What is Digital Signal Processing

What Is Digital Signal Processing (DSP)? And What Does It Mean For Music?

Digital Signal Processing, or “DSP”, is often referred to as a “feature” of modern audio equipment and music gear. Why is that important? Well, two reasons. One, it tells you how they’re handling the audio signal – and the potential trade-offs which that implies. And, two, hopefully they give you confidence in how well they’re using DSP – after all, better processing equals better results.

So What is DSP? Particularly In Audio Processing?

DSP stands for Digital Signal Processing. This term is used for digital encoding of “live” signals such as audio, video, temperature, pressure, position, etc. Digital signal processing allows these live signals to be stored, manipulated, edited, replayed, and transferred much more efficiently and accurately than by using strictly analog methods. For example, think how easy it is to edit video files in iMovie (or even on your phone!) vs cutting and re-attaching film like in the old cutting room.

Sound is naturally an analog phenomenon.  Sound is a function of physical waves traveling through a medium – air or water, or even a hard surface (like putting your ear to a train track to hear the train coming).

Microphones convert this physical sound wave into an analog electronic signal. They do this by use of a small, sensitive diaphragm that vibrates when hit by audio waves. The vibrations of the diaphragm generate a very small, uinque, electronic signal. This signal is music.

Like all electrical signals, it is a flowing sine wave, moving between zero or negative voltage and some other voltage. The rate, frequency, depth, and tiny aberrations of this signal are what make this electronic signal sound uniquely like your voice, or your guitar, or your violin.

 

Sound wave forms - audio signals

Note: Electric guitar pickups function differently. These pickups take advantage of Faraday’s discovery that moving a metal rod through a magnetic field (or a magnetic field over a metal rod.  As Einstein famously pondered… is there any difference?) produces an electric current.  It turns out that when an guitar string (a metal rod) vibrates in the magnetic field of a guitar pickup, a unique electric signal is also produced. The rate, frequency, depth, and tiny aberrations of this signal are what make it sound uniquely like your Telecaster or Les Paul.

The Ups and Downs of Analog

Analog transmission of audio signals require that power amplification and receiver sensitivity each be addressed so that the signal is communicated efficiently. This is because microphones, guitar pickups, and general audio equipment do not put out a strong enough electrical signal to drive a speaker system (such as a guitar amp).  So an electronic amplifier magnifies the signal. The amplified signal is necessary in order to drive the speaker cone to vibrate strongly enough to be heard.

Processing electronic audio signals like this has both advantages and disadvantages:

Advantages

  • An analog signal transmitted this way has the highest fidelity. That is why electric guitars through traditional amps are still the “go to” for recording great guitar sounds.
  • Lower system complexity. This may be arguable (especially among guitarists who have “blown” a tube on a traditional amp), but analog signals are inherently easier to deal with. Software menus, storage devices, encoding errors, OS updates…none of this affects a classic Marshall tube amp.

Disadvantages

  • Analog signals do not transmit easily over long distances. Amplifiers are required to continually boosted.
  • Analog signals are not easily stored. Wax cylinders, vinyl records, and magnetic tape have all been used to store analog music over the years. The process of recording and retrieving the information is generally cumbersome.
  • Analog storage tends to degrade over the years.
  • Manipulating or editing analog audio files is difficult.  They must be played in “real time”, with effects or edits applied during playback, and re-recorded to create the new version. There are, of course, advanced methods for this process, but all are time consuming.

How DSP Handles Audio

Digital Signal Processing converts analog audio signals to a digital stream of binary bits. This digital stream can be transmitted, stored, and modified much more efficiently – with higher speed and quality.

A helpful illustration from the brilliant folks at www.analog.com.

Digital Signal Processing is generally divided into three parts: audio-to-digital conversion (ADC), digital-to-audio (DAC) conversion, and a distinct ‘digital signal processer’ (typically a single microchip).

  1. Audio-to-Digital conversion (ADC) takes incoming analog signals and converts it to a series of binary data points.  In goes electric guitar cable, out comes a string of 1’s and 0’s.
  2. Digital-to-Audio Conversion (DAC) takes the binary data that comprise the music signal (stored as a file), converts to analog, and outputs an analog electronic signal.  In goes 1’s and 0’s, out comes the electric guitar signal again.
  3. A Digital Signal Processor actually performs several functions. This processor includes Program Memory, Data Memory, a Compute Engine, and Input/Output functions.  For our purposes, we can simply say that this is the part of the DSP technology that manipulates the file. If you want to open or close the file, if software wants to access the file, or if you want to transfer it to a hard drive, or perform any of the many the functions that we routinely perform on a computer – the digital signal processor is the part that accepts and processes these commands.

Why is DSP Better?

DSP is ubiquitous in modern audio equipment.  This is because it provides several advantages over traditional analog signal processing, with only minor tradeoffs.

Advantages

  • Digital storage is better in many ways – cheaper, longer-lasting, easy to transfer, easy to duplicate, instantly accessible.
  • Faster, more accurate processing than analog. No need to re-play files in ‘real-time’, and all data is edited directly without the variability of intermediary playback and re-recording equipment.
  • Signals can be transmitted quickly and more efficiently from one place to another, even in real-time performance situations.
  • Signals may also be enhanced or manipulated to improve their quality or provide information that is not sensed by humans.

Disadvantages

  • You lose some data. Like audiophiles who prefer vinyl over mp3’s because there’s more audio “information” in the physical vinyl groove. Since DSP’s convert audio files into binary number representations, some audio fidelity can be lost in the process (the amount of loss can be minimal, depending on your file type and settings).
  • Many recording enthusiasts prefer the idiosyncracies introduced by analog recording and editing equipment. The unique sounds, glitches, and “feel” are all part of the desired sound.

Conclusion

Digital Signal Processing has become the standard for audio processing. While audiophiles and recording enthusiasts may prefer analog methods in many (or even all) situations, the market shows that the vast majority of people prefer the ease and efficiency of DSP for playing, recording, and listening to music.

 


More Reading:

http://www.analog.com/en/design-center/landing-pages/001/beginners-guide-to-dsp.html

http://www.powersoft-audio.com/en/technologies/dsp-technology

https://en.wikipedia.org/wiki/Audio_signal_processing

https://whatis.techtarget.com/definition/digital-signal-processing-DSP