PCM

Have you ever heard about PCM or the Pulse Code Modulation? Basically, PCM or Pulse Code Modulations are digital representations of analog signals where a magnitude of signals are regularly sampled at uniformed intervals before quantized to series of symbols in digital codes, usually in binary from. A PCM is mostly used for digital telephone systems and it is also a standard from for the digital PCM audio in personal computers and compact disc red book formats. Likewise, these pulse code modulations are standard in digital videos using the ITU-R BT.601 PCM tuning. However, the straight pulse code modulations are not normally used for videos in consumer applications like the DVD and the DVR since these require higher bitrates. Instead, there are compressed PCM variants that are normally employed. Frequently, the PCM encodings facilitate digital transmission from one particular point to another point in serial forms.

History of PCM- Pulse code modulations, like any other great inventions appear very simple and very obvious. The history of the electrical communications state that the earliest reasons for sampling signals were to interlace particular samples coming from varied telegraphy sources, and conveying them over single-telegraph cables. Early of 1853, telegraph TDM or time-division American Inventor named M.B. Farmer conveyed multiplexing. In 1903, electrical engineer W.M. Miner electro-mechanical commutatos for those time-division multiplexes of the multiple telegraph signals, while applying this technology specifically for telephony. Miner that obtained intelligible speeches from channels that were sampled at rates that are more the 3500 Hz, and accordingly, below this rate is considered unsatisfactory.

In 1926, Paul M. Rainey of Western electric patented the facsimile machines using optical mechanical analogs to digital converters. These machines though, did not successfully go for production. In 1937 unaware of previous works, Alec Reeves, a British Engineer, conceived using Pulse mode modulations for voice communications while working for the International Telephone and Telegraph from France. Reeves described his theories and advantages but no specific use resulted. However, Reeves still filed for French Patent in 1938, and was granted a US Patent in 1943.

The very first transmission of speeches using digital techniques was SIGSALY vocoder encryption equipments that are used for those high-level allied connections in 1943 during the Second World War. In 1943, engineers from Bells Labs, the same engineers that designed the SIGSALY System, finally became aware of a PCM’s use. In the 1950’s, the pulse code modulations used cathode-ray coding tubes with grids that have encoding perforations. Like the oscilloscope, the beams of these modulations were horizontally swept at sample rates while the vertical deflections were controlled by input analog signals, causing the said beams to pass through either lower or higher portions of the grids that are perforated. The grids generally interrupted the beam, thus producing current variations in binary codes. Rather than those natural binary codes, the grids were perforated so that they can produce Gray codes along transition zones that produce glitches.

The word “pulse” from the Pulse Code Modulations is a bit confusing because no pulses appear anywhere except on the transmission lines. This is actually a natural consequence from these techniques, evolving along two other analog methods called the Pulse width modulation and the Pulse position modulation where the information that will be encoded are represented by discrete signal pulses with varying widths and positions. With this, the Pulse code modulations bear a little resemblance to the other forms of encoding signal, except for the fact that all can actually be used in division multiplexing. Also, the binary numbers from the Pulse-code modulation codes are generally represented as different electrical pulses. The device, likewise, that performs coding and decoding functions of telephone circuits are called codec.

Learning about pulse code modulation is very confusing but if you are driven and resourceful enough to learn, there are actually a lot of sources where you can gather vital details about a PCM.