Any discussion of digital audio must first consider two issues that affect sound quality, noise and timing.

Noise in electronics is an extraneous voltage or current that corrupts an electronic signal.  There are very many internal and external noise sources that affect electronic equipment.  As a result much of the work in audio equipment design involves the avoidance and reduction of noise and its impact on sound quality.

Accurate timing is critical to all aspects of digital audio playback.  Recall that a digital audio signal is produced by sampling the original sound at precise instants.  Timing errors in sampling makes the digital signal an inaccurate representation of the original signal.  Timing errors in handling the digital signal and in digital to analog conversion also reduce the quality of the reproduced sound.  The circuitry responsible for timing is called a clock, and the quality of clock signals has a major effect on the sound quality of digital audio.

When you play your digital music the digital audio data must be converted into an analog signal and delivered to an amplifier which drives the loudspeakers to produce the music.  This requires three main components:

• Transport:  This is involved in retrieving the digital data from where it is stored or located and conveying it to the digital to analog converter (DAC).
• DAC:  The DAC converts the digital data to an analog signal.
• Analog Circuitry:  This circuitry takes the analog signal output by the DAC and delivers it to the rest of the audio system (amplifier and speakers)

Each of these components can affect the quality of the final sound and there are a number of approaches for each.  They may be located within a single piece of equipment, like a CD player, or they may be implemented in a system of multiple, specialized devices.

Let’s look briefly at each component, leaving more detailed discussion for later posts.

Transport:  Digital music data may be stored on a disc like a CD, or on a local hard drive, or somewhere on a computer network.  Whatever the source of the digital music data may be, the function of the transport component is the same:

To deliver the data to the DAC:

• Free of errors
• Without extraneous noise
• With precise, correct timing

There is a wide range of digital audio transports, including:

• An internal CD drive or CD-ROM drive used in a CD player to retrieve the digital audio data from a CD and deliver it to the CD player’s internal DAC.
• A dedicated CD transport, which is a stand-alone piece of equipment which uses a CD or CD-ROM drive and other circuitry and provides a digital audio signal for connection to an external DAC.
• A streaming music player which receives music files over a computer network and converts them into digital music signals for conversion by an internal or external DAC.

DAC:  The DAC must accept the data from the transport and convert it into an accurate analog copy of the original sound.  The DAC is at the heart of digital audio playback.  It may be a implemented as;

• Circuitry within an electronic chip in a piece of equipment.
• A DAC integrated circuit with other associated circuitry.
• A stand-alone piece of equipment with digital input interfaces and analog output interfaces.
• System consisting of multiple pieces of equipment including external clocks and external power supplies.

Analog Circuitry:  The DAC output is a low level analog signal.  The analog circuitry takes this signal, may possibly filter and amplify it, and makes it available to the amplifier without distortion or the addition of noise.

© Wayne Butcher