DATFRAME(4)DATFRAME(4)NAMEdatframe - structure of a frame of audio data on Digital Audio Tape (DAT)
SYNOPSIS
#include <dataudio.h>
DESCRIPTION
The Digital Audio Tape (DAT) recorder system is very complex. This man
page gives the most useful and relevant information for DAT use in
professional systems. It is by no means comprehensive. Conflicts and
questions must be resolved by reference to the definitive DAT Conference
specification referenced below.
Audio on Digital Audio Tapes (DAT) is blocked into frames. A DAT
recording has 33.33 frames every second. One frame of data contains both
audio and non-audio data (known as subcodes). A program reading or
writing a DAT in audio mode must read(2) or write(2) a minimum of one DAT
frame.
The dtframe structure, describing the content of a frame of DAT data, is
defined in <dataudio.h> as follows:
typedef struct dtframe {
char audio[DTDA_DATASIZE];
struct dtsubcode sc;
} DTFRAME
Audio samples in the audio array are linearly encoded in a 16-bit 2's
complement format. Encoding is carried out either without pre-emphasis
or with a first order 50/15 Ms pre-emphasis. Pre-emphasis is not
recommended for professional use. Other encodings are supported by the
DAT format but are not permitted in professional use. The data stream
contains two interleaved channels, usually used as the left and right
channels of a stereo signal. The sample representing the left channel is
first, followed by the sample representing the right channel. The DAT
Conference recommends sampling both both left and right channels
simultaneously during recording. The IRIS Indigo and Personal IRIS 4D/35
audio ADC hardware samples both channels simultaneously.
The least significant byte of each 16-bit value is first. The bytes need
to be swapped in order to match the native byte ordering of the IRIS
Indigo and Personal IRIS 4D/35 and their audio hardware. So each 4-byte
chunk of the data array represents a pair of samples (1 left plus 1
right) as follows. Byte 0 is the least significant byte of the left
channel sample; byte 1 is the most significant byte of the left channel
sample. Byte 2 is the least significant byte of the simultaneous right
channel sample; byte 3 is the most significant byte of the simultaneous
right channel sample.
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DTDA_DATASIZE is 5760 bytes.
Subcodes
The dtsubcode structure contains subcode information that is recorded on
several different areas of the tape. struct dtsubcode is defined in
dataudio.h as follows:
typedef struct dtsubcode {
struct dtsubcodepack packs[7];
struct dtsubid {
unchar ctrlid:4, dataid:4;
unchar pno1:4, numpacks:4;
unchar pno2:4, pno3:4;
unchar ipf;
} sid;
struct dtmainid {
unchar fmtid:2;
unchar emphasis:2;
unchar sampfreq:2;
unchar numchans:2;
unchar quantization:2;
unchar trackpitch:2;
unchar copy:2;
unchar pack:2;
} mid;
} DTSUBCODE;
In referring to the DAT Conference specification, you need to know that
the mid field contains the bits from the ID field of the main data area
otherwise known as the Main ID bits. The sid field contains the bits
from the ID field of the sub data area, otherwise known as Sub ID bits.
The packs field contains all seven data packs from the sub-data area of
the tape.
Main ID Bits
The bits of the mid field have the following meanings and values:
value meaning
fmtid: 0 for audio use
1 reserved
2 reserved
3 reserved
value meaning
emphasis: 0 off
1 50/15 Msec
2 reserved
3 reserved
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value meaning
sampfreq: 0 48kHz
1 44.1kHz
2 32kHz
3 reserved
value meaning
numchans: 0 2 channels
1 4 channels
2 reserved
3 reserved
value meaning
quantization: 0 16-bit linear
1 12-bit non-linear
2 reserved
3 reserved
value meaning
trackpitch: 0 normal track mode
1 wide track mode
2 reserved
3 reserved
value meaning
copy: 0 copy permitted
1 reserved
2 copy prohibited
3 reserved
These bits are always present and must be recorded when making your own
recordings.
The pack field is optional. It should be filled with zero when not being
used. The bits of the pack field are assembled with the bits of the pack
fields from 31 other frames to build one 8-byte subcode pack. These
packs are duplicated in the sub data area from where they are much easier
to recover. All seven packs can be found in the packs array in DTFRAME.
