> Sure; consider the very common Samsung SFD-321B,
particularly page 5:
> . . .
> Another type of "tri-Density" drive is the fairly common Teac FD-235J,
> which can do 720K, 1.44M and 2.88M. I've got a couple installed in
> older systems.
On Tue, 28 Mar 2023, Ali via cctalk wrote:
I don't know if we are talking about different
things? You are speaking
about a 3 Mode drive that allows 1.2MB on 3.5" drives. I am asking about
a Tri Density (as in SD, HD, ED, TD) drive. Supposedly these drives were
going to use longitudinal recording (vs. the perpendicular recording
used in ED drives) to fit 12.5MB on a 3.5" disk. I had linked a press
release in the OP:
I always thought the Mode 3 drives recorder 1.2MB on a standard 1.44MB
floppy (kind of like how QD drives recorded 720KB on a 1.2MB disk).
Not quite the same way as recording 720K on 1.2M drive.
Far more than you wanted to go through:
720K on a 1.2M drive is using all 80 tracks, but recording each track at
the density used by 360K drives. a 360K drive, in PC format, has 9 512
byte sectors, for 4,608 bytes per track, plus sector headers, gaps and
other overhead.. About 6.25K unformatted capacity. Other disk formats,
with different number of bytes per sector and/or sectors per track can
haveup to 5120 bytes of data per track (5 x 1024). It normally does that
by using 250,000 bits per second at 300RPM. Some 1.2M drives switch back
and forth between 300 RPM and 360RPM. If the drive is at 300 RPM then you
need 250,000 bits per second to do 360K, or "720"/"QD". If the drive
at 360RPM, then you need 300,000 bits per second.
360K on a 1.2M drive is double stepping, to get 48tpi, and using
250,000bps at 300 RPM, or 300,000bps at 360RPM
1.2M is typically 15 512 byte sectors, with 360 RPM and 500,000 bits
8"DD is the same.
What NEC did was to have 8 1024 byte sectors as their format on their 8"DD
and 5.25" HD disks, with the usual 360RPM and 500,000 bits per second.
8 x 1024 gives slightly more capacity then 15 x 512.
"1.4M" is usually 18 512 byte sectors, with 500,000 bits per second at 300
But, THEN what NEC did differently than everybody else, is that they sped
up their 3.5" drives to 360 RPM, so that they had the same format on their
3.5" disks as on their 5.25"HD and their 9"DD.
A "mode 3" 3.5" drive needs to be able to do both 300 RPM for
be able to do 360RPM for the NEC format.
"SD"/"Single Density" is FM. There is a clock pulse and a data pulse
1, and a clock pulse and gap (no pulse) for a 0. So, there are two pulses
(or a pulse and a missing pulse) for each bit. Some engineers call that
"DD"/"Doble Density" is MFM. MFM permits almost twice as much data to
crammed into the same space. It does that by leaving out the clock pulse
when it is between two data pulses. Since you, therefore, won't have
continuous clock and data pulses, the distance between pulses is almost
double, permitting using a higher data transfer rate, for almost twice as
much data per track. The marketing people called that "double density",
and AFTER they started doing that, they began to refer to FM as "single
density". Therefore, the term "double density" first occurs in the
literature BEFORE the term "single density" was ever used (much like
"World War 2" occured in newspapers of the time, BEFORE anybody ever
called the previous one "World War 1")
"HD"/"High Density" is MFM. Improvements in hardware and media made
that you could get away with twice as much data per track. It is ACTUALLY
just "double density", with twice the data rate.
"ED"/"Extended density" is MFM. Use of barrium ferrite media and
perpendicular recording permitted squeezing more. It is 1,000,000 bits
8" is 48tpi, with an unformatted capaciy of 500K SS or 1M DS
"360K" 5.25" is 48tpi, with an unformmatted capcity of 500K.
"1.2M" is 96tpi, with an unformatted capacity of 1.5?M
"720K", "1.4M", and "2.8M" are all 135 tracks per inch
"720K" has an unformatted capacity of 1M
"1.4M" has an unformatted capacity of 2M
"2.8M" has an unformatted capacity of 4M
The disk that you linked a picture of says "406TPI"!
Therefore, it presumably has about three times as many tracks. (240?),
which, if it does a 1,000,000 bits per second, with track density the same
as "2.8M", would give it an UNFORMATTED capacity of 12M, and a formatted
(data) capacity of about 8.5M.
A "TD" drive, if it also handles "720K"/"1.4M" would need
both types of
heads, and be able to handle both 135tpi and 406TPI. And the FDC would
need to do 250,000 bps, 500,000 bps, and 1,000,000 bps.
Grumpy Ol' Fred cisin(a)xenosoft.com
The drives were going to be called: PC FD810.1. Doing
a Google search bring up some pictures of disks so maybe the drives did make it out in
Or am I missing something?