Can the Media Recognition System information be restored to a Digital Audio Tape?
Moin (Est-Frisean for "Good Morning", "Good Day", "Good Evening"), I have run into trouble with Digital Data Storage tapes.
Every kind of magnetic tape has two points or positions of interest: the load point or Beginning of Media, and the End of Media; everything in between is accessable from the drive.
Before CompactDisks became famous and available, another digital system tried to enter the market: digital audio tape, a down sized version of video tapes at .15"-wide tapes. It was stillborn, because of the price: $500+ in mid-80s. The idea was picked up in the computer world, though: digital audio tape streamers, DDS. So as not to be able to use dat tapes, a media recognition system was intoduced, both on the tape and the drive. The MRS could be disabled on the drive. If you did that, and inserted a DDS tape, the MRS was overwritten, and the tape was rendered write-locked on MRS-enabled drives.
A lot of my tapes are now write-locked because I put them into a MRS-disabled, which I was unaware of at the time, drive, enough to think about MRS.
So here is my question: can this MRS information be restored to a DDS tape?
What could this information be?
Digital tapes are handled a little bit differendly from audio tapes. On audio tapes, a carrier signal is modulated with the audio frequencies (Amplitude modulation), that magnetizes the tape with variing strength. This varying strength magnetization is picked up during playback, amplified and filtered, and results in audible sounds. With two recording/playback assemblies (tracks) you could record stereo sound. And if you turned the tape around, and the tracks were positioned so that now two other tracks were used, you could record in both directions - 4-track. 8-track tape drives, which move only in one direction, move the record/playback assemblies (heads) up and down.
Digital tapes use the same physical setup: carrier, modulation, magnetization, induction, amplification, filtering. Since being digital, the modulation knows two states: up/down, or left/right. They use 7 (old) or 9 tracks. Since the tape moves, one write of one byte (track 9 is vertical parity) results in a rectangle of common magnetization, called a rung. One write of data results in a block of rungs; the minimum number of rungs that can be written in one write is 6, the maximum number (block length) depends on the drive (and system software) used, usually 64K. The - stationery - tape needs to be accelerated to operating speed, then the data is written, and the - moving - tape needs to be decelerated to a stop. This brings gaps about, so a tape is a sequence of blocks and gaps.
But how to mark the end of a file - that could easily consist of hundreds of thousands of blocks. So a special block is used: since it should not be mistaken for data, it could not be of data blocks format. A two rung block - tapemark - of the two ASCII characters ctrl/s (transmitter of) are used, along with special electronics to recognize them at every possible speed and every possible density (rungs per inch or bpi). And that gives an easily recognizeable end-of-data marker: two consecutive tape marks, resulting in an empty file on a read; end-of-media, along with the load point, are signalled by reflective stickers on the backside of the tape.
DDS follows along the same lines. But as the head assembly rotates with high speed and the tape crawls at 120m/2h (play time of tape), no gaps are neccessary, because ac-/deceleration is easily achieved and no stress on the tape, and the tape can be easily back-/forwardspaced, so continous recording is possible, and since the rungs are slanted and thus provide a natural blocking factor, after a backspace, the tape can move forward and hit exactly the next after last rung, avoiding overwriting the last rung and wasting of space.
So, what could this MRS information be, that - intentionally or accedentially - gets overwritten on a MRS-disabled drive? It must be distinguishable from any data and should be recognizable at every density.
How about some tapemarks? Like four tapemarks? With MRS enabled, the drive always skips four tapemarks, so the load point is always after the fourth tape mark, making them save. With MRS disabled, load point is at Beginning-of-Media (BOM), and the four tapemarks get óverwritten on a write to tape. This tape, on a MRS-enabled drive, would, maybe, also have four tapemarks, but not the associated empty files. Reads of a MRS-tape would yield: empty file, empty file, empty file, empty file, non-empty file ... Reads of a nonMRS-tape would yield; non-empty file, non-empty file, ... Or, when the drive can do no operation analysis, time could be another indicator: measure the time between tapemarks, and electronics and micro controller are excelent at this; if to long, non-empty file, thus non-MRS, thus write-locked?!