Sub ID Bits
The ctrlid field is a bit field with the following meanings:
bit name meaning
0 TOC ID set if this program is stored
in the table of contents
1 Shortening ID set for 33 _ 3 frames at the
start of shortening play
2 Start ID set for 300 _ 30 frames at
the start of a track
3 Priority ID set when the program number is
recorded and is well defined
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The dataid field is zero for audio use. All other values are reserved.
pno1, pno2 and pno3 are 3 BCD digits indicating the current program
number. pno1 is the most significant digit. Program numbering starts
with the value 001 and has to increase by increments of one. The maximum
program number is 799 so pno can have values from 0 - 7, and pno2 and
pno3 can have values from 0 - 9. The program number can also be one of
three special values 0AA means the program number is not currently valid.
0BB means we are in the lead-in area. 0EE means we are in the lead-out
area.
The program number is only required to be valid when startid is 1, i.e.
for 300 _ 30 frames. so it is likely to disappear and be replaced by 0AA
shortly after a track starts.
numpacks is the number of subcode packs actually used in this frame. It
usually indicates that all seven packs are recorded even when they are
all zero.
ipf contains the interpolation flags, one for each channel as follows:
Bit Channel
0x40 Left IPF
0x20 Right IPF
The interpolation flags indicate when uncorrectable errors have been
detected during playback leading the DAT drive to interpolate the missing
data. It is usually advisable not to play the audio data from a frame
with these flags set.
Most of Sub ID is optional. Only dataid is required and must be zero.
If the bits of ctrlid are zero, the bits of the rest of Sub ID may also
be set to zero and will be ignored.
Subcode Packs
Nine different pack items are currently defined and seven are reserved.
The nine items are:
Pack ID Content
0000 no information
0001 Program Time
0010 Absolute Time
0011 Running Time/Pro R-Time
0100 Table of Contents
0101 Date
0110 Catalog Number
0111 International Standard Recording Code
1000 Pro Binary
Program Time
Program Time indicates the time from the beginning of the program. It is
stored in a dttimepack structure defined in dataudio.h as follows:
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/*
* The timecode structure looks like this:
*
* struct dttimecode {
* unchar hhi:4, hlo:4;
* unchar mhi:4, mlo:4;
* unchar shi:4, slo:4;
* unchar fhi:4, flo:4;
* };
*
* This structure is identical to mtaudtimecode in sys/mtio.h so
*/
#define dttimecode mtaudtimecode
/* pack for ptime, atime, rtime and pro-rtime */
struct dttimepack {
unchar id:4, flag:1, pno1:3, pno2:4, pno3:4;
struct dtpackedbcd {unchar dhi:4, dlo:4;} index;
struct dttimecode tc;
unchar parity;
};
The program number is stored in pno1, pno2, and pno3 as 3 BCD digits
exactly like the Sub ID field. That information is duplicated here.
index is the index number of the current subdivision within the program.
The index number is expressed as 2 BCD digits. AA indicates that the
index number is invalid. A program may be preceded by an index number of
00 with the same program number. This represents the music interval or
pause between tracks. Valid index numbers are 01 to 99. Within a
program, the first value of the index number is 01 and the value of the
index number has to increase by one.
The program time is represented by 2 BCD digits each for hours, minutes,
seconds and frames. Hours range from 00 to 99, and minutes and seconds
from 00 to 59. Over a three second period, frames ranges from 00 to 32,
00 to 32, and 00 to 33. In other words if (seconds modulo 3) is 0 or 1,
frames ranges from 00 to 32. If (seconds modulo 3) is 2, frames ranges
from 00 to 33. AA indicates the the program time is invalid. The
program time is set to zero at the start position of a program and
increases within the program. The start position of a program is the
first position where the program number is renewed (changed) and the
index number is not 00.
Absolute Time
Absolute time is stored in a struct dttimepack identical to program
time's. Absolute time is set to 00 at the start of the first program on
the tape and increases throughout the tape except on the lead-in area
(program number 0BB). The time decreases on the lead-in area and is set
to zero at the end point of the lead-in area.
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When the first program cannot begin after the time is set to zero, the
time should be zero continuously until the first program begins. The
program number of this area should be 0BB and this area should be less
than 33 frames.
