Method of position detection on a recording medium

A method of position detection on a recording medium is performed by moving the recording medium alternately in normal and reverse directions, while searching for a target position, and decreasing the travel speed of the recording medium with each change in direction. Immediately after a target position has been finally passed, a report signal of the search completion is generated, thereby shortening the search time.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an apparatus and method for recording 
and/or reproducing a digital signal, and more particularly to such an 
apparatus and method suitable for use in recording and/or reproducing a 
digital signal transferred from a computer or the like. 
2. Description of the Prior Art 
In practice, data stored in a hard disk or the like of a computer can be 
transferred to and recorded by a data streamer (data-recorder) once a day 
so as to protect the data or back up the same. 
For this operation, or as a data recorder, analog audio tape recorders have 
been conventionally used in many cases. However, analog tape recorders 
have disadvantages in that they need an excessive amount of a recording 
medium or magnetic tape for recording and operate at a quite low data 
transferring rate upon recording, so that it takes too much time to 
transfer and record such data information. Moreover, analog tape recorders 
have problems, e.g. the starting point of a desired portion of the 
recorded data information cannot be rapidly searched for, and so on. 
Thus, to overcome the above-mentioned problems, it is thought to utilize a 
helical-scan type DAT (digital audio tape recorder) using a rotary head, 
that is, a so-called recently commercialized DAT as a data recorder. Such 
data recorders utilizing a DAT are described in U.S. Pat. Nos. 4,873,589, 
4,899,232, and 4,876,616 and in co-pending U.S. patent application Ser. 
No. 210,229 filed June 23, 1988. by one of the assignees of the present 
application. 
To utilize the DAT as a data recorder, data transferred from a host 
computer is transformed in accordance with a DAT format before recording. 
In the DAT format, one frame is made up of two oblique tracks formed by 
one rotation of two heads having different azimuth angles. 16-bit PCM 
audio data, which has been interleaved, and auxiliary sub-data are 
recorded in this one frame area as a unit. During recording, there are 
formed in each track a main area for recording the PCM data and a sub-area 
for recording the sub-data. 
The DAT has a high-speed search function. The high-speed search operation 
in the conventional DAT is performed by the method shown in FIG. 1. 
In FIG. 1, when a target position of a magnetic tape 15 on which position 
signals "1", "2", "3", . . . are recorded is designated as, e.g., "4" in 
performing a search operation, a position detection signal is detected 
while driving the tape 15 at a 200-times normal playback speed. When the 
target position "4" is detected, a stop signal is supplied, and the tape 
15 passes by the target position and is then stopped. The tape 15 then 
travels in the reverse direction at 16 times normal speed, and when the 
target position is detected, a stop signal is supplied. In this case, the 
tape 15 passes by the target position and is again stopped. The tape 15 is 
again reversed and travels at 3 times the normal speed. When the target 
position is detected, the stop signal is supplied. The tape 15 is again 
reversed and travels in the reverse direction at the normal speed. When 
the target position is detected and the stop signal is supplied, the tape 
15 slightly passes by the target position, and is then stopped. In this 
case, a search end signal is output, and the next command signal is 
awaited. When the command signal is input in this waiting state, the 
magnetic tape 15 travels in the opposite direction at the normal speed, 
and the target position "4" is accurately detected. 
The conventional high-speed search operation is performed as described 
above. Since the tape is kept stopped after the search end signal is 
output until the next command signal is input, there is a time loss, thus 
impairing the efficiency of the search operation. 
SUMMARY OF THE INVENTION 
It is a primary object of the present invention to provide a search method 
for a recording medium, capable of performing a search operation without 
wasting time. 
The above and other objectives are achieved by a search method for a 
recording medium in which said recording medium recorded with a position 
signal is alternately driven in normal and reverse directions while 
detecting the position signal and decreasing the travel speed each time 
the travel direction is switched so as to detect a target position on said 
recording medium, wherein after it is detected that the target position 
has been last passed, a report signal indicating that the target position 
has been searched is generated. 
