The 8970 or 8973 bitpump chip from Brooktree aka Rockwell aka Conexant is the
invariant among all flavors of 2B1Q SDSL.  Bt8970 was used in the first wave of
SDSL deployment and RS8973 is a subsequent improvement.  These chips were
preceded by Bt8952 (required an external analog front end) and Bt8960 (maxed
out at 416 kbps), but neither of the latter was used for SDSL.

Both Bt8970 and RS8973 have proper datasheets documenting all functions and
registers of the chip.  However, they require non-trivial control firmware for
operation.  One has to use BT software and cannot just write all necessary
control code from the datasheet alone for the following two reasons:

1. The DSP inside the bitpump has microcode and the microcode image is
buried inside BT software.

2. 2-wire 2B1Q transmission is not a simple matter and will not automagically
"just work" after some registers have been set right.  There is a non-trivial
activation sequence that must be followed, and the controlling firmware has to
do stuff related to the analog/DSP nature of 2B1Q transmission.  In other
words, the analog/DSP work is not all done and finished when the chip has been
made and is in your hand, in order to actually work that chip needs to be
driven by firmware written by an analog/DSP transmission line engineer and not
by a hacker.

The datasheets claim that the BT software package is available in source code
form, but in actuality obtaining it has been anything but easy.  The relevant
part of Conexant is now a separate company called Mindspeed; and Mindspeed, an
enemy of the people, is keeping a stranglehold on the essential BT software
package and withholding it from the people.  When Harhan Engineering Co. had
purchased 5 RS8973EPFSE parts and asked Mindspeed for the source code that is
required to use them, Mindspeed taunted us by sending us the source code in an
encrypted ZIP and refused to tell us the key to decrypt it!

IFCTF has finally succeeded in recovering and freeing the BT source code by
brute force cracking of the encrypted ZIP; the effort took 5 months.  The
free-at-last source code can be found in bp44cu62.zip in this directory.