Running Time
Running time is stored in a pack with ID 3, and the flag bit zero. The
pack is a time pack identical to program and absolute times'. The
running time is a continuously increasing time code beginning from some
value at the start point of each recording. The time increases within
one recording.
Pro R Time
Pro R time is a variation on the running time pack for professional use.
Pro R time is indicated when the flag bit is one. Pro R time is stored
in a dtprotimepack defined in dataudio.h as follows:
/* pack for pro-rtime */
struct dtprotimepack {
unchar id:4, flag:1, fill:1, sid:2;
unchar freq:2, xrate:3, msb:3;
unchar lsb;
struct dttimecode tc;
unchar parity;
};
Pro R time stores a running time in the tc field just like running time.
The other fields give addition information about the stored time value.
The sid field indicates the type of time code recorded as follows:
value meaning
00 IEC (SMPTE) timecode (IEC 461)
01 local sample address code of Pro DIO time code.
10 time-of-day code of Pro DIO time code (IEC 958)
11 reserved
The freq field indicates the sampling frequency used for the time
conversion to Pro R time as follows:
value meaning
00 48kHz
01 44.1kHz
10 32kHz
11 reserved
When the sid field indicates IEC (SMPTE) time code, the xrate field
indicates the transmission rate of the time code as follows:
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value meaning
000 30Hz
001 29.97Hz, non-drop frame
010 29.97Hz, drop frame
011 25Hz
100 24Hz
101 reserved
110 reserved
111 reserved
For any other value of sid, this field must have the value 000.
The msb, and lsb fields provide 11 bits of binary data. When sid
indicates IEC (SMPTE) time code these bits record a Time code Marker (TC
Marker). TC Marker is the time difference between the IEC time code frame
and the DAT frame.
When sid indicates one of the Pro DIO time codes, the binary data records
the 32-bit binary code corresponding to the first sample of a DAT frame
converted into hour, minute, seconds, frame and sample (called the sample
number).
The range of value indicated by these 11 bits is different for each value
of freq as shown in the following table:
frequency range
48kHz 0 - 1439
44.1kHz 0 - 1322
32kHz 0 - 959.
Table of Contents
The TOC pack contains one entry from the table of contents. The TOC is
stored in a struct dttocpack defined in dataudio.h as follows:
/* pack for table of contents */
struct dttocpack {
unchar id:4, flag:1, pno1:3, pno2:4, pno3:4;
struct dtpackedbcd point;
struct dttimecode atime;
unchar parity;
};
The table of contents may be recorded repeatedly over the entire length
of the tape or only at the beginning of the tape. In the first case it
is called repeated TOC (R-TOC). In the latter case it is called user's
TOC (U-TOC).
The standard table of contents entry has a program number in pno1, pno2
and pno3, and an index number in point. The starting position of that
program, index pair is given in atime. Several special values of pno and
index with specific meanings are also used. The following table shows
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these values.
Point Program Number Atime
AA 0BB all "0"
BB PNO of TOC area all "0"
B0 No. of TOC entries all "0"
A0 No. of 1st program start of 1st program
A1 No. of last program start of last program
C0 No. of a skip area start of skip area
C1 No. of a skip area end of skip area
CC 0BB all "0"
EE PNO of TOC area all "0"
Point BB is the beginning of the table of contents. Point CC is a
continuation of the table of contents. Point EE is the end of the table
of contents.
A location in absolute time is found by scanning the tape at 150X play
speed. Since it is possible to do exactly the same kind of search for a
program number, the table of contents is not terribly useful.
Date
The date pack is a time stamp giving the time and date of recording. It
is stored in a struct dtdatepack which is defined in dataudio.h as
follows:
/* pack for date */
struct dtdatepack {
unchar id:4, dayow:4;
struct dtpackedbcd year;
struct dtpackedbcd month;
struct dtpackedbcd day;
struct dtpackedbcd hour;
struct dtpackedbcd min;
struct dtpackedbcd sec;
unchar parity;
};
Catalog Number
The catalog number pack gives the catalog number expressed in 13 BCD
digits according to the UPS/EAN-Code. It is stored in a struct
dtcatalogpack defined in dataudio.h as follows:
/* pack for catalog number */
struct dtcatalogpack {
unchar id:4, n1:4;
struct dtpackedbcd nrest[6];
unchar parity;
};
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The catalog number does not change on a tape. When this item is present
it occupies at least one of 100 successive frames.
