Welcome to this tutorial on digital audio signals. Before we start, and to ensure there is no confusion, I will be referring to non-optical signals as digital electrical or coaxial. The term coaxial refers to a simple cable with an inner core and earth shield braided around it.

Digital audio signals consist of a data stream that represents line level signals in binary code. They therefore cannot be said to be audible until they are converted, or decoded, back to an electrical signal. The process of converting an electrical line level audio signal to digital is known as analogue to digital conversion, or A to D. The process of converting an digital audio signal to electrical line level is known as digital to analogue conversion, or D to A.

Caption - Digital audio signal types

There are 4 primary digital audio interconnect signal types used in the project and home studio. They are ..

• ADAT light-pipe optical carrying between 2 and 8 channels
• SPDIF optical carrying a stereo pair
• AES3 electrical carrying 2 channels
• SPDIF coaxial, an electrical version of SPDIF optical

Caption - Optical and electrical (coaxial) signals

Digital audio signals can be divided into 2 categories, optical and electrical.

Caption - Digital optical signals

Optical signals are transmitted with fibre optic Toslink cables. The signal is sent as light and is therefore inaudible until converted by a receiving device. They are commonly used to send or receive one of 2 optical signal formats.

Caption - 1. ADAT optical (light-pipe)

The first format, known as ADAT light-pipe, can carry up to 8 channels of audio in a single data stream. It is used for a wide variety of professional and semi-professional applications, such as connecting a digital mixing desk to a DAW via an audio interface. Another popular application is for connecting multi-channel analogue to digital audio converters to audio interfaces.

Demonstration This is an 8 channel A to D converter, which can be fed with any line level signal. It's ADAT light-pipe output is connected to an audio interface from which signals can be sent to a DAW to be recorded.

Caption - 2. SPDIF optical

The second optical signal format, known as SPDIF optical, carries a stereo pair, and because it was designed to interconnect hi-fi devices, can be found on CD players and digital amplifiers. Although its origins were in consumer electronics, SPDIF quality is perfectly acceptable for professional applications and there is no obvious quality difference between it and a professional digital audio format. It has been adopted by many manufactures as an affordable way of enabling digital interconnection on their devices.

Caption - Did you know? (non-essential but interesting fact)
The Toslink cable was invented by Japanese electronics company Toshiba for connecting their hi-fi products, but the two most popular current signal formats it carries were developed by others. Alesis created the ADAT format for connecting their revolutionary 8-track recorders, and Sony and Phillips collaborated to create an optical version of their Sony Phillips Digital Interface Format, SPDIF.

SPDIF is an acronym for Sony Phillips Digital Interface.

Caption - Digital electrical (coaxial) signals

Digital electrical signals are pulse waves at ‘computer’ level, and to all intents and purposes inaudible. Too weak to drive a loudspeaker or headphone, even if you could hear one you would not hear the encoded soundwave, just the digital 'chatter' of the binary code.

There are 2 common electrical formats for digital audio ..

Caption - 1. AES3 electrical (coaxial)

AES3 is widely used for professional applications, in particular the digital interconnection of 2-track recorders and outboard processors to digital mixing desks and audio interfaces. AES3 signals are balanced, 2 channel, or stereo, and require a 3-core cable and connector.

Caption - Example AES3 enabled device
This is a microphone pre-amplifier with a built-in A to D converter. The digital output is being sent directly to an audio interface via an AES3 connection.

Caption - 2. SPDIF electrical (coaxial)

The second format is SPDIF coaxial, an electrical version of SPDIF optical. Due to its origins as a consumer hi-fi interconnect, SPDIF coaxial is a lower level unbalanced version of AES3, but in all other respects quality is identical.

Caption - AES10 / MADI (Multichannel Audio Digital Interface)

SPDIF, ADAT, and AES3 are not the only formats available, there are others but they are not commonly used in home and project studios. Nevertheless, it's worth saying something about MADI.

Essentially MADI is a multi-channel version of AES3, capable of 28, 56 or 64 channels over a single BNC coaxial or fibre optic connector. Cables can be 100's of meters long so it has found favour in broadcast, live sound and large studio complex installations. MADI does not carry a word clock signal so a separate BNC sync connection must be used.

Caption - Lead lengths

Optical, coaxial and AES digital audio leads are limited to relatively short lengths, of between several meters and several 10's of meters, but this is not usually an issue in home and project studios. If greater channel counts and distances are required, formats such as MADI and audio over IP can be used.

Caption - Did you know? (non-essential but interesting fact)
Digital computer level signals are actually pulse waves, turning on and off to represent 1s and 0s. In raw unconverted form they can be amplified to audible line level if they need to be sent down an analogue audio cable. Examples are phone modems and analogue SMPTE synchronisation signals. Essentially, a ‘computer’ level signal is amplified to allow it to be transmitted over a phone line or recorded by an audio recorder such as a multi-track tape machine. Pulse waves are also created at audio level by synthesiser oscillators and are a useful waveform for a range of sounds.

Caption - Elevated sample rates

The quality of digital audio is defined by 3 primary factors. They are ..

• sample rate
• word length
• and whether the data has been compressed or not, and if so by how much

In the project and home studio most owners record, process and mix using ..

• a sample rate of 44.1KHz or 48KHz
• 24-bit word length
• uncompressed .aiff or .wav files

The ADAT, AES and SPDIF formats allow for elevated sample rate modes, typically 96KHz, but because for many the quality improvement is negligible, and the burden on CPUs and hard discs is doubled, they are rarely used by project and home studio users. These modes are called, Dual-wire and S-mux.

Caption - Did you know? (non-essential but interesting fact)
SPDIF coaxial signals can easily be converted to optical with a device such as this. This is useful if you have 2 devices you need to connect such as this CD player and this audio interface.

Caption - Combining digital signals types

In this tutorial we have talked about 4 primary types of digital audio signal.

• ADAT light-pipe optical carrying between 2 and 8 channels
• SPDIF optical carrying a stereo pair
• AES3 electrical carrying 2 channels
• SPDIF coaxial, an electrical version of SPDIF optical

An interconnected system can utilise all these signal types simultaneously provided they share the same sample rate. Although not always necessary, its is also good practice to ensure they are all at 24-bit.

Caption - Thanks for watching

The script for this video, with accompanying images, can be found at projectstudiohandbook.com 

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