PCM
Common digital audio file formats like wav, aiff, sd2, ISO, and others are all PCM, pulse code modulation, files.
Pulse code modulation is a sample and hold method to capture analog signals and convert them into digital signals. The sample rate (the horizontal resolution), is the sampling frequency used in the recording. The bit depth (the vertical resolution), is the number of quantized steps that capture the amplitude of that particular sample.
The format was actually invented way back in 1937 by British engineer Alec Reeves, for telecommunication applications.
PCM was used as encryption to encode audio during World War II, in order to broadcast high-level messages without the enemy intercepting them.
Then, in the 1960s, broadcasters BBC in Great Brittan and NHK in Japan as well as the Japanese record company Nippon Columbia were all developing digital tape recorders using PCM. In 1971, in conjunction with Denon, their tape recorder division, Nippon Columbia released the first digitally recorded LP, "Something" by Steve Marcus.
Later that decade in the US, Thomas Stockham of Soundstream developed the first 16-bit digital multi-track tape recorder and the first digital audio workstation using PCM.