The FD1771 was succeeded by many derivatives that were mostly software-compatible: • The FD1781 was designed for
double density, but required external modulation and demodulation circuitry, so it could support
MFM,
M2FM,
GCR or other double-density encodings. • The FD1791-FD1797 series added internal support for
double density (
MFM) modulation, compatible with the
IBM System/34 disk format. They required an external data separator. • The WD1761-WD1767 series were versions of the FD179x series rated for a maximum clock frequency of 1
MHz, resulting in a data rate limit of 125
kbit/s for single density and 250 kbit/s for double density, thus preventing them from being used for 8-in (200 mm) floppy drives or the later "high-density" or 90 mm floppy drives. These were sold at a lower price point and widely used in
home computer floppy drives. • The WD2791-WD2797 series added an internal data separator using an analog
phase-locked loop, with some external passive components required for the
VCO. They took a 1 MHz or 2 MHz clock and were intended for and drives. • The WD1770, WD1772, and WD1773 added an internal digital data separator and write precompensator, eliminating the need for external passive components but raising the clock rate requirement to 8 MHz. They supported double density, despite the apparent regression of the part number, and were packaged in 28-pin DIP packages. • The WD1772PH02-02 was a version of the chip that Atari fitted to the
Atari STE which supported high density (500 kbit/s) operation. After production at WD could not be sustained, Atari decided to license the design and modify it to get high density and extra density (1 MBit/s) operation. The chip was codenamed "Ajax", had the number C302096 and was produced by Toshiba. == Compatible chips ==