DAT Tapes and Tips
DAT Recorders Tapes & Tips
by Bruce Bartlett,
with additional material from Art Munson and Mike Metlay
© 1994 Deltamedia
DAT RECORDERS AND DAT TAPE
The DAT (Digital Audio Tape) recorder is a great format to work in. It delivers CD-quality sound (or better) in a small, tape-based format. And, since DAT has become a mastering and audio backup standard, it's easy to exchange tapes with other studios and musicians.
The older-style analog tape recorders degrade the audio they record. They add tape hiss, tape distortion, frequency response errors, and speed variations (wow and flutter). Digital recorders get rid of these problems. Thanks to digital recording, the original goal of tape recording has finally been achieved: to accurately store and reproduce our audio creations.
In this booklet we'll give a brief overview of digital recording, DAT recorders, DAT tape, and digital editing.
Like an analog tape recorder, a digital recorder (such as a DAT machine) records audio on magnetic tape, but in a different way. Here's what happens.
- The signal from your mixer is run through a lowpass filter (anti- aliasing filter) which removes all frequencies above 20 kHz.
- Next, the filtered signal passes through an analog-to-digital (A/D) converter. This converter measures the voltage of the audio waveform several thousand times a second.
- Each time the waveform is measured, a binary number (made of 1's and 0's) is generated that represents the voltage of the waveform at the instant it is measured. Each 1 and 0 is called a bit, which stands for binary digit.
- These binary numbers are stored magnetically on tape in the form of a modulated square wave recorded at maximum level.
The playback process is the reverse:
- The binary numbers are read from tape.
- The digital-to-analog (D/A) converter translates the numbers back into an analog signal made of voltage steps.
- An anti-imaging filter (lowpass filter) smooths the steps in the analog signal, resulting in the original analog signal.
Since the digital playback head reads only two binary numbers, it is insensitive to tape hiss and tape distortion. Numbers are read into a buffer memory and read out at a constant rate, eliminating speed variations.
The resulting freedom from noise, distortion, print- through, wow, and flutter makes digital recordings sound extremely clean and clear. Unlike analog recordings, digital recordings can be copied with little or no degradation in quality. Lost data is restored by error-correction circuitry.
As we said, the audio signal is measured several thousand times a second to generate a string of binary numbers. The longer each binary number is (the more bits it has), the greater the accuracy of the measurement. In other words, short binary numbers provide poor resolution of the waveform's amplitude or voltage; long binary numbers provide good resolution. A quantization of 16 bits is considered to be adequate for high-fidelity reproduction. It is the current standard for digital tape recording and compact discs.
The rate at which the waveform is measured is called the sampling rate, measured in samples/sec. At a sampling rate of 48 kHz, 48,000 measurements are generated for each second of sound.
The higher the sampling rate, the wider the frequency response of the recording. The upper frequency limit is slightly less than half the sampling rate. If the sampling rate is 44.1 kHz, this is adequate for high-fidelity reproduction up to 20 kHz.
Currently, open-reel digital recorders are quite expensive. For stereo mastering, there's an alternative to an open-reel digital recorder: a DAT or R-DAT (Rotating-head Digital Audio Tape) Recorder. Costing $700 and up, a DAT recorder records audio digitally on a small cassette. Its sound quality is at least as good as a compact disc. What you put in, you get out, virtually without any added hiss, distortion, or wow & flutter.
The DAT transport is very fast. A two-hour tape winds in about 45 seconds. The DAT machine records on a small cassette about half the size of a standard analog cassette. The shell has a hinged door that flips open to expose the tape.
When you load the cassette, two spindles enter the two hubs from below the cassette and lock it into place. Then the tape is drawn into the machine and wraps around a rotating drum, which holds the record and playback heads.
As the tape moves past the rotating heads, they scan diagonally across the tape as they write the digital signal. The tracks laid down by the rotating heads are about one-tenth the thickness of a human hair! Although the linear tape speed is slow, the tape speed as seen by the record head is very high, so the ultrasonic digital signal can be recorded. Tape drop-outs can cause data losses, but most of these are restored by a circuit that corrects the errors.
While the audio program is recorded, absolute time is written on tape. It's the running time in hours, minutes, and seconds, where 00:00:00 is at the head of the tape. You use it to locate selections and time them.
As we said, a DAT cassette is about half the size of a standard analog cassette, measuring 73 x 54 x 10.5 mm. Its tape is 3.81 millimeters (1/8") wide. On one end of the shell is a hinged door that flips open to expose the tape so it can be drawn around the rotating head drum.