In carrying out this data search method, a target count value and a count 
value reproduced from said recording medium are compared with each other, 
the direction and speed of the recording medium are controlled in 
accordance with the comparison output, and when the count value reproduced 
from the recording medium becomes close to the target count value, the 
report signal is generated after detecting a division of the unit count 
signal. 
The foregoing and other objectives, features and advantages of the 
invention will be more readily understood upon consideration of the 
following detailed description of certain preferred embodiments of the 
invention, taken in conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 2 shows an arrangement when a DAT is used as a data recorder. 
Reference numeral 1 denotes a DAT; 2, an interface bus; 3, a host 
computer; and 4 and 5, inner buses. The DAT 1 is mainly constituted by a 
recording/reproducing section 6, a recording amplifier 7, a reproducing 
amplifier 8, a signal processing circuit 9, a RAM 10, a data controller 
11, an interface board 12, a system controller 13, a servo and motor drive 
circuit 14 and the like. 
The system controller 13, the signal processing circuit 9 and the data 
controller 11 are arranged to exchange predetermined signals such as an 
absolute frame number AFNO, a mode instruction, a logical frame number 
LFNO, a data transfer instruction and the like. 
Although not shown, the recording/reproducing section 6 is provided with a 
rotary head drum. A magnetic tape is wound around the drum in an angular 
range of about 90.degree. and is fed by a capstan. The drum has two heads 
A and B having different azimuth angles. During one revolution of the 
drum, two oblique tracks are recorded or reproduced on or from the tape. 
Digital data supplied from the host computer 3 through the buses 5, 2 and 4 
are input to the interface board 12, and are then subjected to 
predetermined signal processing in the data controller 11, the RAM 10, the 
signal processing circuit 9 and the like in accordance with instructions 
from the system controller 13. In this manner, conversion to the DAT 
format described above is performed. The converted signal is supplied to 
the recording or reproducing section 6 through the recording amplifier 7, 
and is recorded on the magnetic tape by the heads A and B. 
The signal recorded on the magnetic tape can also be reproduced by the 
heads A and B. The reproduced signal is supplied to the signal processing 
circuit 9 through the reproducing amplifier 8. Digital data which is 
obtained by reconverting the reproduced signal by the signal processing 
circuit 9 is supplied to the host computer 3 through the data controller 
11, the interface board 12 and the buses 4, 2 and 5. 
In the above apparatus, the DAT format on the magnetic tape is as shown in 
FIG. 3. 
In FIG. 3, during one revolution of the heads A and B, two oblique tracks 
T.sub.A and T.sub.B are formed on a tape 15 from its lower side, as 
indicated by an arrow a. The two tracks T.sub.A and T.sub.B constitute one 
frame. The track T.sub.A (or T.sub.B) consists of 196 blocks, and one 
block consists of 288 bits. 34 blocks at each end portion serve as a sub 
area, and 128 central blocks serve as a main area. 
Each sub area is further divided into sections. More specifically, there 
are provided, from the lower end of the track, a margin section, a PLL 
preamble section of a sub code, a first sub-code section constituted of 8 
blocks, a postamble section, a gap section for a block section, a tracking 
(ATF) signal section, a gap section between adjacent blocks, and a PLL 
preamble of data. Then, after the main data section there follows a gap 
section between adjacent blocks, an ATF signal section, a gap section 
between adjacent blocks, a PLL preamble section of a sub code, a second 
sub-code section constituted by 8 blocks, a postamble second sub-code 
section, and a margin section. The other blocks are constituted by 
predetermined numbers of blocks, respectively. Note that in FIG. 3, the 
measure of lengths of sections is not accurately depicted. 
