Chapter - 1

Key Words: signalsaudio signalsdigital audioanalog

Signals

🎯 Learning Objectives:

This topic explains the fundamentals of digital audio, how audio signals are expressed in the digital domain, how they're converted and transformed, and the limitations of working with digital signals.

Background and History

The world of audio processing has changed dramatically over the last few decades. What used to be an expensive and time-consuming discipline — recording on magnetic tape, applying effects with fixed hardware, physically cutting tape to edit — has been largely replaced by flexible digital workflows.
With the arrival of personal computers and digital audio converters, analog pressure waves could be represented as numbers.
That abstraction unlocked powerful editing, non-destructive processing, and exact replication.
However, converting between physical, analog, and digital domains still carries costs and limitations that engineers must understand and manage.

Signal domain illustration
Figure 1. Signal domains: physical, analog, and digital.

Signal Paths

A signal path describes the sequence of conversions and processing a sound undergoes from source to listener.
We commonly consider three domains:

  1. Physical — acoustic pressure waves moving through air.
  2. Analog — continuous electrical voltages (microphone output, analog mixers).
  3. Digital — discrete-time numeric representations (samples/words stored in memory).

Every time a signal crosses between domains (physical ↔ analog, analog ↔ digital) a conversion occurs (microphone, ADC, DAC, speaker).
Conversions introduce limitations: dynamic range, quantization noise, aliasing, latency, and bandwidth constraints.

Minimize unnecessary domain crossings when possible. For example, keep processing in the digital domain when you already have the signal captured digitally.
Each conversion potentially degrades quality (though modern converters are extremely good).

Audio Processing

Audio processing modifies signals within a domain. Typical operations include:

  • Amplification
  • Filtering (frequency-selective attenuation/boost)
  • Delay / Reverb
  • Dynamic range processing (compression/limiting)
  • Time-stretching and pitch-shifting

Many digital processors are effectively lossless — their numerical operations can be reversed. However, conversion steps (ADC/DAC) are where inherent information loss and noise are introduced.

🧪 Interactive Examples

📝 Key Takeaways

  • Audio exists in physical, analog, and digital domains — conversions are required between them.
  • Each conversion (mic/ADC, DAC/speaker) can introduce noise, latency, and limits (dynamic range, aliasing).
  • Prefer fewer domain crossings and use appropriate sampling/bit-depth to preserve quality.

🧠 Quick Quiz

Test your understanding - select and submit an answer.

1) Which device converts acoustic pressure waves into an electrical signal?

2) The Nyquist criterion states the sampling frequency must be at least:

3) Which of the following is typically considered a random signal?

4) What happens when you sample below the Nyquist rate?

5) Which two conversions happen when recording and then playing back audio through a computer?

References: