Method and apparatus for recording and reproducing a digital signal with a stationary head

In a stationary head type PCM recorder having an A/D converter for sampling an analog signal and converting the analog signal to a digital signal, a signal processing circuit including data delay means for adding an error detection and correction code and a predetermined signal to the digital signal for each error correction group of a predetermined number of samples, and a multi-track head for recording an output of the signal processing circuit on a plurality of tracks of a magnetic record medium and reproducing the signals recorded on the magnetic record medium; incorrectability of the error detection and correction code for a burst error in the output analog signal due to a burst error in the reproduced output is reduced by delaying parity data by the error detection and correction code and the digital signal data by different delay times such that the digital signal data are dispersely recorded in a track direction and a tape transport direction, and allotting the delayed data to the multi-track head such that the data of the adjacent sample points are spaced from each other by at least the distribution length of the parity data and the parity data is arranged between the distributed adjacent data.

BACKGROUND OF THE INVENTION 
The present invention relates to recording and reproducing technique on a 
digital audio tape by a stationary head, and more particularly to method 
and apparatus suitable for correcting and concealing a data error such as 
a dropout in a record medium. 
A multi-track, Stationary-head Digital tape recording/reproducing apparatus 
Audio Tape-recorder (S-DAT) has been well known in the art of digital 
audio system. 
Many audio data recording formats have been proposed to prevent or correct 
data errors in recording and reproducing the data. As an example, an 
interleaved format is shown in an article entitled "Stationary Head 
Digital Audio Tape Deck" by Uchida et al, Technical Report of Institute of 
Electronics and Electrical Communication of Japan, Vol. 81, EA 81-64, p. 
38 (1981). In the disclosed audio recording/reproducing method, a CIRC 
(cross-interleaved Reed Solomon code) is used to correct the data error. 
By nature of a magnetic tape, a burst error which is much longer than a 
bit length, that is, a data dropout frequently occurs. Accordingly, the 
data is interleaved to convert the burst error to a random error. FIG. 2 
shows an interleaved data in the above system. The interleaved data is 
arranged in a parity track located at a center of the tape and a data 
signal track located at an edge are of the tape. Twelve words (1 word=16 
bits, one sampling data) of left (L) and right (R) channels sampled from a 
two-channel stereo signal are represented by 24 symbols (1 symbol=8 bits, 
error correction unit) to which 4 parity symbols are added to form 28 
symbols, which is arranged on the tape. After the interleave, four parity 
symbols are further added on the tape to form a 32-symbol data. The 
correction is made in two steps for the widthwise data and the interleaved 
28-symbol data. Up to four symbol errors can be corrected by an erasure 
correction method. In this system, a correction length is approximately 
2.1 mm and an interpolation length is approximately 5 mm. However, no 
consideration was made in this system to a low error rate at the edge of 
the tape and a long burst error generated in one of the tracks. 
An article entitled "On PCM Multi-Channel Tape Recorder using Powerful Code 
Format" by K. Tanaka et al, presented at the 67th Convention of the Audio 
Engineering Society (AES), October 31-Nov. 3, 1980 New York, discloses 
development of a semi-separate code format. Particularly, FIG. 7 thereof 
shows a format of odd/even sample data interleaved by a skew pattern with 
respect to a tape transport direction. 
An article entitled "A Multitrack Digital Audio Recorder for Consumer 
Applications by W. J. Van Gestel et al, in J. AES, vol. 30, No. 12, 1982, 
December pp. 889-895, presented at the 70th Convention of the AES, New 
York, October 30-Nov. 2, 1981, particularly FIG. 9 and page 892 disclose a 
record format on a tape in which the distance between data words 
terminates in each block and parity check words are added in the block 
longitudinally and widthwise of the tape. 
This error correction technique for correcting the random and burst errors 
is not sufficient to prevent and correct the data dropout in recording and 
reproducing the two-channel audio signal on the tape. 