The main area consists of 128 data blocks. As shown in FIG. 4, each block 
is constituted by an 8-bit sync signal, an 8-bit PCM-ID (W.sub.1), an 
8-bit block address (W.sub.2) and an 8-bit parity, and main data is stored 
in the following 256-bit section. The main data is 16-bit PCM data for L 
and R channels when an audio signal is processed. The 16-bit main data are 
interleaved in the main areas of the tracks T.sub.A and T.sub.B (one 
frame) together with the parity. In this case, in the main areas in one 
frame, about 5760-bytes of data is recorded. When the DAT is used as a 
data recorder, the data sent from the host computer 3 are converted into 
16-bit data and are processed in the same manner as the PCM data. These 
data are formatted as shown in FIG. 5, and are recorded in the main area 
of each frame. 
More specifically, in FIG. 5, the above 5760 bytes are divided into words 
(0 to 1439), each consisting of 4 bytes (32 bits). These words are divided 
into 16-bit (2-byte) L and R channels to comply with the audio signal 
format. In this format, a header portion is provided in the first 1 word 
(4 bytes). 4 bits near the most significant bit (MSB) of the first half 
byte in the L channel in this header portion serve as a format ID 
indicating the format of the data recorder, and the remaining 4 bits of 
this byte are indefinite. The remaining one byte in this L channel are 
used as logical frame number (LFNO) areas. Each 8-bit area LFNO provides a 
binary value indicating one of the series of numbers 1 to 23 to designate 
each frame in units of 23 frames. In the R channel in the header portion, 
the same data as that in the L channel are provided. 
A total of 5756 bytes of a data portion is provided in the following words 
"1"to "1439", and data signals from the host computer 3 are sequentially 
stored in respective words in units of 4 bytes. 
The logical frame number LFNO will be described below. Each LFNO area 
indicates one of the serial numbers 1 to 23 of the frames in units of 23 
frames. That is, the frame numbers 1 to 23 repeatedly appear every 23 
frames. 
By designating a unit numbered by such LFNOs, the divisions for each 
predetermined amount of data can be easily detected, and signal processing 
and high-speed searching can be facilitated. 
The data formats in the first and second sub-code sections in the sub area 
will be described. Each of the first and second sub-code sections consists 
of 8 sub-code blocks, and can record 2048-bits of data. 
FIGS. 6A and 6B show, respectively, the constructions of the even-numbered 
sub-code block (EVEN block) and the odd-numbered sub-code block (ODD 
block), in each of which a synchronizing signal, the areas W.sub.1 and 
W.sub.2 and a parity, respectively formed of eight bits, and 256 bits of 
sub-code data including a parity are located in this order. The sub-code 
data is divided into four packs formed of 64 (8.times.8) bits (eight 
symbols), respectively. 
As shown in FIGS. 6A and 6B, the contents of W.sub.1 and W.sub.2 in the 
EVEN block are different from those in the ODD block, and the packs in the 
EVEN and ODD blocks are alternately numbered from "1" to "7". The eighth 
pack is assigned to record an error detecting code C.sub.1. 
W.sub.1 of the EVEN block consists of a 4-bit area ID and a 4-bit data ID, 
and W.sub.2 thereof consists of an upper bit "1", a 3-bit pack ID, and a 
4-bit block address. W.sub.1 in the ODD block consists of a 4-bit 
indefinite portion, and a 4-bit format ID, and W.sub.2 thereof consists of 
an upper bit "1", a 3-bit all "0" code, and a 4-bit block address. 
Each of the packs "1"to "7"is divided into 8 words in units of 8 bits. Each 
word includes, together with a parity, various codes such as a code 
indicating a read-in area of a recording start portion on a tape, a code 
indicating a read-out area of a recording end portion, a code indicating a 
recording date, an absolute frame number, a logical frame number and the 
like. 
FIG. 7 shows the format of the pack "1" of the seven packs, and FIG. 8 
shows a format of the pack "2". 
In FIG. 7, the pack "1" consists of eight 8-bit words PC1 to PC8. The upper 
four bits of the word PC1 are assigned to a pack number (in this case, 
"0001" indicating pack "1"), and the next 2 bits are indefinite. The 
following lower two bits (P,M) provide a repeat ID (R-ID) associated with 
multiple writing of data for a plurality of units. The following words PC2 
and PC3 (16 bits) are assigned to a group count GPC. The group count GPC 
is a value obtained by counting the number of units of 23 frames (called 
groups hereinafter) from the leading end of a tape. 
