Power Computing PowerTower Pro User Manual page 315

Blown Sessions
288
The most common way to create a IIBlown Session
ll
is data underrun when mastering.
Data underrun is a condition where a device that is busy recording a single long spiraling
CD-Track consumes all the data in its buffer and has to terminate the session as a blown
session. The way to prevent it is to either provide more data to the recorder at a faster
and consistent rate, or to operate the recorder at a slower recording rate. The slowest
recording rate is normally called IISingle Speed
ll
and is 75 blocks per second where each
block is 2352 bytes of data per block (the recording technology actually uses 7,203 bytes
per block within the recorder). If a CD Recorder is rotating at 75 blocks per second the
Macintosh must provide about 173K/sec. If the device has a large buffer (enough to hold a
few seconds) then the recording software and technology can be lax in feeding the
recorder. Some CD Recorders however uses a rather small 64K buffer that exhausts
almost three times per second. This means that the Macintosh must always provide data
to the recorder without fail without ever delaying for a third of a second. If a whole CD is
recorded this must be maintained for an hour without fail. If not ... Kaboom, the session
is blown and you will need to try recording again in a new session.
In addition, any CD created with the some CD Recorders' software containing even a
single Blown Session is referred to as a IIFrisbee ll because it has no value except on the
CD Recorder that created it, or as a psychedelic flying discus. The root cause is that the
Blown Sessions are marked bad using a technique in the software.
It
marks the Table of
Content entry as MISSING for that track entry. Normal CD-ROMs may not contain
missing references to tracks, though the CD Recorder that created it does not mind read-
ing them. Not all CD recording software uses this questionable technique.
Thermal recalibrating hard drives, and busy network tasks can cause problems, so it is
recommended that the recording computer be unencumbered while performing the
recording, and that the thermal recalibration be disabled. Thermal recalibration is an
annoying but necessary function of high-temperature fast rotating hard disks that period-
ically evaluate the horizontal expansion of platters so that future head positioning can be
performed accurately at maximum speed. This recalibration should not be performed if
the SCSI device or bus appears very active but the devices do not include such useful
logic and abruptly seize up every now and then to perform thermal recalibration. FWB's
HammerTime™ Extension and FWB drivers can temporarily disable thermal recalibra-
tion on key FWB products when capturing QuickTime video, but HammerTime does not
help in this instance. Luckily, owner's of FWB's Hard Disk ToolKit can use the IIWorld
Control ll program on Seagate Drives and DEC drives if they are skilled power-users and
comfortable manipulating vendor-unique SCSI MODE SELECT PAGES. Note that using
World Control is only a temporary 15 minute reprieve from future thermal recalibration
delays.
CD-ROM ToolKit User Guide