peaks. You won't lose any meaningful signal-to-noise ratio, and you'll end up with a cleaner recording, especially when sending it for mastering. At the mastering studio, a tape which is too hot can cause a digital EQ or sample rate converter to overload. There are ways around that, but not without complicating the mastering engineer's life.
Contrary to popular belief, the levels on a digital meter have (almost) nothing to do with loudness. For example, you're doing a direct to two-track recording (some engineers still work that way!) and you've found the perfect mix. Now, keep your hands off the faders, watch the levels to make sure they don't overload, and let the musicians make a perfect take. During take one, the performance reached -4 dB on the meter; and in take two, it reached 0 dB for a brief moment during a snare drum hit. Does that mean that take two is louder? If you answered "both takes are about the same loudness", you're probably right, because in general, the ear responds to average levels, not peak levels when judging loudness. If you raise the master gain of take one by 4 dB so that it, too reaches 0 dBFS, it will now sound 4 dB louder than take two, even though they both now measure the same on the peak meter.
Do not confuse the peak-reading meters on digital recorders with VU meters. Besides having a different scale, a VU meter has a much slower attack time than a digital peak meter. A digital peak meter has one sample (11 microseconds at 44.1 kHz) attack time, while an ANSI standard VU meter takes 300 ms to respond, closer to the response of the ear. But do not rely on the VU meter to judge loudness. The VU meter does not discriminate between low frequency and high frequency sounds, but the ear reacts differently to each. The VU meter can be fooled by different amounts of compression; it is possible to apply compression to an audio signal so that it sounds much louder than the apparent reading on a VU meter. So use your ears, not your meters!
Did you know that an analog and digital recording of the same source sound very different in terms of loudness? Make an analog recording and a digital recording of the same music. Dub the analog recording to digital tape, peaking at 0 dB. The analog dub will sound about 6 dB louder than the all-digital recording! That's a lot. This is because the typical peak-to-average ratio of an analog recording is about 14 dB, compared with as much as 20 dB for an uncompressed digital recording. Analog tape's built-in compressor is a means of getting recordings to sound louder (oops, did I just reveal a secret?). That's why pop producers who record digitally may have to compress or limit to compete with the loudness of their analog counterparts.
Judging Loudness the Right Way
Since the ear is the only judge of loudness, is there any objective way to get a handle on how loud your CD will sound? The first key is to use a single D/A converter to reproduce all your digital sources. That way you can compare your CD in the making against other CDs, in the digital domain. Judge DATs, CDs, workstations, and digital processors through this single converter. Another important tool is a calibrated monitor level control with 1 dB per step settings. In a consistent monitoring environment, you can become familiar with the level settings of the monitor control for many genres of music, and immediately know how far you are (in dB) from your nearest competitor, just by looking at the setting of the monitor knob. At Digital Domain, we log all monitor settings used on a given project, so we can return to the same setting for revisions.
The Moving Average Goes Up and Up...
Some of the latest-model digital processors permit making louder-sounding recordings than ever before. Today's mastering tools could make a nuclear bomb out of yesterday's firecrackers.
Producers today are convinced that they have to make loud records to attract radio program directors. This has inspired a conscious race to be the loudest on the block (repeating the fiasco of the distorted 45 RPM single, for those who remember that far back). I am very concerned about this new loudness race, because there can be no winners, only losers. If we keep compressing sound so each CD sounds louder than its competition, we create an incremental cycle whose end is distortion, whether the music is acoustic-based or electric. Furthermore, this practice doesn't help the consumer, who has to turn his volume control down to get the same apparent loudness out of his speakers; in recent years the loudness race has greatly increased the differences among the existing loudest and softest CDs, forcing the consumer to make extreme changes in his volume control every time he changes discs. And many records which were engineered to sound great on the radio sound like horse manure on home systems (from mid-fi on up). This is madness! Records are supposed to sound great at home!
Here's my recommendation: When mastering, use a calibrated monitor control so you know exactly how loud your CD will be. You can still make a hot CD and get any sound your heart desires (and a good mastering engineer will help make that CD sound incredible). Just try to make the apparent loudness of the musical material fall within, not above the average of other CDs that are out there (in the particular musical genre); while it seems the macho thing to do, you don't have to make your CD louder than the loudest current CD. By doing so, you're only creating a moving target for the next guy to shoot down, which is a disservice to the entire industry.
That concludes our production discussion. This section is intended primarily for the maintenance engineer. Let's talk about alignment of studio audio levels. Stick around for a fresh perspective on level setting in the hybrid analog-digital studio.