The following words PC4 to PC7 (32 bits) are assigned to a file mark count 
FMC. A file mark indicates a division of a predetermined amount of data 
sent from the host computer. The file mark count FMC is a value obtained 
by counting the number of file marks from the leading end of the tape 
until counting of the count GPC is completed for group of 23 frames. The 
word PC 8 is assigned to a parity for the words PC1 to PC7. 
In FIG. 8, the pack "2" consists of 8 words PC1 to PC8. The upper 4 bits of 
the word PC1 are assigned to a pack number (in this case, "0010" 
indicating the pack "2"). The words PC2 and PC3 (16 bits) are assigned to 
a save set mark count SSMC. Data recorded by the data recorder during one 
back-up operation is called a save set, and a save set mark is sent from 
the host computer for each save set. The save set mark count SSMC 
indicates a value obtained by counting the number of save set marks from 
the leading end of the tape until counting of the count GPC is completed 
for each group of 23 frames. The following words PC4 to PC7 (32 bits) are 
assigned to a record mark count RMC. A record mark is a mark sent from the 
host computer for each division of a predetermined amount of data. The 
record mark count RMC indicates a value obtained by counting the number of 
record marks from the leading end of the tape until counting of the count 
GPC is completed for each group of 23 frames. The word PC8 is assigned to 
a parity for the words PC1 to PC7. 
In this embodiment, as described above, the packs "1" and "2" provide four 
count values GPC, FMC, SSMC, and RMC respectively indicating divisions of 
data. In other words, these count values represent four types of units. 
These units are not especially associated with data lengths (recording 
lengths on a tape), and have predetermined independent lengths. The counts 
FMC, SSMC, and RMC are represented in association with the count GPC. 
FIG. 9 shows the relationship between the counts GPC and SSMC. The number 
of save set marks from the leading end of the tape until each time when a 
group of 23 frames is completed is recorded as the count SSMC in each of 
the 23 frames constituting the group including the timing. In the case of 
FIG. 9, when the counting of the group GPC 1 is completed, since seven 
save set marks are counted, SSMC 7 is recorded in each of the 23 frames of 
the group of GPC 1. The same set of SSMC 8 extends over two groups of GPC 
2 and GPC 3. Therefore, counts SSMC 8 are recorded in each of the 46 
frames of these groups. The relationships between the other counts FMC and 
RMC and the count GPC are also determined in the same manner as in FIG. 9. 
FIG. 10A shows examples of the indications of the counts GPC, FMC, SSMC, 
and RMC for the respective groups. 
Therefore, the above-mentioned counts GPC, FMC, SSMC, and RMC can be 
selectively detected, so that a high-speed search operation can be 
performed. 
A detailed description of the method of the high-speed search operation 
carried out by the system controller 13 according to the present invention 
will be described with reference to FIGS. 10B, 10C, 11A and 11B. The 
target position is given as FMC=4, and in order to detect the target 
position, the count GPC is used to first detect a boundary point between 
GPC8 and GPC9. 
The high-speed search operation includes a fast forward (FF) search mode in 
which the search operation is performed while fast-forwarding the tape at 
200 times normal playback speed in a FWD direction (normal direction). 
After the target position (TGT POSN) is detected, a REW search mode ensues 
in which the search operation is performed while rewinding the tape in a 
REW direction (reverse direction) at 16 times normal speed. Shortly after 
passing back over the target position a report is returned to the computer 
that the target position is detected. In this example, the computer 
instructs the DAT to forward the tape at normal speed. 
