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C
C
o
BIT CELL 0
USB
-----------------------B~-----------------------
BINARY REPRESENTATION OF:
DATA BITS
CLOCK BITS
HEXADECIMAL
REPRESENTATION
OF
DATA BITS
CLOCK BITS
t
BIT CELL 0 OF BYTE 0 IS
FIIRST DATA TO BE SENT
TO THE DRIVE WHEN
WIfIITING AND FROM THE
Df~IVE
WHEN READING
o
o
FIGURE
5.
BYTE
FIGURE
6.
DATA BYTES
1.2.3 RECORDING FORMAT (DOUBLE DENSITy)
.0
C D C
t
BIT CELL 7 OF BYTE 17 IS
LAST DATA TO BE SEIH TO
THE DRIVE WHEN WAITING
AND FROM THE DRIVIE
WHEN READING
Double capacity can be obtained by use of MFM (modified frequency modulation) and M2FM (modified,
modified frequency modulation) rather than FM (frequency modulation) which is the standard method of en-
coding data on the diskette.
The differences between FM, MFM and M2FM encoding are shown in Figure 7. Note that MFM and M2FM
result in a 1 to 1 relationship between the "flux changes per inch" and the bits per inch recorded
Oln
the
diskette. This also results in a doubling of the data transfer rate, from 250 to 500 KBS, when compared to
FM . .
Data error rate performance equal to standard capacity diskettes using FM encoding can be achieved by us-
ing:
•
The SA850/851 diskette drive with its proprietary ceramic/ferrite read/write head.
•
Phase locked loop (VFO) data separator
•
Write precompensation.
Provision of the phase locked loop data separator and write precompensation circuitry is the responslibility
of the user of the SA850/851 diskette drive.
Shugart Associates will provide design information, as required, to SA850/851 users who desire to incor-
porate double capacity diskette drives in their products.
The bit cell for MFM and M2FM encoded data is one half the duration of the bit cell for FM encoded data.
Also, unlike FM, and MFM and M2FM bit cell does not always contain a clock bit at its leading edge. This lack
of clock bit makes data separation more complex Also, the window size is half the FM window size, which
results in less tolerance to bit shift. The only reliable method to separate MFM and M2FM encoded data is
through use of a phase locked loop (VFO) type of data separator. The VFO, once synchronized, tracks the
data and generates clock and data windows, improving the bit shift tolerance over the conventional "hard"
data separators commonly used in FM recording, which use windows of fixed timing.
4
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