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Digital Audio

Digital Audio for Hifi

Many hifi listeners have opted to use an external DAC, or digital to analog converter, instead of the one built into their CD player. The reason for this could be that they only have a CD transport hence the only output is digital or they wanted to improve the performance of the DAC stage in the player by using a newer or better external one. Of course not all external DAC’s are as good as internal ones, it depends on the design and componentry used. But the main purpose of this article is to look at different ways of connecting the CD transport to the DAC itself.

Types of Digital Outputs
There are three main types of digital outputs available on most consumer hifi audio gear: TOSLINK, coaxial and AES/EBU. The later was developed by the Audio Engineering Society and the European Broadcasting Union in the 1980’s and is mainly used in professional audio. Some very high-end domestic products use it but it is rare so we wont cover it here. New digital interfaces have also been developed such as HDMI (high definition multimedia interface) and DVI (digital visual interface) but these are currently used mostly for surround sound and video systems so we won’t cover them here. However expect these and even newer technologies to start appearing on hifi products in the future.

TOSLINK is an optical interface that uses mainly plastic optical cable. High-end versions use glass fibre and of course the more you spend, the better quality the construction and optical characteristics of the cable. It was developed by Japanese company Toshiba but can now be found on a huge range of hifi products. TOSLINK usually has a small square plug or folding cover installed to keep the lens free from dust. You can see it is operating by the red glow.

Coaxial is generally in the form of a standard RCA (or occasionally the better quality BNC) connector and uses coaxial wire as it’s interconnect. An RCA connector is the most common coaxial output connector. Again quality differences occur, as you go up a range, better quality RCA sockets / plugs are used and the cable has better quality materials used during construction.

How it Works
Audio that you hear is inherently analog. The digital audio data stored on a CD must first be converted to analog for your ears to be able make sense of it. The digital data stored on CD's is of course computer data, ones and zeroes. After it's read from the disc it is converted to SP/DIF (Sony - Philips Digital Interface Format). SP/DIF is a standard for exchanging digital audio information between different pieces of equipment and has become the defacto standard for domestic hifi. This signal format can be carried either by TOSLINK or coaxial cable.

General consensus is that coaxial is preferable over TOSLINK where possible for a few different reasons. Many people claim they cannot hear the difference, this could be due to a poor quality coaxial output using low speed parts and not the correct impedance (75 Ohm) matching also the system must be good enough to resolve the differences. So why does coaxial tend to be better, all else being equal?

TOSLINK utilises an optical connection which you would initially think would be better, but being often based on plastic optical cable it is prone to jitter (the timing of the digital to analog conversion becomes skewed). The electrical coaxial digital interface has lower levels of jitter. Also there is less conversion of the signal with coaxial, with optical it is converted from electrical to light then back again to electrical, with coaxial there is no such optical conversion as it remain electrical the whole way in most cases. (Some high-end coaxial interfaces use small special transformers to obtain the correct impedance.)

In Practice
There are optical to coaxial (and visa-versa) converters available but beware cheap versions that introduce huge amounts of jitter. It is not advisable to use a converter when a straight run is possible. You can always buy a better quality cable to improve the signal, especially if you are using an RCA cable intended for audio. They often don’t have the frequency bandwidth capabilities to allow the high-frequency digital signals through. Also keep the length in mind as the longer the cable, the great the signal loss.

So what differences can you expect to hear between different digital signals? Some say that digital is digital, no matter how it is carried. That may be true in regards to the actual data, the ones and zeros may be the same (assuming no losses from very poor quality, faulty or very long cables) but the timing differences between those ones and zeros are very important as well. It is these timing differences that are referred to as jitter. They can cause problems with the actual processing of the digital signal including the all-important digital to analog conversion.

The sound will typically have less staging, less clarity and definition and bass can be muddier. One trouble is measuring jitter, the equipment to measure it costs many thousands and is not easily available so it is difficult to know how much jitter you are experiencing. The main thing is to try and use the best digital cable you can and even experiment between optical and coaxial to see which you prefer. After all, every system is different and ever person hears things differently!

Written by Leon Gross, originally published in Audio & Video Lifestyle magazine.