FIG. 10C shows the case wherein the REW search mode is executed. The count 
FMC=4 is detected while rewinding the tape at 200 times normal speed. The 
rewinding operation is performed until it detected that the tape has 
passed by the target position. Thereafter, a stop signal is sent to the 
tape drive system as well as a signal for driving the tape at a 16-times 
speed in the forward direction. Thus, the tape has passed by the target 
position and stopped, and is then reversed to travel at a 16-times normal 
forward speed while detecting the FMC. The 16-times speed traveling is 
continued until it is again determined that the tape has passed by the 
target position. Then the tape travel direction is reversed and the tape 
travels at a 3 times normal reverse speed until it is determined that the 
tape has passed by the target position. The host computer is informed by 
the DAT through the bus line that the target position is detected. After a 
200-msec standby time has passed, the next command signal is awaited. When 
the next command signal is received, the FMC is detected while feeding the 
tape at a normal speed to detect the target position "4". 
FIGS. 11A and 11B are a flow chart of the steps taken by the system 
controller when the FF and REW search modes are executed. Reference symbol 
tp indicates a target position represented by the FMC and pp represents a 
present position on the tape represented by the FMC. Reference symbol GPC1 
denotes a present GPC and GPC.sub.2 represents an immediately preceding 
GPC. Therefore, during traveling of the tape, the present GPC.sub.1 serves 
as the GPC.sub.2 when the next GPC.sub.1 is detected. 
In FIGS. 10B and 11A, after a high-speed search routine is started in step 
(1), it is checked whether tp &gt;pp in step (2), i.e. whether the present 
position is located before or after the target position. If tp &gt;pp then 
the present position is located before the target position, and the FF 
search mode in step (3) and subsequent steps is executed. If tp .ltoreq. 
pp then the present position is located after the target position, and the 
REW search mode in step (24) (FIG. 11B) and subsequent steps is executed. 
In the FF search mode, the count FMC is detected while fast-forwarding the 
tape at 200-times normal speed. The fast-forwarding operation is performed 
until it is detected in step 4 that tp .ltoreq. pp, indicating that the 
tape has arrived at the target position or has passed by the target 
position. If the determination is YES in step (4), a stop signal is sent 
to the tape drive system in step (5) as well as a signal for driving the 
tape at a 16 times normal speed in the opposite direction. Thus, the tape 
has passed by the target position and stopped, and is then reversed, so 
that it travels at a 16 times normal speed while detecting the FMC. In 
step (6), as the tape is traveling the GPCs are detected and sequentially 
set to be GPC.sub.1 and GPC.sub.2. The 16 times normal speed travel is 
performed until tp&gt;pp in step (7), i.e., until it is determined that the 
target position has been passed again. If the determination is YES in step 
(7), it is checked in step (8) if GPC.sub.2 -GPC.sub.1 ="1". If GPC.sub.2 
-GPC.sub.1 =1 immediately after tp &gt; pp is established in step (8), then 
GPC.sub.2 =9 and GPC.sub.1 = 8 (see FIG. 10A), and it is regarded that the 
tape has reached the target position. In step (9), the host computer is 
informed by the sending of a "Good Report" signal through the bus line 
that the target position is detected. After a 60-msec standby time has 
passed in step (10), the next command signal is awaited in step (11). When 
the next command signal is received in step (11), an operation is 
performed in accordance with the command signal in step (12). In this 
operation, the FMC is detected while forwarding the tape at a normal speed 
to search the target position "4". If no command signal is input in step 
(11), the tape is stopped in step (13), and the routine is ended in step 
(14). 
If GPC.sub.2 -GPC.sub.1 .noteq.1 is established in step (8) due to drop-out 
of data or the like, the flow advances to step (15) to forward the tape, 
and the GPC is detected in step (16). Step (16) is repeated until tp 
.gtoreq. pp in step (17), i.e., until the tape has passed by the target 
position again. If tp .gtoreq. pp is established, it is checked in step 
(18) if GPC.sub.1 -GPC.sub.2 =1. lf the determination is YES in step (18), 
it is determined that the target position is detected, and the host 
computer is informed of this fact in step (19). In step (20), the tape 
direction is reversed, and after 600 msec have passed, the flow advances 
to step (11). If GPC.sub.1 -GPC.sub.2 .noteq. 1 is established in step 
(18), the tape is stopped in step (21), and the routine is ended in step 
(23). 
