What Is Pulse Code Modulation (PCM)? How Does It Work?

Pulse code modulation (PCM) brings digital audio to life. Phones, televisions, and computers all use PCM in some way to deliver crisp, high-resolution sound. Keep reading to learn precisely how this process works, alongside its significant advantages and disadvantages.

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The Importance of Modulation

Modulation is when a message signal modifies a carrier wave to aid long-distance transmission. This means organizations could send signals with smaller antennas, among other benefits. The PCM format has two main categories: analog and digital.

Analog modulation includes:

• Amplitude modulation, where the wave’s height matches the message signal.
• Frequency modulation, which changes the wave’s cycles per second instead.
• Phase modulation, where the wave’s starting point depends on the message signal.

A Closer Look at Digital Modulation

In digital modulation, the carrier waves contain digital data. As with analog, changes to the wave or signal lead to different types of modulation, including:

• Pulse amplitude modulation, which again involves changing the wave’s height.
• Pulse width modulation, where the width changes to fit an analog signal’s height.
• Pulse code modulation, which transforms the analog signal into binary code.
  
  
  

  What Is Pulse Code Modulation?

  Pulse code modulation, or PCM, is a widespread way of encoding analog signals into a digital format. PCM is actually the standard format for digital audio. Modern telephone systems, studio recorders, DVDs, and more rely on PCM.

  Businesses and individual users alike utilize PCM every day. For example, conference calls using AV over IP implement PCM audio due to its high fidelity.

  The Types of Pulse Code Modulation

  Several pulse code modulation varieties build upon PCM’s foundations. The end result is a more efficient signal with a reduced data rate. Here are the two main PCM sub-types:

  Differential Pulse Code Modulation (DPCM)

  DPCM only encodes the difference between individual signal values, not the entire signal. This still represents the full signal but with fewer samples, letting it shed data redundancies.

  Adaptive Differential Pulse Code Modulation (ADPCM)

  This adjusts the “gap” between values to capture more details while still reducing the bitrate. In doing so, ADPCM creates a more efficient signal at a higher quality.

  What Is the Pulse Code Modulation Process?

  PCM follows three key steps: sampling, quantization, and encoding. Here’s a closer look at each one:

  1. Sampling

  This step converts continuous-time analog signals into discrete-time ones. It samples the signal regularly at a relatively high frequency. This must be twice the signal’s highest frequency to limit distortion.

  2. Quantization

  Quantization then changes these sampled values into amplitude levels. Uniform quantization is when the levels have even spacing. Non-uniform quantization, however, can give higher-quality audio at a smaller bit depth.

  3. Encoding

  Encoding transforms the quantized information into a digital binary format, making storage and transmission easier. It also limits noise susceptibility and ensures the signal works with digital systems.

  

  (Source: commons.wikimedia.org)
  
  
  

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  PCM Process Details

  Pulse code modulation involves many moving parts. Each one is essential to the overall process and allows for a higher-quality output. PCM’s main components include:

  1. Low Pass Filter

  The LPF removes high-frequency components to limit aliasing and distortion. It ensures only the most legible samples remain, leading to crisper audio.

  2. Sampler

  A sampler measures a signal’s amplitude at the defined sampling rate. According to the Nyquist theorem, this rate should be twice the highest frequency.

  3. Quantizer

  Quantizers map a sample’s amplitude to the nearest discrete level. Though this allows for easier digital representation, it also introduces potential quantization noise errors.

  4. Encoder

  The encoder converts quantized values into binary data. The file’s levels each receive their own binary code, which digitally represents the signal’s components.

  5. Regenerative Repeater

  When transmitting PCM signals long-distance, a regenerative repeater amplifies it and removes noise. By interpreting the original binary data, it creates a clean signal.

  6. Decoder

  A decoder does the opposite of an encoder; it turns binary data back into quantized values. The decoding process is essential for playing the file.

  7. Reconstruction Filter

  After decoding, a reconstruction filter changes the quantized signal back to analog. The filter will ensure the resulting signal is suitable for playback.

  What Are the Advantages of PCM?

  PCM is commonplace because of its many advantages. These include:

  • Noise immunity: PCM signals are more resilient to distortion and noise. A regenerative repeater ensures the original signal quality remains.
  • Flexibility: PCM is incredibly versatile and commonplace. The format’s standardization makes it compatible with audio and video technology.
  • Efficiency: Differential and adaptive differential PCM offer better compression. The end result is a smaller file without significant quality losses.
  • High fidelity: PCM is a critical component of high-definition audio. With a good sample rate and quantization resolution, the signal always remains strong.

   

  What Are the Disadvantages of PCM?

  However, the format is far from flawless. Here are a number of limitations present with PCM:

  • Bandwidth requirements: PCM needs a high sampling rate to guarantee a crisp signal. The format takes up a lot of bandwidth to achieve this.
  • Complexity: The PCM process covers several complicated stages. Signals will naturally take longer to process, possibly leading to minor delays.
  • Quantization errors: Pulse code modulation isn’t free from errors. Amplitude issues can lead to a lower-quality signal, especially with low resolution.
    
    
  

  
  Where and How Do We Use Pulse Code Modulation? 

  PCM is incredibly commonplace, especially in audiovisual technology. It helps digitize telephone voice signals, making it essential for mobile networks and Voice over Internet Protocol.

  A CD’s uncompressed audio also uses PCM with 16-bit quantization alongside a 44.1 kilohertz sample rate. Streaming services transform this into MP3 files for easier distribution. Major video compression formats (such as MPEG and H.264) similarly contain PCM audio.

  Even hearing aids and ultrasound machines use PCM to digitize their signals. Any industry that uses sound could implement pulse code modulation for greater quality and flexibility.

  Final Thoughts

  Pulse code modulation is vital for modern, high-quality audio systems. Much of the digital audio you hear each day depends on PCM. Despite its limitations, PCM is still highly versatile, making it invaluable for AV equipment across many industries.
  
  

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