QAM (Quadrature Amplitude Modulation) is the well-known technique used in dial-up modem standards such as v.32 and v.34. It works, but it is rather complex, and thus the AuDSL receiver currently has some trouble keeping up with an incoming QAM data stream at 48 kHz.
PR-PAM (Partial Response Pulse Amplitude Modulation) is an alternative modulation technique. It is less well known than QAM in the context of data communications, although it is commonly used for magnetic recording on disks and tapes. PR-PAM can actually achieve the same spectral efficiency as QAM (or better), and often with significantly simpler modulator/demodulator structures and less computational effort. When using PR-PAM, the AuDSL modem is already capable of running in real time at a 48 kHz sampling rate and a 96 kbps data rate.
Here are some references on partial response modulation:
Peter Kabal, Subbaryan Pasupathy: Partial-Response Signaling. IEEE Transactions on Communications, vol com-23, no 9, September 1975.Amir Said, John B. Anderson: Bandwidth-Efficient Coded Modulation with Optimized Linear Partial-Response Signals. IEEE Transactions on Information Theory, vol 44, no 2, March 1998.
Nambirajan Seshadri, John B. Anderson: Decoding of Severely Filtered Modulation Codes Using the (M, L) Algorithm. IEEE Journal on Selected Areas in Communications, vol 7, no 6, August 1989.
Krishna Balachandran, John B. Anderson: Reduced Complexity Sequence Detection for Nonminumum Phase Intersymbol Interference Channels. IEEE Transactions on Information Theory, vol 43, no 1, January 1997.