International Standard Recording Code
The ISRC gives a unique identification code to a recording. It is stored
in a struct dtidentpack which is defined in dataudio.h as follows:
/* pack for ISRC */
struct dtidentpack {
unchar id:4, point:2, fill:2;
union {
struct {
unchar country[2];
unchar owner[3];
unchar fill;
} point0;
struct {
struct dtpackedbcd year;
struct dtpackedbcd serial[3]; /* serial is 1st 5 digits */
unchar fill[2];
} point1;
} data;
unchar parity;
};
When point is zero the pack contains the country and owner codes. When
point is one the pack contains the year and serial number. Country and
owner are encoded in a six-bit code representing digits and upper case
letters. [See cdframe(4) for the code.] Year and serial number are BCD
digits.
The ISRC can only change immediately after the program number changes.
When this item is present it occupies at least one of 100 successive
frames.
Pro Binary
The Pro Binary pack contains either the binary group data of IEC (SMPTE)
time code or the status data of Pro DIO time code. It is stored in a
struct dtprobinarypack which is defined in dataudio.h as follows:
/* pack for pro-binary */
struct dtprobinarypack {
unchar id:4, fill:2, sid:2;
union {
struct bgd {
unchar fill;
unchar tcbits;
struct dtpackedbcd bg[4];
} bgd; /* binary group data of IEC time code */
struct csid {
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unchar csbytes[6];
} csid; /* channel status id of Pro DIO time code */
struct cod {
unchar fill[2];
unchar cod[4];
} cod; /* alphanumeric origin data of Pro DIO */
struct cdd {
unchar fill[1];
unchar flags;
unchar cdd[4];
} cdd; /* alphanumeric destination data of Pro DIO */
} data;
unchar parity;
};
sid is a subpack identification identifying what is in this pack as
follows:
value content
00 binary group data of IEC (SMPTE) time code
01 channel status ID of Pro DIO time code
10 alphanumeric origin data of Pro DIO
11 alphanumeric destination data of Pro DIO
The IEC (SMPTE) binary group data consists of binary group and time code
bits. The binary groups are stored as BCD digits in bg. The eighth
binary group is stored in bg[0].dhi. The seventh binary group is stored
in bg[0].dlo. The groups continue in descending order through the array
with the second group in bg[3].dhi and the first group in bg[3].dlo.
The time code bits are used as follows:
bit IEC bit 525/60 625/50
0 10 drop frame unassigned
1 11 color frame color lock
2 27 phase correction binary group flag
3 43 binary group flag binary group flag
4 58 unassigned unassigned
5 59 binary group flag phase correction
6 not used
7 not used
When the IEC (SMPTE) time code is recorded for a given frame, the binary
group data must also be recorded.
The channel status ID consists of bytes 0 to 5 of the channel status data
of the IEC serial digital interface stored in bytes 0 to 5 of
data.cdid.csbytes.
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The alphanumeric channel origin data consists of bytes 6 to 9 of the
channel status data of the IEC serial digital interface stored in bytes 0
to 3 of data.cod.cod.
The alphanumeric channel destination data consists of byte 22 of the
channel status data of the IEC serial digital interface stored in
data.cdd.flags and bytes 10 to 13 of the channel status data stored in
bytes 0 to 4 of data.cdd.cdd.
Pack Parity
The last byte of a pack is a parity checksum which is computed as the
exclusive-OR of the first seven bytes. During playback, the DAT drive
executes a parity check and only returns packs with correct parity.
During recording, the correct parity value must be computed and written
into the parity field before the pack is written to the tape.
SEE ALSO
The Digital Audio Tape Recorder System published by The DAT Conference.
c/o Electronic Industries Association of Japan, Engineering Department.
Tokyo Chamber of Commerce & Industry Building 2-2, 3-chome, Marunouchi,
Chiyoda-ku, Tokyo 100, Japan
DAT for Professional Use. The DAT Conference. c/o Electronic Industries
Association of Japan, Engineering Department. Tokyo Chamber of Commerce
& Industry Building 2-2, 3-chome, Marunouchi, Chiyoda-ku, Tokyo 100,
Japan
AUTHOR
Mark Callow
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