If the determination is NO in step (2), the REW search mode is executed 
(FIG. 11B). In step (24), the count FMC is detected while rewinding the 
tape at a 200 times normal speed. The rewinding operation is performed 
until tp &gt; pp is established in step (25) to determine that the tape has 
passed by the target position. If the determination is YES in step (25), a 
stop signal is sent to the tape drive system in step (26) as well as a 
signal for driving the tape at a 16 times normal speed in the opposite 
direction. Thus, the tape has passed by the target position and stopped, 
and is then reversed to travel at a 16 times normal speed while detecting 
the FMC. The 16 times normal speed traveling is continued until tp &lt; pp in 
step (27), that is, until it is again determined that the tape has passed 
by the target position. If the determination YES is obtained in step (27), 
the tape travel direction is reversed in step (28), and the tape travels 
at a 3 times normal speed. In step (29), the GPCs are detected and 
sequentially set to be GPC.sub.1 and GPC.sub.2. The 3 times normal speed 
traveling is continued until tp &gt; pp is established in step (3), i.e., 
until it is determined that the tape has passed by the target position. If 
the determination is YES in step (30), it is checked in step (31) if 
GPC.sub.2 -GPC.sub.1 ="1". If GPC.sub.2 -GPC.sub.1 =1 in step (31) 
immediately after tp &gt;pp is established in step (30), then GPC.sub.2 =9 
and GPC.sub.1 =8 and it is regarded that the target position has been 
reached. In step (32), the host computer is informed by the sending of a 
"Good Report" signal through the bus line that the target position is 
detected. After a 200-msec standby time has passed in step (33), the next 
command signal is awaited in step (34). If the next command signal is 
received, an operation corresponding to the command signal is executed in 
step (35). In this operation, the FMC is detected while driving the tape 
at a normal speed to detect the target position "FMC=4". If no command 
signal is received in step (34), the tape is stopped in step (36), and the 
routine is ended in step (37). 
If GPC.sub.2 -GPC.sub.1 .noteq.1 is established in step (31) due to 
drop-out of data or the like, the flow enters the routine after step (15), 
and the above-mentioned operation is executed. 
As described above, immediately after it is determined that the target 
position is detected in step (8) in the FF search mode or in step (31) in 
the REW search mode, a "good report" is transmitted to the host computer 
in steps (9) and (32) so that the next command can be received early while 
the tape is being driven. Therefore, no time is wasted and the search time 
can be shortened. In this embodiment, the counts GPC and FMC are used as 
position signals of the recording medium. However, the search operation 
can be performed using the above-mentioned SSMC and RMC, as a matter of 
course. 
In summary, according to the prior art data search method illustrated in 
FIG. 1, if a target position is set, for example, FMC=4, then during the 
search after the tape is driven at fast forward, fast rewind, forward, 
rewind, etc., and then stopped at the target position FMC=4, the search 
end signal is transmitted. This search method requires a long period of 
time until the search end and also a time period for the next operation 
command. In other words, there are required a time during which the tape 
is stopped and a rising-up time when the tape is stopped and then 
accelerated to its predetermined speed. 
On the other hand, according to the present invention, when the target 
position FMC=4 is searched for, before the tape is stopped at the target 
position, a Good Report signal indicating that the target position is 
detected is supplied to the host computer. That is, in practice, in the 
high speed search mode, the target position FMC=4 is detected and the 
speed is changed to 16 times normal speed. 
When the boundary of the Group including FMC=4 is detected by comparing the 
difference of two successive Group Counts to one (e.g. "GPC.sub.2 
-GPC.sub.1 =1"), the Good Report signal is fed to the host computer. Thus, 
without stopping the tape, the next command from the host computer can be 
responded to. Thus, a high speed search becomes possible. 
Although the present invention has been shown and described with respect to 
preferred embodiments, various changes and modifications which are obvious 
to a person skilled in the art to which the invention pertains are deemed 
to lie within the spirit and scope of the invention.