Magnetic Recording and Playback 1087ting and buffering to handshake with the host computer
system.
The hard disk data path is much faster than the
DASH tape recorder. Read/write circuits for hard drives
are pushing frequencies of a gigahertz, yielding data
rates of 100 megabytes/s. At 3 bytes per 24-bit audio
word and a sample rate of 96 ksamples/s, this represents
about 250 audio channels. This number is best case, and
the throughput drops drastically if the drive must read
and write simultaneously while seeking various tracks
of data.
The trend is toward higher levels of integration in
disk drives, with more of the very high-speed circuitry
moving closer to the read/write head to avoid delays
and waveform distortions due to wire lengths and induc-
tances. (Electricity travels about 1 foot in a nanosecond,
and 1 ns is the period of one cycle of a gigahertz signal.)
The adoption of fluid dynamic bearings permits
higher disk rotation speeds that reduce the latency time
for a desired block of data to rotate to the head’s loca-
tion. The average latency is half the rotation period for
the disk. For a 15,000 rev/min (250 rev/s) disk drive,
the average latency is 2 ms.
28.5.9 Formatting Media
Digital media typically require one or two stages of
preparatory recordings of control information before
user data can be recorded. Low-level formatting
involves basic housekeeping tasks that allow the drive
to properly and accurately move the media and heads to
the correct physical locations. In addition, high-level
formatting defines the nature of the digital data blocks
regarding sector and block lengths. Formatting also
checks for media defects, marking bad sectors and relo-
cating data to good sectors.
Formatting may also include writing control tracks
with synchronization and address information. To illus-
trate why, consider a helical scan digital audio tape
recorder. We can start with a blank tape and begin a
recording. The machine records the helical stripes of
data with embedded address information and a control
track along the edge of the tape to facilitate synchro-
nizing the linear tape speed with the rotations of the
helical drum during subsequent playbacks. If we stop
the recording, we also stop the recording of all of the
address and synchronizing data.
If we wish to restart the recording by punching in at
our previous exit point, we must seamlessly append
address and synchronization data to the ends of the
previously recorded tracks. But what happens if the
recorder is running at a slightly different speed, perhaps
due to the recorder warming up, when we punch in?
Whenever we play back the tape, the recorder must
abruptly change the tape speed at the punch-in point.
A better technique is to prerecord or format the
entire tape with address and synchronizing information.
This will allow us to locate any address on the tape in a
continuous manner, and the tape speed will be constant
throughout the tape.
Formatting a tape or disk can be a very time-
consuming task. Tapes typically must run through the
machine at normal speed; hence a 30 minute tape would
require 30 minutes for formatting. Preformatted tapes
with prerecorded address and synchronization tracks are
now available from the tape manufacturers for some of
the digital audio formats. In addition to saving time, the
preformatted tapes also reduce wear on the recorder.
Hard disk formatting may vary from minutes to
many hours, depending on the operation. The lengthiest
operation is repacking all of the data on a disk. After a
file has been changed a number of times, the physical
file may be many sections scattered widely across the
surfaces of the disks, leaving unusable islands of
updated and deleted data. The repacking operation relo-
cates and reassembles the files as contiguous data,
freeing up the wasted space. Repacking also checks the
disk surface for defects. If a sector is contaminated with
errors, the drive may try several read operations to
recover the data. Some drives will also move the head
off-track slightly to recover poorly written tracks. Bad
sectors are marked so that they will not be reused.
Although audio tape recordings do not contain any
addressing and synchronizing information embedded
within the audio recording, some applications require
adding a track of SMPTE/EBU timecode for synchroni-
zation or editing. For live production work, the time-
code track will be recorded on all of the audio and video
machines to allow later synchronization of multiple
machines.
Timecode is also used during the editing process to
identify segments that are to be assembled onto a master
reel. Timecode is first prerecorded or striped onto the
master reel to allow the editing computer to precisely
locate the destination addresses of all of the edits. The
computer then locates the appropriate segments in the
timecodes on the source reels and copies the audio
and/or video onto the designated timecode section of the
master reel.28.5.10 Long-Term StorageA common question is “How do I store my digital data
when I finish a project?” Many users have decided that