AUDIO INTERFACES – A PRACTICAL GUIDE

For users who want to know more on how to optimally connect the sound source to the amplifier – we have prepared an audio interface guide. It will tell you what the most popular types of audio interfaces are and – importantly – what to look at when picking one, i.e. what audio connector to choose depending on the source you use. We will also show you our own solution, the SAI link.

Audio Interface – What Is It and What Is It Used For?

The audio interface is used to connect the signal from the source output (e.g. CD player or TV) to the power amplifier input. To explain the functioning of the interface, we will use a simplified description of layers. The audio interface consists of three basic layers:

• hardware layer,

• connection layer,

• transport layer.

The hardware layer describes what type of connector (plug and socket) and what type of carrier (electric or optical fibre) are used within a given interface. 

The data link layer refers in turn to the method of signal transmission by the carrier and determines whether the interface is analog or digital. By the way, let’s clarify that an analog interface is one where the transmitted signal corresponds directly to the acoustic wave reaching the listener's ear, while in a digital interface the transmitted signal is converted to numbers is transmitted as zeros and ones.

The transport layer is present only in digital interfaces. Usually it’s quite complex. It defines how devices compete for a link (which device and when has the right to transmit a signal), how data is divided into packages, and whether and how data is protected against loss, e.g. in the event of interference.

Digital Interfaces – Pros and Cons

As a rule, digital interfaces are much more resistant to interference or loss of transmitted signal compared to analog interfaces. However, in order to use their advantages to the fullest, an analog signal needs to be digitised. Note that if this process is not performed properly, it may degrade the sound quality. Nevertheless, analog-digital processing techniques have matured by now and practically all musical signals are digitised right at the first stage of their recording. A doubtless advantage of digital interfaces is the additional transport layer, which means that in the event of damage, the data can be resent and then recreated in the correct order – exactly as if the disturbance had not occurred. A drawback of digital interfaces is the occurrence of so-called jitter, which is phase noise that, in the event of transmitter and receiver desynchronisation, may cause loss of signal. However, we would like to point out that this issue only applies to the simplest interfaces transmitted without signal synchronisation or to those that do not have a built-in data recovery mechanism.

Interfaces – Common Types Used To Transmit Audio Signals

Here we need to consider what you need as a user and what is most important to you when choosing the right connection. There are several common interface types, and each of them has some peculiar features, properties, advantages and drawbacks. Here are the most popular solutions available:

Analog interface with RCA connector (aka CINCH)

This is the most popular interface, not resistant to interference, suitable for transmitting signals over short distances. Two RCA connectors for the right and left channel are used to transmit the stereo signal. The analog electrical signal typically has an amplitude of 1 Vrms, or 2.82 Vpp. Phono inputs have significantly lower sensitivity, for which the selection of connection cables has a particular impact on the sound of the audio set.

Analog interface with XLR connector

Interference-resistant differential analog interface sometimes used in top-end home audio equipment. Two XLR connectors are used to transmit the stereo signal at a distance of up to several dozen metres. The analog electrical signal typically has an amplitude of 1 Vrms, or 2.82 Vpp.

SPDIF digital interface with RCA connector (CINCH)

The simplest digital interface, very common, provides fairly high resistance to interference, but is not resistant to jitter and does not have a mechanism for recovering disrupted data. One RCA connector is enough to transmit a high-quality (or multi-channel but compressed) stereo signal.

SPDIF digital interface with XLR connector

Interference-resistant digital interface sometimes used in top-end home audio equipment. High resistance to interference is achieved through differential signal transmission. Not resistant to jitter. One XLR connector is enough to transmit a high-definition stereo signal (or multi-channel but compressed) at distances of up to several metres.

SPDIF digital interface with TOSLINK connector (optical)

The basic digital interface, very common, provides very high resistance to interference, but not resistant to jitter. One TOSLINK connector is enough to transmit a high-definition stereo signal (or multi-channel but compressed).

USB interface

A highly interference-resistant interface, perfectly suited for transmitting a sound signal of very good quality from the computer. If the interface is in asynchronous mode, it is resistant to jitter. Data can be transmitted at distances of up to several metres (USB standard specification limits the length of the cable to 5 m, but there are so-called active cables available, which can transmit a signal to much larger distances).

Bluetooth

A popular wireless interface supporting audio signal transmission of limited quality. Currently, all standards of audio data transmission via the Bluetooth interface use audio data compression, but thanks to high popularity, the interface is dynamically developing and its output signal has an increasing quality. However, it is not true to the original.

HDMI

The popular interface used in TVs and video players allows you to transmit high-definition digital audio signals in the LPCM (Linear Pulse Code Modulation) standard. It also has an additional VESA EDID communication channel, which allows for exchange of information and mutual configuration of connected devices (the transmitter can adjust signal parameters, e.g. number of audio channels, to the capabilities of the receiving device). By default, the audio signal is transmitted only from the HDMI output to the HDMI input, but if the sockets support the ARC (Audio Return Channel) or eARC standard, it is also possible to transmit audio data in the opposite direction (e.g. from the TV input to the amplifier). The HDMI interface is very resistant to interference thanks to differential data transmission and a built-in error correction mechanism. The length of the HDMI cable can reach about 15 m, it is also possible to use "active" cables of greater length. Generally, the data transfer standard used in the HDMI interface is resistant to jitter, but the length of the cable has a negative impact on the quality of the output signal.

Synchronous Audio Interface (SAI) – Solidele's proprietary solution

The connector developed by our company is distinguished by the fact that it allows you to connect multiple power degrees to a single amplifier using a shielded RJ45 socket and an Ethernet CAT6 cable. Its key quality is that the SAI interface is extremely resistant to interference and jitter. It enables lossless transmission of stereo audio signal in digital form at distances of up to 300 m. However, the distance between successive power stages must be no greater than 50 m. Furthermore, the SAI interface has an additional communication channel for mutual data transmission, enabling control and configuration and software upgrades in the connected power degrees.

Audio Interface – What To Choose?

Remember that you pick the method of connecting the audio signal depending on the source. The computer is best connected to the amplifier via the USB interface (especially if the amplifier USB port supports asynchronous audio data transmission mode). Connect the TV using an HDMI connector supporting the ARC/eARC standard or an optical TOSLINK SPDIF. To connect other sources playing the signal from a digital carrier, such as a CD player, decoder or console, use TOSLINK SPDIF (preferably optical). On the other hand, pick analog interfaces when using analog sources, such as a turntable or a spool tape recorder.