The sound of the dialup, pictured (2012)
I really like how you can figure out the phone number dialed with the representation of the DTMF tones. This was basically an audio coordinate system (or just a 2D array): an X tone and a Y tone played at the same time.<p>The upper component has three positions representing the columns on a phone keypad.<p>The lower four positions represent the rows.<p>For example, zeros are represented by upper tone 2 for the middle column (2,5,8,0) and lower tone 4 for the bottom row (*,0,#).