Pulse Code Modulation
PWM and PPM both served their purpose as data transmission systems in space telemetry in the 1940s, early 1950s and even the 1960s to some degree. Newer technology uses Pulse Code Modulation (PCM). Data transmission by this method later found its way into the music and video entertainment industry and is how compact discs (CDs), digital video discs (DVDs) and similar media are encoded. There are many methods of implementing PCM, but the basic principle is the same.
PCM is an encoded binary signal of ones and zeros and is used to send both analog and digital information. The accuracy of analog information sent by PCM relies on the bit depth and sampling rate of the analog to digital encoder. Bit depth is the number of bits used to represent a value, while sampling rate determines how often a sample of the information is taken.
For an analog example, consider a photograph taken with a digital camera. At high resolution the file size is large since there are many pixels (sample rate) and colors (bit depth). At low resolution with fewer colors , the bit depth and sample rate are low with lower image quality and a lower file size.
Sending a high resolution photograph of Saturn's rings back to Earth using PCM requires a lot of data. On the other hand, reporting the temperature of the spacecraft's battery might require just a few bytes of data, along with the overhead required to send it. Besides the capacity for sending a lot data quickly, a major advantage of PCM is the ability to encrypt information.