RDMUX is an analog signal that represents the level of magnetization of each track. On an analog oscilloscope the signals will change so fast that you’ll just see a blur, represented in the picture in Max’s post as a sequence of blocks. On a digital scope you will see the levels of the 9 heads, and if you put it on hold and look at multiple blocks, you will see that each head changes between high and low at least every few blocks. If one of the heads is defective or dirty, you will see a gap in the blocks. If the entire head is dirty, or there’s not enough head to tape contact, or if the amplification needs to be adjusted, the blocks will look flatter.
At recording time, the tracks are magnetized to full saturation in either direction by sending current through the recording heads in either forward or backwards direction, where the polarity can change up to 96000 times per second per track (except the auxiliary track which runs at 1/12th of that frequency). Because of a system called “eight-to-ten modulation”, there can only be a few (I don’t recall how many) consecutive 0-bits or 1-bits per track. The tracks are recorded in a staggered pattern: no two heads change polarity at the same time; first track 0 changes polarity (if necessary), then track 1, then track 2 etc.
At playback time, the system has to recover the bits from the tape of course, but it doesn’t know where each bit begins. So it reads the tracks repeatedly at a fast enough pace to get a “slice” of each track (in the form of the RDMUX signal) at a high sample rate (not 96000 Hz). Because the system knows that the number of transitions per second per head is between 96000 and 96000/n (where n is the maximum number of consecutive unchanged bits according to the right-to-ten modulation algorithm), it can recover the clock of the original signal (96000 x 8 = 768000 bits per second). It uses this to adjust the capstan speed. And it uses it to sample the bits.
The bit stream of 768000 bits per second includes synchronization patterns, SYSINFO data, error detection/correction codes and music data. The music data is exactly half of the bandwidth: 384000 bits per second.
A very clever system! For more info about how it works, see the DCC system description on archive.org