Every kind of magnetic tape has two points or positions of interest: the load point or Beginning of Media, and the End of Media; everything in between is accessable from the drive.
Before CompactDisks became famous and available, another digital system tried to enter the market: digital audio tape, a down sized version of video tapes at .15"-wide tapes. It was stillborn, because of the price: $500+ in mid-80s. The idea was picked up in the computer world, though: digital audio tape streamers, DDS. So as not to be able to use dat tapes, a media recognition system was intoduced, both on the tape and the drive. The MRS could be disabled on the drive. If you did that, and inserted a DDS tape, the MRS was overwritten, and the tape was rendered write-locked on MRS-enabled drives.
A lot of my tapes are now write-locked because I put them into a MRS-disabled, which I was unaware of at the time, drive, enough to think about MRS.
So here is my question: can this MRS information be restored to a DDS tape?
What could this information be?
Digital tapes are handled a little bit differendly from audio tapes. On audio tapes, a carrier signal is modulated with the audio frequencies (Amplitude modulation), that magnetizes the tape with variing strength. This varying strength magnetization is picked up during playback, amplified and filtered, and results in audible sounds. With two recording/playback assemblies (tracks) you could record stereo sound. And if you turned the tape around, and the tracks were positioned so that now two other tracks were used, you could record in both directions - 4-track. 8-track tape drives, which move only in one direction, move the record/playback assemblies (heads) up and down.
Digital tapes use the same physical setup: carrier, modulation, magnetization, induction, amplification, filtering. Since being digital, the modulation knows two states: up/down, or left/right. They use 7 (old) or 9 tracks. Since the tape moves, one write of one byte (track 9 is vertical parity) results in a rectangle of common magnetization, called a rung. One write of data results in a block of rungs; the minimum number of rungs that can be written in one write is 6, the maximum number (block length) depends on the drive (and system software) used, usually 64K. The - stationery - tape needs to be accelerated to operating speed, then the data is written, and the - moving - tape needs to be decelerated to a stop. This brings gaps about, so a tape is a sequence of blocks and gaps.
But how to mark the end of a file - that could easily consist of hundreds of thousands of blocks. So a special block is used: since it should not be mistaken for data, it could not be of data blocks format. A two rung block - tapemark - of the two ASCII characters ctrl/s (transmitter of) are used, along with special electronics to recognize them at every possible speed and every possible density (rungs per inch or bpi). And that gives an easily recognizeable end-of-data marker: two consecutive tape marks, resulting in an empty file on a read; end-of-media, along with the load point, are signalled by reflective stickers on the backside of the tape.
DDS follows along the same lines. But as the head assembly rotates with high speed and the tape crawls at 120m/2h (play time of tape), no gaps are neccessary, because ac-/deceleration is easily achieved and no stress on the tape, and the tape can be easily back-/forwardspaced, so continous recording is possible, and since the rungs are slanted and thus provide a natural blocking factor, after a backspace, the tape can move forward and hit exactly the next after last rung, avoiding overwriting the last rung and wasting of space.
So, what could this MRS information be, that - intentionally or accedentially - gets overwritten on a MRS-disabled drive? It must be distinguishable from any data and should be recognizable at every density.
How about some tapemarks? Like four tapemarks? With MRS enabled, the drive always skips four tapemarks, so the load point is always after the fourth tape mark, making them save. With MRS disabled, load point is at Beginning-of-Media (BOM), and the four tapemarks get óverwritten on a write to tape. This tape, on a MRS-enabled drive, would, maybe, also have four tapemarks, but not the associated empty files. Reads of a MRS-tape would yield: empty file, empty file, empty file, empty file, non-empty file ... Reads of a nonMRS-tape would yield; non-empty file, non-empty file, ... Or, when the drive can do no operation analysis, time could be another indicator: measure the time between tapemarks, and electronics and micro controller are excelent at this; if to long, non-empty file, thus non-MRS, thus write-locked?!
Comments
You might ask this question on the vintage computing federation forums: https://forum.vcfed.org/index.php?whats-new/posts
There are various tape drive experts there who might have a solution.