The technical trend of the S-DAT is described in the article entitled 
"Activities of DAT Association", ELECTRONICS published by Electronic 
Industries Association of Japan, vol. 24, No. 10, 1984, pp. 36-42, 
particularly pp. 40-42. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a stationary head 
digital audio tape recording and reproducing system which resolves the 
problems encountered in the prior art stationary head digital audio tape 
recording and reproducing system and can reproduce an analog signal which 
is sufficient for practical use even if dropouts occurs in the track width 
direction and travel direction of the tape. 
It is another object of the present invention to provide a stationary head 
digital recording method and apparatus having a tape recording format of a 
high error correction capability for a burst error in converting a 
multi-channel audio signal to a digital signal and recording it on a 
multi-track tape. 
In one aspect of the present invention, odd data and even data of two 
sampled channels, that is, sample data adjacent to each other are 
separated from each other longitudinally of the tape, and an error 
detecting and correcting parity data generated from the odd and even data 
is inserted between those channels, and the parity data is arranged in a 
track on a tape edge. 
In another aspect of the present invention, each track on a tape includes 
L-channel data and R-channel data. Accordingly, if a burst error occurs 
within the track, error data is distributed to both channels. The parity 
data is arranged in the track on the edge of the tape where errors 
frequently occur and the audio data is arranged in inner tracks where the 
occurrence of errors is relatively low. The data at the odd sample points 
and the data at the even sample points of the audio data are separated and 
the parity data is inserted therebetween so that the error concealment 
capability with respect to the burst error is enhanced. 
A delay of the parity data is set to be larger than a delay of the audio 
data to prevent the reduction of the burst error correction capability due 
to the concentrated arrangement of the parity data.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
One example of the S-DAT is described in U.S. patent application Ser. No. 
416,244 by Hoshino et al filed on Sept. 9, 1982, entitled "A PCM Tape 
recording and Reproducing Apparatus Having a Dropout-Immune Data Recording 
Format", now U.S. Pat. No. 4,539,605, issued Sept. 3, 1985 which 
corresponds to Japanese Patent Application No. 56-142292 and European 
Patent Application No. 82108375.5 filed on Sept. 10, 1982. 
In a multi-track PCM recorder, PCM data is recorded on a plurality of 
tracks, e.g. 20 tracks. Left and right time-serial audio signals L.sub.in 
and R.sub.in from the two-channel audio signal source are alternately 
selected by a channel selection circuit MPX, and they are time-division 
multiplexed as left and right sample signals in the order of L.sub.0, 
R.sub.0, L.sub.1, R.sub.1, L.sub.2, R.sub.2, . . . , which are then 
transmitted as a serial signal. The analog signal from the channel 
selection circuit MPX is converted to a digital signal by an A/D 
converter, and the digital signal is sent to a digital signal processing 
circuit, where the time-serially transmitted data is stored in a RAM and 
the data is read from the memory in a different sequence. This is called 
interleaving. The interleaving is a data distribution processing to allow 
error correction of the data even if a dropout or burst error occurs in 
the recorded data. This process is carried out by an interleaver shown in 
FIG. 1 which is a data delaying and rearranging circuit. The interleaved 
signals on the input data L.sub.i, R.sub.i -L.sub.i+15 and R.sub.i-15 are 
arranged to form an error detection and correction block, and by encoding 
using error detection and correction code such as the Reed-Solomon code 
parity words, for example, Q.sub.0, Q.sub.1, . . . , and Q.sub.7 or 
P.sub.0 and P.sub.1 are produced. The signals processed in this manner are 
recorded on the magnetic tape by a multi-head (20-track head, for example) 
through recording amplifiers. In the reproducing operation, the signals 
read from the magnetic tape by the multi-head are amplified and reshaped 
by reproducing pre-amplifiers, outputs of which are sent to a reproducing 
digital signal processing circuit, in which the input data is arranged to 
form the error detection and correction block. After the detection and 
correction of the data error, the data having the sequence thereof changed 
by the interleaving is rearranged in the original sequence by a 
de-interleaver. This is called de-interleave processing. The 
de-interleaved digital signal is converted to an analog signal by a D/A 
converter, and the analog signal is divided into left and right channels 
by a demultiplexor to reproduce the two-channel audio signals L.sub.out 
and R.sub.out. 
