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Theory of operation - DAT Recorders

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Theory Of Operation



DAT recorders record on a very narrow, slowly moving tape. They achieve the bandwidth necessary for this trick with a rotating head (Actually two heads on a rotating cylinder). Individual bits of data take up a microscopic area of tape; therefore the tape must be treated very gently, and never touched by human hands. The tape is normally hidden inside the plastic cassette out of reach: when it is inserted into the DAT recorder, the case is opened by the mechanism and threaded around the head spool. This is known as loading.



The head will spin whenever tape is loaded, and the tape is always contacting the head, even in fast forward and rewind. If you don't run the tape for a period of ten minutes or so, most decks unload to prevent head wear. This means an extra delay when you press play.

The tape travels at 8.15 mm per second, but the head rotation of 2000 rpm gives an effective speed of 3.15 meters per second. (124 ips).

Data Encoding:
Even at this speed the data has to be processed heavily to allow error free recovery at the other end. Some bit patterns, such as 00010000, would give a very narrow blip in the playback signal that is especially hard to detect. To avoid these, the usual 8 bit data words are recoded as selected 10 bit words, with the difficult ones left out. (This is called ETM for Eight to Ten Modulation. CDs use Eight to Fourteen Modulation.)

Each track (with a rotary head the tracks run across the tape at an angle) is just under an inch long. This is enough for 56448 bits after ETM demodulation. These are divided into 196 blocks of 288 bits. 256 bits of a block are used for data, the others for synchronization and error detection. Within a track, 128 blocks are for audio data and 16 for sub code data (IDs, Time and so forth); the rest are used to precisely control the way the tape moves across the heads.

A single 16 bit sample occupies 0.0003 inches. Naturally, any kind of a hole or dropout on the tape is going to have disastrous consequences. To prevent this, the data is scattered around the tape, a technique known as interleaving. Within each block, data bytes are mixed with parity bytes and error correcting bytes for other blocks. A total is kept of the data and parity, and if they don't match, that block is marked as containing an error. If the damaged region isn't too big (less than 22 blocks) the error correction bytes can be used to completely reconstruct the data. Within a somewhat larger region (74 blocks) interpolation can keep the music going. With more damage than this, the machine usually shuts off.

Error Indicators:
Many decks have a light that flashes when errors are detected. Others have a hidden feature that gives some statistic, like errors per second. Errors are inevitable. Even a calibration tape will have two or three per second, and fresh recordings will read in the 20s or 30s. It is a good idea with a new machine to make a recording on a fresh tape and note the error rate. Then put the tape away. When doubt arises, play the reference tape and see what the error rate is. If it has gone up sharply, clean the heads. If you didn't happen to make a reference tape, you can tell the heads need cleaning when no tape will play properly. If a single tape stops playing, the tape is damaged or simply worn out. They can go surprisingly fast. In fact, enough tapes are unusable right out of the box that most engineers use two DATs when making critical recordings.

Head Cleaning on DATS:
Proper cleaning of a DAT head requires disassembly of the machine and should be done by a qualified technician. Manufacturers used to include abrasive type head cleaners with the machines, but these should be used sparingly (if at all). Only clean when you know it's necessary.

About Time:
The time from the beginning of tape is recorded in every track of the tape as part of the subcode. If you start recording in the middle of a tape, the machine reads the time at that spot and keeps recording time subcode appropriately. If the tape is blank, this won't work. Therefore, every inch of a DAT tape should have something recorded on it. If you want silent spots on the tape, record zeros; either with the REC MUTE or by turning the input down. To find the end of recorded space, simply hit FAST FORWARD. You will be left cued up to the first blank. (Some decks have a special End Search function to do this.)

Start IDs:
Start IDs are also recorded in the subcode data of the tape. They run for about 9 seconds (so they are easy to find in fast motion) and contain a digital code indicating a start point.

On most decks, a start ID is recorded every time you hit the RECORD button and new one will be added if you Pause. Some decks feature an AUTO mode, where a Start is added if the signal stops for a second or so and restarts. Generally, you can record a start ID anywhere you want one, even while playing back. They are also easily removed.

Program Numbers:
A program number is another chunk of data recorded in the subcode. These are the numbers searched for during Previous Play or AMS operations Most of the time the deck includes a program number with each start ID, but not always. ( For instance, there will be no program numbers when you add IDs during play.) To sort out program numbers, most DATs have a RENUMBER function.

Skip IDs:
Skip IDs instruct the deck to find the next start ID. You may write them in Record or Play mode.

End ID:
An End ID marks that spot as the end of tape. The deck will not play or fast forward past the End ID. Adding them is just like adding START IDs. You should avoid having more than one of these on the same tape.

SCMS:
The Serial Copy Management System is designed to enforce copyright protection of some kinds of material. Decks that follow the SCMS standard are locked out of digital recording under certain circumstances. This is accomplished by means of a two bit code known as ID6 and a byte called category code, which identifies the source of a digital input.

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