552 Chapter 17
whole number of time code frames. It is then possible to link each disc block address
used during a recording with the time code at which it occurred. The time codes and the
corresponding blocks are stored in a table. The table is also recorded on the disc when the
recording is completed.
In order to replay the recording, the table is retrieved from the disc, and a time code
generator is started at the fi rst code. As the generator runs, each code is generated in
sequence, and the appropriate data block is read from the disc and placed in memory,
where it can be fed to the convertor.
If it is desired to replay the recording from elsewhere than the beginning, the time code
generator can be forced to any appropriate setting, and the recording will play from there.
If an external device, such as a video recorder, provides a time code signal, this can be
used instead of the internal time code generator, and the machine will automatically
synchronize to it.
The transfer rate and access time of the disc drive are such that if suffi cient memory
and another convertor are available, two completely independent playback processes
can be supplied with data by the same drive. For the purpose of editing, two playback
processes can be controlled by one time code generator. The time code generator output
can be offset differently for each process, so that they can play back with any time
relationship. If it is required to join the beginning of one recording to the end of another,
the operator specifi es the in point on the second recording and the out point on the second
recording. By changing the time code offsets, the machine can cause both points to occur
simultaneously in data accessed from the disc and played from memory. In the vicinity of
the edit points, both processes are providing samples simultaneously and a cross fade of
any desired length can be made between them.
The arrangement of data on the disc surface has a bearing on the edit process. In the
worst case, if all the blocks of the fi rst recording were located at the outside of the disc
and all of the blocks of the second recording were located at the inside, the positioner
would spend a lot of time moving. If the blocks for all recordings are scattered over the
entire disc surface, the average distance the positioner needs to move is reduced.
The edit can be repeated with different settings as often as necessary without changing the
original recordings. Once an edit is found to be correct, it is only necessary to store the
handful of instructions which caused it to happen, and it can be executed at any time in the
future in the same way. The operator has the choice of archiving the whole disc contents