Referring to FIG. 1, one embodiment of the present invention is explained. 
FIG. 1 shows an interleave circuit for interleaving the data in the record 
mode. Numeral 1 denotes uninterleaved data, numeral 2 denotes interleaved 
data, numeral 3 denotes a track number on a tape on which the interleaved 
data is recorded, numerals 4-8 denote data delay circuits, and numerals 9a 
and 9b denote a C.sub.2 -parity encoder and a C.sub.1 -parity encoder 
which use a doublyencoded error correction code. An example for 
contructing such encoders is disclosed in Odaka et al U.S. Pat. No. 
4,413,340. PCM data to be recorded on the tape is distributed to 20 
tracks. Accordingly, the interleaving process must take the data delay in 
the tape transport direction and the distribution of the data among the 
tracks into consideration. In the interleave circuit of FIG. 1, the data 
is delayed by the delay circuits 4-8 and then the data is distributed to 
the tracks as shown by numeral 3. After the sampling of the L and R 
channel audio signals, the A/D converted L - R two-channel PCM signal 1 is 
delayed by the delay circuit 4 only for the odd data, and the error 
correction code generator 9a which may use the Reed-Solomon code carries 
out the C.sub.2 -encoding by the Reed-Solomon code for the 32-word data 
(40, 32, 9) and adds parities Q.sub.0i, Q.sub.1i, . . . , Q.sub.7i. A 
delay d.sub.1 by the delay circuit 4 may change from data to data. In this 
manner, the C.sub.2 -parity encoder is constructed. The data delayed in 
the delay circuit 4 and the parities generated by the error correction 
code generator 9a are further delayed, that is, interleaved for each word 
by the delay circuits 5-8. The Reed-solomon code encoding (C.sub.1 
-encoding) is carried out for the 27 words of control signal words and 
data words of one block or C.sub.2 parity words by the C.sub.1 -encoder 9b 
(29, 27, 3) to produce C.sub.1 parity words P.sub.o,j,k and P.sub.i,j,k, 
and it is recorded in the tracks shown by 3. 
FIG. 3 shows a record track pattern on the tape. Numeral 10 denotes a 
magnetic tape, numeral 11 denotes a center of the tape and numeral 12 
denotes a record track. The record track comprises 22 tracks, that is, 20 
tracks for recording 20 PCM signals, an AUX track for recording an 
auxiliary signal other than the PCM signal, and a CUE track for recording 
a fast access control signal. The track width may be 65 .mu.m and a track 
pitch may be 80 .mu.m. The 22 tracks are arranged in one half widthwise 
area of the tape. The remaining half area is used to record signals when 
the tape is transported in the opposite direction. In this manner, 
reciprocal recording and reproducing is attained. In the following 
description, only the 20 tracks for recording the PCM signals are 
explained. 
