Advanced Taper's Guide | | -----Equipment------ |
Test Modes
Error Rate Counters
Dropouts
Channel Loss
1/2 Size Heads
Tapes

### Why do I get dropouts when playing back some tapes ?

Dropouts can occur for several reasons, erratic back tension, dirty or worn heads, tape problems, machine alignment or poor error interpolation. Dirty or worn heads are the most common cause of dropouts followed by machine alignment. The alignment problem can be in the record or playback deck. Tape problems are the second most common cause. Low grade tapes or 90 meter tapes of any quality are the primary culprits here and should be avoided for mastering applications. Erratic back tension is often the cause with low end DAT decks that do not control back tension properly. Erratic back tension can also occur with 90 meter tapes and the 1/2 size heads found in some portable DAT machines.

The digital system allows for low error rates and normal playback sound even when the tape or playback deck is performing marginally. Once the performance of the machine or tape drops below a given threshold, problems become audible. Dropouts can occur with tapes that playback with 0000 error rates, until a problem becomes severe enough to prevent data recovery. It is possible to have a tape playback fine on one deck and dropout when played back on another deck. This could be a result of the tape, the record machine or the playback machine. DAT machines have two basic schemes in place to minimize these problems. They are Error Correction Code and Automatic Track Finding systems.

The DAT error correction code format is the product code type, with two codes for one symbol. The symbols C1 and C2 are know as Reed Solomon Code. C1 interleave is on two adjacent blocks and C2 interleave is one entire track of data every four blocks. One entire track of data must be stored (and readable) for C1 to C2 and C2 to C1 encoding and decoding to be performed. DAT ECC is the same mathematical structure for all DAT machines. The correction of errors is handled in the same way by all DAT machines. The DAT ECC scheme alone, however is not sufficient for error correction in the DAT system. One reason for this is that ECC interleave is complete in one track and PCM data is complete in two tracks. The other is that preformatted media is not supplied and must be formatted on machines with different alignments. Data is double interleaved using inter track interleave and intra track interleave. This interleave technique scatters data over seperate areas on the tape so that a bad spot on the tape does not result in a complete loss of the audio signal. Data is written to tracks on the tape which contain 196 blocks, each block contains 288 bits. Of these 288 bits, 32 are Sync, Main ID, Block Address and Parity (each of these use 8 bits). The remaining 256 bits (or 32 symbols) contain PCM data (music). The same basic structure is used to store sub-code data ( track number, start ID, skip ID, absolute time, etc.). Data recovery requires two scans and two tracks to complete an interleave sequence. Even with proper ECC decoding there is no guarantee that dropouts will not occur. If data dropouts occur consecutively along the length of the tape, up to 22 blocks or 796 symbols can be corrected. Data dropouts of longer duration requires a countermeasure known as error interpolation. Up to 74 blocks or 2664 symbols can be interpolated. It is in error interpolation that manufacturers' schemes differ and produce various levels of sound quality. In theory one head can be lost completely and sound can still be heard without glitches or moments of silence. We have seen the Panasonic DAT machines do this quite often (there is a slight loss of fine detail).

The Sony machines do better error interpolation than the Tascam DAT machines. The exception is the DA20MKII which uses a Sony chip in place of the Pioneer chip (used in the Fostex D5 and Tascam DA20). The Panasonic machines do the best job of error interpolation.

Another factor in playing back tapes without dropouts is tracking, handled by the ATF system. This Automatic Track Finding data is used to sync the drum assembly (head) to the capstan and the reels. This allows for compatibility between machines. The track pitch is 13.6 microns so normal manufacturing variations must be corrected by the servo circuits of the playback deck. This explains the second design challenge imposed by the lack of preformatted media.

The ATF system consist of four signals, F1 (130.67khz), F2 (522.67khz), F3 (784khz) and F4 (1.568mhz). The 130.67 khz signal provides the major ATF function. The F2 and F3 signals are sync signals which allow the servo circuit to control tracking so that inter-track crosstalk is equal. This insures the servo is tracking the proper path or azimuth. F4 is know as the erase signal.

Most manufacturers align the tape path using the RF output waveform. It is set for the most rectangular wave possible. They then set the pilot signal with a separate adjustment. The Panasonic system uses the ATF signals to align the tape path. This is the most reliable method in use and explains why the Panasonic DAT machines recover data so well. If the tape path alignment is out of adjustment or not EXACTLY aligned, you can still get good playback results on most tapes. If the system cannot properly follow the ATF signals, dropouts can occur on tapes that otherwise play with 0000 error rates. This is not due to error correction or interpolation, but rather a momentary loss of sync due to alignment problems.