DAT tapes can record up to two hours of audio on a tape about 60 meters long. They come in various lengths. Blank tapes use a metal-powder oxide, while prerecorded tapes use a barium-ferrite oxide. The cassette includes a sliding safety tab on one end to prevent accidental erasure. If the little door is closed, you can record on the tape. If it's open, you can't.
Computer-grade DAT tapes are more reliable than audio-grade DAT tapes. They were originally designed for computer backup systems but make an excellent audio recording tape. They use a different formulation, run cleaner are easier on your heads and are certified error free.
Subcodes are information written on tape, independent of the audio signal, that tells the machine the number of each selection or program, where each selection starts, and whether or not to play each one. You can record or erase subcodes without affecting the audio program. The three main types of subcodes are Program Numbers, Start IDs, and Skip IDs.
- Program Numbers are assigned to selections in order, and can then be used to locate them.
- A Start ID marks the beginning of each selection. It can be written manually or automatically. Manual Start IDs can be placed anywhere, say, to mark cues. Automatic Start IDs are put on tape whenever there is enough signal applied, after a silence of three or more seconds. You'll want to record manual Start IDs after recording all your mixes onto a DAT tape.
Like other subcodes, Start IDs can be recorded or erased without altering the audio program. You can enter them during recording or playback. If the cassette's safety tab is set to prevent accidental erasure, you can't record or erase subcodes. Most DAT machines can automatically re-number the Start IDs in consecutive order.
- A Skip ID makes the machine skip the selection. This ID can only be written manually. Whenever the machine senses a Skip ID during playback, it stops and fast winds to the next Start ID, and begins playing. This function can be turned on and off.
A DAT has conventional cassette transport controls, plus a number of unique features, indicators, and controls. Some DAT machines include these features:
- Search: When you enable the search function, the machine fast winds to the selected program number.
- Memory Rewind or Return to Zero: When enabled, this function rewinds the tape to a preset "0" position on the tape counter. * SMPTE time code: This is available on a few models, digitally encoded as part of the subcode. It's used to synchronize the DAT audio program with a video tape or a MIDI sequencer.
- Mic inputs: These let you plug two mics directly into the DAT recorder. Generally the sound quality of the DAT mic preamps is not as good as an external mixer or mic preamp. You have a choice of pro-type balanced XLR connectors or consumer-type unbalanced phone jacks.
- Digital inputs and outputs: These let you make digital copies with no A/D-D/A conversion required. A copy made from a digital in/out is a clone of the original recording, with no loss in sound quality. There are several types of connectors available: AES/EBU or SDIF-2: Pro type with 3-pin XLR connectors. IEC 958 or SPDIF: Consumer type with RCA phono jacks. Optical (EIAJ CP-340): Pro or consumer; highest quality data transfer.
- Confidence head playback: Some units have an extra head so you can hear the tape playback as you're recording.
Other features might include a built-in monitor speaker, a time- clock/calendar, phantom power for condenser microphones, input attenuator to prevent overload, and a stereo microphone. Some DAT recorders are very small and portable, and they run off batteries as well as AC.
The DAT recorder has an LED or LCD display window which shows various aspects of the recording.
- Time: Absolute time, remaining time on the tape, or elapsed time for the current selection.
- Meters: Peak-reading bargraph meters that show recording level. Unlike with an analog tape deck, "0" on the meter is absolute maximum recording level.
- Error: If this indicator flashes, there was a loss of data. Usually the electronics can correct for this loss. Some units show the error rate. * Sampling rate: The frequency at which the machine is sampling audio signals, such as 44.1 kHz. This indicator works both during recording and playback.
- Subcodes: Program number, Start ID, and Skip ID.
- Search mode: This shows whether the search mode is activated. CONTROLS
- Sampling Frequency: This is the rate at which the DAT's A-to-D convertor samples or measures the analog waveform during recording. On most models you can switch between 48 kHz and 44.1 kHz. Some units can also record at 32 kHz.
A rate of 44.1 kHz is preferred if your tapes will be duplicated on compact disc, because no sample-rate conversion is needed. 48 kHz gives slightly better sound quality. The mastering engineer can avoid sampling- rate conversion by using the DAT's analog outputs. A 32 kHz sampling rate provides either longer playing time or four channels.
During playback, a DAT recorder senses the sampling rate and automatically switches to the correct rate.
* Analog/digital input selector: This chooses between analog or digital input signals.
* Emphasis: Used in older consumer DAT recorders, this boosts high frequencies during recording and turns them down during playback. The result is lower noise. If a DAT tape was recorded with emphasis, the DAT machine detects it during playback and switches in de-emphasis automatically.