FIGS. 4A, 4B and 4C show a frame format of the track on which the PCM 
signal is recorded, a data record format in a field, and a block 
configuration. Numeral 13 denotes a frame, numeral 14 denotes a one-byte 
synchronization signal, numeral 15 denotes a three-byte control signal 
including a one-bit ID code, a 7-bit frame address and information (16-bit 
sub-code) relating to the PCM signal, numeral 16 denotes the 24-byte PCM 
signal, and numeral 17 denotes an error detection and correction code 
(C.sub.1 -parity word) added to each 2-byte block. One track of data in 
one frame thus constructed is defined as one block which includes 240 bits 
(FIG. 4C). One frame comprises 20 blocks. In each block area for PCM 
signal 16, the interleaved data 2 shown in FIG. 1 is recorded. After the 
addition of the parities by the error correction code generator 9a and the 
delay of the data by the delay circuits 4-8, the C.sub.1 -parity word 17 
is further added to each block by the C.sub.1 -parity encoder 9b. By 
doubly encoding the data by the interleaving, a high error detection and 
correction capability is attained with relatively simple code arrangement 
and decoding algorithm. Examples of the decoding algorithm for the 
Reed-Solomon code (C.sub.2) having a code length of 40, the number of 
information words of 32 and a minimum distance of 9, as the code generated 
by the error correction code generator 9a, and Reed-Solomon code (C.sub.1) 
in circuit 9b having a code length of 29, information words of 27 and a 
minimum distance of 3 are shown in FIGS. 5 and 6. FIG. 5 shows the 
algorithm of the first error detection and correction (C.sub.1 -encoding) 
for each block. N(E) denotes the number of errors detected. If one error 
is corrected and a further error is detected, a flag F.sub.0 is set to 
"1", and if two or more errors are detected and are not correctable, a 
flag F.sub.1 is set to "1". By utilizing the flags F.sub.0 and F.sub.1, 
the error is detected and corrected by the second error detection and 
correction algorithm (C.sub.2 -decoding) for the Reed-Solomon code having 
a minimum distance of 9. FIG. 6 shows the error detection and correction 
algorithm. N (F.sub.0) and N (F.sub.1) denote the numbers of flags F.sub.0 
and flags F.sub.1. Erasure correction, erasure and one-error correction or 
erasure and two-error correction is carried out depending on the number of 
flags. For the erasure for which the error location is known, a word 
having the flag F.sub.0 or F.sub.1 added thereto is used. F denotes an 
incorrectable flag which is added when the error is incorrectable. When 
F=F.sub.0, the flag F.sub.0 is used as the incorrectable flag. When the 
error is incorrectable, the flag F.sub.0 is used as the incorrectable 
flag. When N (F.sub.0 .ltoreq.6, all words are incorrectable. In the 
algorithms shown in FIGS. 5 and 6, a probability of incorrectability to 
the random error is approximately 6.times.10.sup.30. P.sub.s.sup.18 where 
P.sub.s is an error rate per symbol unit. When P.sub.s =10.sup.-3, the 
probability of incorrectability is 6.times.10.sup.-24. Thus, a high 
correction capability is attained by a relatively simple correction. By 
the combination of the Reed-Solomon code having a high random error 
correction capability and the interleaving which converts the burst error 
to the random error, the correction capability to the burst error is 
enhanced. A technique to enhance an error correction capability in 
decoding the doubly-encoded error correction code is described in U.S. 
patent application Ser. No. 665,378 by Okamoto et al, entitled "Decoding 
Method and System for Doubly-Encoded Read-Solomon Codes", filed on Oct. 
26, 1984 based on Japanese Patent Application No. 58-202602 filed in Japan 
on Oct. 31, 1984, and included herein by reference. 
FIG. 7 shows a data record pattern on the magnetic tape recorded in the 
present embodiment. One section corresponds to one block delays of data 
are d.sub.1 =0, d.sub.2 =30 frames, d.sub.3 =2 frames, d.sub.4 =4 frames 
and d.sub.5 =32 frames. Those values allow the interleave delay by a 
commercially available 256K-bit RAM. 