Marking Tapes
dBm and dBv do not travel from house to house. These are measurements of voltages expressed in decibels. I once received a 1/4" tape in the mail marked "the level is +4 dBm". +4 dBm is a voltage (it's 1.23 volts, although the "m" stands for milliwatts). The 1/4" tape has no voltage on it, it doesn't have any idea whether it was made with a semi-pro level of 0 VU = -10 dBv or a professional level of +4. Voltages don't travel from house to house, only nanowebers per meter on analog tapes, and dBFS on digital tapes.
That doesn't diminish the importance of the analog reference level you use in-house. It's just irrelevant to the recipient of the tape. Just indicate the magnetic flux level which was used to coordinate with 0 VU. For example, 0 VU=400 nW/m at 1 KHz. Most alignment tapes have tables of common flux levels, where you'll find that 400 nW/M is 6 dB over 200 nW/m. Engineers often abbreviate this on the tape box as +6dB/200.
Deciding On an In-House Analog (voltage) Level
Just use the level provided by your console manufacturer, right? Well, maybe notŠ +4 dBv (reference .775 volts) may be a bad choice of reference level. Let's examine some factors you may not have considered when deciding on an in-house standard analog (voltage) level.
When was the last time you checked the clipping point of your console and outboard gear? Before the advent of inexpensive 8-buss consoles, most professional consoles' clipping points were +24 dBv or higher. A frequent compromise in low-priced console design is to use internal circuits that clip around +20 dBv (7.75 volts). This can be a big impediment to clean audio, especially when cascading stages (how many of those amplifiers are between your source and your multitrack?). In my opinion, to avoid the "solid-state edginess" that plagues a lot of modern equipment, the minimum clip level of every amplifier in your system should be 6 dB above the potential peak level of the music. This means at least +30 dBv (24.5 volts RMS) if 0 VU is +4 dBv.
How Much Headroom is Enough?
Have you noticed that solid-state equipment starts to sound pretty nasty when used near its clip point? All other things being equal, the amplifier with the higher clipping point sounds better, in my opinion. Perhaps that's why tube equipment (with their 300 volt B+ supplies and headroom 30 dB or greater) often has a "good" name and solid state equipment with inadequate power supplies or headroom has a bad name.
Traditionally, the difference between average level and clip point has been called the headroom, but in order to emphasize the need for even more than the traditional amount of headroom, I'll call the space between the peak level of the music and the amplifier clip point a cushion. In the days of analog tape, a 0 VU reference of +4 dBv with a clipping point of +20 dBv provided reasonable amplifier headroom, because musical peak-to-average ratios were reduced to the compression point of the tape, which maxes out at around 14 dB over 0 VU. Instead of clipping, analog tape's gradual saturation curve produces 3rd and 2nd harmonics, much gentler on the ear than the higher order distortions of solid state amplifier clipping.
But it's a different story today, where the peak-to-average ratio of raw, unprocessed digital audio tracks can be 20 dB. Adding 20 dB to a reference of +4 dBv results in +24 dBv, which is beyond the clipping point of many so-called professional pieces of gear, and doesn't leave any room for a cushion. If you adapt an active balanced output to an unbalanced input, the clipping point reduces by 6 dB, so the situation becomes proportionally worse (all those headroom specs have to be reduced by 6 dB if you unbalance an amplifier's output). Be particularly suspicious of consoles that are designed to work at either professional or semi-pro levels. To meet price goals, manufacturers often compromise on headroom in professional mode, making the so-called semi-pro mode sound cleaner! You'll be unpleasantly surprised to discover that many consoles clip at +20 dBv, meaning they should never be using a professional reference level of +4 dBv (headroom of only 16 dB and no cushion). Even if the console clips at +30 dBv (the minimum clipping point I recommend), that only leaves a 6 dB cushion when reproducing music with 20 dB peak-to-average ratio. That's why more and more high-end professional equipment have clipping points as high as +37 dBv (55 volts!). To obtain that specification, an amplifier must use very high output devices and high-voltage power supplies. Translation better sound.
One of the most common mistakes made by digital equipment manufacturers is to assume that, if the digital signal "clips" at 0 dBFS, then it's OK to install a (cheap) analog output stage that would clip at a voltage equivalent to, say, 1 dB higher. This almost guarantees a nasty-sounding DAT recorder, because of the lack of cushion in its analog output section.
To summarize, make sure the clip point of all your analog amplifiers is at least 6 dB (preferably 12 or more dB) above the peak level of analog material that will run in the system. I call this additional headroom the cushion.
How can you increase the cushion in your system, short of junking all your distribution amplifiers and consoles for new ones? One way to solve the problem is to recalibrate all your VU meters.
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Mcmix Production Services Inc.
2878 Jonquil Drive
Smyrna, GA 30080
(770) 436-9620
http://www.mcmix.com
Email: mcmix@mcmix.com
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