* Copy protect: This puts a code on tape that prevents the tape from being digitally copied. A DAT deck will identify data that has been recorded with a copy-inhibit flag in its subcode, and will not digitally copy that recording. Be sure copy inhibit is OFF if you want to duplicate your DAT master digitally!
Consumer DAT machines have a copy inhibit system called SCMS (Serial Copy Management System). This feature lets you make a digital-to-digital copy, but prevents regenerations from that copy. That is, you can't make digital copies of the copy.
EDITING DAT TAPES
You can't edit DAT tape by splicing it because the tracks won't align perfectly after the splice, and you may remove some data. Instead, you can copy from one DAT machine to another, changing the order of selections during the copying process. Tight edits are impossible unless you use a digital-audio editing system. It includes a circuit card you plug into your home computer, an input/output box, and editing software. To use it, you play your DAT tape, record it on hard disk, use a mouse and computer screen to edit the selections, and record a new edited DAT tape off the hard disk. When you edit the program, you remove noises and count-offs just before and after each song, and add several seconds of silence between songs. You can even do crossfades and fadeouts.
Editing systems cost about $1500 and up, not including the hard disk. You need about 11 megabytes per minute of stereo program. A 660 megabyte hard drive will record about an hour.
This list of DAT-user's tips was gleaned from Art Munson's own experience and from a TASCAM User Group magazine article whose author was, sadly, left uncredited there. I've retyped it, reordered it, and shortened some of the entries, to make a listing suitable for putting up on a wall where it's easy to see in your studio. Most folks will know some or all of these already, but they serve as a handy reminder every time you use your DAT machine. Enjoy, and happy DATting!
Mike Metlay - Atomic City
THE TWENTY COMMANDMENTS TO THE DAT USER
by Art Munson of Deltamedia and the TASCAM User Group
rewrites by Mike Metlay of Atomic City
THE TEN COMMANDMENTS OF DAT USE
- When using a DAT for the first time, fast forward it to the end and rewind it, to repack the tape on the reels and ensure smooth transport.
- Start every tape with one or two minutes of recorded silence, and don't go to the end of a tape either. Error rates are much higher at the ends.
- Avoid SCMS if at all possible, by recording to pro decks and dubbing from consumer decks to pro decks rather than vice versa.
- Record at 44.1 kHz if you think your tapes will be moved to CD; sample rate conversion is costly and can introduce artifacts into the sound.
- The 0 VU mark is more critical on DAT than on analog tape, but if your only overloads are very brief percussive transients, let your ears judge.
- Use Cue and Review only when Fast Forward and Rewind won't do your job for you at all; you'll save head and tape wear, which can add up fast.
- Ditto the Pause function; use Stop if possible, and reserve Pause for time-critical applications like punch-ins for building archive tapes.
- Don't re-record multiple takes on the same part of a DAT; record them in order, and pick the best one later. It's much better for the tape.
- Listen to every track after recording to be sure there are no glitches; this inspection can be done while recording ONLY with a 4 head recorder.
- Never leave a DAT (or any other kind of recording tape) in a machine when you power it off. It's bad for the tape, and jams transports.
THE TEN COMMANDMENTS OF DAT UPKEEP
- Make backups or suffer the consequences. DAT audio is digital data, and can be corrupted. Use a second DAT to make digital copies; beware SCMS.
- If you don't have a second DAT, many hard-disk recording systems have features that allow for backup to DAT, and you can use them for transfers.
- For critical material, have rotating backups. Use write-protect tabs to differentiate between the latest backup (protected) and older ones (not).
- Label your DATs with track indexes, titles, times, sources, and sampling rates. Keep a notebook if you can. It's easy to lose track of songs.
- When you label your DATs, date them by first use. Tapes that are five years old should probably be digitally copied to preserve the material.
- When you label your DATs, use felt-tip pens only. Ball-point pen ink and pencil graphite leave solid residues that can fall into transports.
- Store your DATs in a cool, dry, dust-free environment. DAT tape is metal; it'll rust if you store it in a humid spot.
- Store and use your DATs away from strong magnetic fields. They're somewhat sturdier in this regard than analog tape, but they're not indestructible.
- Clean your DAT deck (when? watch error rates, or read the manual). If you don't know how to PROPERLY clean by hand, use a dry cleaning cassette.
- Keep updated on software updates and other revisions to your DAT machines; new features are rare but sometimes performance can be improved.
We hope that this booklet has been of some help in your exploration of the DAT world. We would like to thank Bruce and Mike for their excellent contributions, without whose effort this booklet would not have been possible. If you have any comments or suggestion we would be happy to hear them.