The history of DHA can be divided into three stages. The first stage began in the 1960s with the widespread use of digital computers for simulation of audio processing for the analysis of systems and algorithms. The work was conducted with the help of the very large digital computers of that era. These efforts were not actual digital hearing aids because the computers were not fast enough for audio processing in real time and the size prevented them from being described as wearable, but they allowed successful studies of the various hardware circuits and algorithms for digital processing of audio signals. The software package Block of Compiled Diagrams (BLODI) developed by Kelly, Lockbaum and Vysotskiy in 1961 allowed simulation of any sound system that could be characterized in the form of a block diagram. A special phone was created so that a person with a hearing impairment could listen to the digitally processed signals, but not in real time. In 1967, Harry Levitt used BLODI to simulate a hearing aid on a digital computer.