In the record pattern shown in FIG. 7, the L-channel and R-channel even 
data (L.sub.0, R.sub.8), L.sub.2, R.sub.10) . . . (R.sub.6, L.sub.14) or 
odd data (R.sub.1, L.sub.9), (R.sub.3, L.sub.11), . . . (L.sub.7, 
R.sub.15) are arranged on each of the first to sixteenth tracks except the 
edge tracks, and parity data (words) (Q.sub.1, Q.sub.5), (Q.sub.3, 
Q.sub.7), (Q.sub.0, Q.sub.4), (Q.sub.2, Q.sub.6) are arranged on the 
seventeenth to twentieth tracks on the edges of the tape. In the 
reproducing mode, one block is constructed by those data. The odd data and 
the even data are spaced by 30 block length in the tape transport 
direction so that they are completely separated. In this separation area, 
the parity data (Q.sub.0 -Q.sub.7) which are selectively delayed are 
obliquely and dispersely arranged with respect to the track. The data is 
also selectively delayed and obliquely distributed with respect to the 
track. In order to enhance the error correction capability to the burst 
error in the track width direction (widthwise of the tape), the first half 
of the even data (L.sub.0, L.sub.2, L.sub.4, L.sub.6) and the record half 
of the odd data (L.sub.9, L.sub.11, L.sub.13, L.sub.15) are distributed in 
the first to eighth tracks in the center of the tape, and the second half 
of the even data (L.sub.8, L.sub.10, L.sub.12, L.sub.14) and the first 
half of the odd data (L.sub.1, L.sub.3, L.sub.5, L.sub.7) are distributed 
in the nineth to sixteenth tracks on the edge of the tape. The right 
channel sample data R.sub.0 -R.sub.15 are distributed symmetrically to 
L.sub.0 -L.sub.15 with respect to the tracks of those two groups. The 
distance between the sample data which are adjacent to each other in the 
original time sequence, for example, the distance between the even data 
L.sub.0 and the odd data L.sub.1 after the separation can be adjusted by 
the distribution of the parity data Q.sub.0 -Q.sub.7. Accordingly, the 
adjustment for the audio data is not necessary. 
FIG. 8 illustrates the burst error correction capability for the 20 tracks. 
An ordinate represents the number of occurrence of burst errors, an 
abscissa represents a burst error length (in mm), an area A represents a 
correctable area, an area B represents a first order interpolation (i.e. 
concealment) area and an area C represents an incorrectable area on 
correction and concealment. 
A two-dimension interleave method in which a series of data is obliquely 
distributed at a constant pitch with respect to the tape transport 
direction is disclosed in the above-mentioned patent application by 
Hoshino et al. 
In setting the pattern, defects in the tape transport direction and the 
tape width direction and abnormal output due to the defect of the head or 
the deposition of dust were studied with respect to the record error on 
the tape and the defects of the tape and it was proved that the above data 
allocation, particularly the distribution in the tape transport direction 
is effective to suppress the error. In the present embodiment, the 
L-channel data and the R-channel data are recorded in each track so that 
the burst error in a specific track is distributed to both channels. The 
parities are arranged in the tracks near the tape edge having a low error 
rate and the PCM signals are recorded in the inner tracks so that a 
probability or error of the PCM signals is reduced. The odd data and the 
even data are separated and the adjacent data, for example, the data 
L.sub.0 and L.sub.1 are arranged to be spaced as much as possible in the 
tape transport direction and the tape width direction so that the error 
correction capability to the burst error is enhanced. 
When the above error correction method is applied in the embodiment of FIG. 
7, all track errors in seven frames or burst errors in four tracks can be 
corrected. When an error correction method by an average interpolation is 
used, all track errors in 57 frames or burst errors in seven tracks can be 
interpolated (FIG. 8). In FIG. 7, d.sub.1 is set to 0. By making a delay 
in d.sub.1, the correction capability is further enhanced. 
FIG. 9 shows other embodiment of the present invention for a 10-track 
record. The only difference from FIG. 1 is the track distribution means 
39. In the present data distribution method for the ten tracks, the even 
data is arranged on the odd tracks and the odd data is arranged on the 
even tracks. FIG. 10 shows a record pattern on the tape in the embodiment 
of FIG. 9. The delays are equal to those shown in FIG. 7. The interleave 
method of the present invention can be applied to the ten-track record 
without reducing the error correction capability by merely changing the 
track distribution means.