A digital magnetic recording system is described that doubles the linear data bit density of an existing head/medium interface. It uses the combination of the following three techniques: an efficient new run-length limited code (3PM), a special kind of ac-bias recording, and pulse slimming by write current shaping. The new code has the inherent potential of increasing linear data density over MFM by 50 percent. This potential can be fully utilized only if the analog waveshape is equalized to optimum spectral shaping. The present paper uses pulse slimming by double step write current shaping to achieve optimum equalization. Write current shaping requires linear operation. This is accomplished by ac-bias recording, assuring symmetrical single pulse shape that allows a large degree of pulse slimming. The parameters of ac-bias recording are chosen so that large signal amplitude is used. This, in conjunction with write current shaping, maximizes signal-to-noise ratio in the read channel, further increasing the bit density. An additional increase is realized by using a low inductance head that yields narrow PW50value. The combination of all these factors results in 8000 BPI density on the original 4000 BPI MFM system.