PCM Audio in Home Theater

Audio speaker
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PCM stands for pulse code modulation. However, what does it mean and is it important?

PCM technology is a means by which analog audio signals (which are represented by waveforms) are converted to digital audio signals (which are represented by 1's and 0's—much like computer data) with no compression. This allows the recording of a musical performance or movie soundtrack in digital form, which can fit in a small space (compare the size of CD to a vinyl record), while still approaching the quality of the original performance.

PCM Basics

The PCM analog-to-digital audio conversion process can be quite complex, depending on what content is being converted, the quality needed or desired, and how the information is stored, transferred, or distributed. However, here are the basics.

A PCM file is a mathematical interpretation of an analog sound wave. The goal of the PCM process is to replicate the properties of an analog audio signal as closely as possible.

The way the analog-to-PCM conversion is done is via a process called sampling.

As mentioned above, analog sound moves in waves, while digital audio (including PCM) is a series of 1's and 0's. In order to capture analog sound using PCM, specific points on the sound wave must be sampled (frequency). In addition, how much of the waveform is sampled at the given point (bits) is also part of the process. More sampled points, and larger pieces of sound wave sampled at each point, means more accuracy on the listening end.

For example, for CD audio, an analog waveform is sampled 44.1 thousand times per second (or 44.1kHz), with points that are 16bits in size (depth). In other words, the digital audio standard for CD audio is 44.1kHz/16bits.

PCM Audio and Home Theater

One type of PCM, linear plus code modulation (LPCM), is used in CD, DVD, Blu-ray Disc, and other digital audio applications.

In home audio and home theater, LPCM (usually referred to as just PCM) is applied in the following ways.

In a CD, DVD, or Blu-ray Disc player, a PCM signal is read off a disc and can be transferred in two ways.

One way is by retaining its digital form and sending that signal to a home theater receiver via a digital optical, digital coaxial, or HDMI connection. The receiver then converts the PCM signal to analog so that it can be sent through the amplifiers and on to the speakers. The reason the PCM signal has to be converted to analog is that the human ear works by hearing analog audio signals.

The second way is for the CD, DVD, or Blu-ray Disc player to convert the PCM signal back to analog form internally, and then transfer the re-created analog signal to a home theater or stereo receiver via standard analog audio connections. In this case, the stereo or home theater receiver does not have to perform any additional conversion steps in order for you to hear the sound.

Most CD players only provide analog audio output connections, so it is required that the PCM signal on the disc be converted to analog by the player internally. However, some CD players (as well as almost all DVD and Blu-ray Disc players) provide the ability to transfer the PCM audio signal directly using the above-mentioned digital optical or digital coaxial connection option.

In addition, most DVD and Blu-ray Disc players can also transfer PCM signals via the HDMI connection.

PCM, Dolby, and DTS

Another trick that most DVD and Blu-ray Disc players can do is to read undecoded Dolby Digital or DTS type audio signals. Dolby and DTS are digital audio formats that use coding that compresses the information in order to fit all the surround sound audio information digitally onto a DVD or Blu-ray Disc. Usually, undecoded Dolby Digital and DTS audio files are transferred to a home theater receiver for further decoding to analog—but there is another option.

Once read off the disc, many DVD or Blu-ray Disc players can internally convert Dolby Digital and DTS signals to uncompressed PCM, and then either pass that decoded signal directly to a home theater receiver via an HDMI connection, or convert the PCM signal further to analog for output via two or multichannel analog audio outputs to a home theater receiver that has the corresponding compatible inputs.

However, there is a twist. Since a PCM signal is uncompressed, it takes up a lot of bandwidth transmission space. So, if using a digital optical or coaxial connection, there is only enough room to transfer two channels of PCM audio. For CD playback that is perfectly fine, but for Dolby Digital or DTS surround signals that have been converted to PCM, you need more capacity. HDMI provides the solution, as it can transfer up to eight channels of PCM audio.

For more on how PCM functions between a Blu-ray Disc player and a home theater receiver, refer to Blu-ray Disc Player Audio Settings: Bitstream vs PCM.

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