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The Right Match: Is “Digital” Cable Required For Digital Audio Interfaces?

Going beyond opinions and theories to weigh out the pros and cons of those "special" cables...

The audio interface provides the means of getting the signal from one component to the next. It includes the source (output) circuit, the cable, and the load (input) circuit.

There are consumer (2-wire, unbalanced) and professional (3-wire, balanced) versions, the major difference being robustness against electromagnetic interference in its various forms (Figure 1). Does a digital audio interface require a digital-grade audio cable?

Since increasing the cable length can have a detrimental affect on both analog and digital audio signals, the use of balanced interfaces is mandated for larger systems. A cable contains multiple wires, or conductors; those designed for an unbalanced interface have two conductors, one of which is a shield, while those designed for a balanced interface have three conductors consisting of a twisted-pair and shield (commonly called “shielded twisted pair” or STP for short).

For this reason, the cable itself may be referred to as unbalanced or balanced. I’ll limit this article to a discussion of balanced interfaces and cables. Do balanced analog and digital interfaces require different cables? First let’s lay some groundwork.

Figure 1 – Unbalanced and balanced audio interfaces.

Analog Audio

An analog interface passes the signal as a time-varying voltage, the shape of which must be preserved by the interface, which includes the cable (Figure 2). Assuming that the input and output circuits are good designs, that leaves the cable as a source of signal degradation.

Cables have electrical properties, and capacitance tends to be the major one that can affect signal quality. Cable capacitance is a property that shorts out the signal with increasing frequency, resulting in a loss of the signal’s high frequency content.

While that sounds devastating, audio interfaces for both consumer and professional products are designed to push the effects of cable capacitance beyond the audible frequency range.

If decent cables are utilized and kept to a practical length, audio should not be affected. But the cable effect is not zero, and there is no consensus on the high frequency limits of human hearing. This opens the door to claims that “special” cables are needed to preserve the fidelity of the analog signal.

Figure 2 – An analog audio waveform has a complex shape that must be preserved by the interface.

Digital Audio

The digital signal consists of packets of data that are streamed down the cable and recovered at the load (input). The AES3 digital format consists of two channels of audio data traveling one way on a STP cable. As with a balanced analog interface, the cable shield serves only as a shield and is not required for passing the signal between source and load.

While the analog signal can take on a unique value at any specific instant in time, there are only two states for the digital signal. A “bit” is either a zero or it’s a one. The objective is to preserve this distinction at the digital input. Cable capacitance still exists, and can reshape the signal by rounding the transitions between the bits (Figure 3).

Figure 3 – A digital waveform. The information is carried by the transitions, so minor rounding of the waveform does not reduce the data integrity.

A major difference between the analog and digital signal is the bandwidth, or frequency range. Analog audio waveforms contain useful information to about 20 kHz. Digital signal bandwidth extends well into the MHz (mega or millions of Hz) region. This is necessary because the data only has two states, so a lot of bits have to flow to carry the information from the original analog waveform.

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The high frequency losses due to cable capacitance tend to round the otherwise sharp edges of the digital waveform. Under normal circumstances this rounding has no effect on the information carried by the signal, because the zero or one is actually determined by the transitions, or zero-crossings. This means that the digital signal is far “less corruptible” by the wire than the analog signal, because it’s not audio, it’s data. This is a major advantage of digital audio over analog.

The take-away is that a cable isn’t analog or digital, it’s just a cable that has impedance characteristics that can reshape the signal that travels down it. Both analog and digital waveforms are affected by the cable’s capacitance as the cable length is increased. Both interface types are very forgiving cable-wise at short distances.

Added Considerations

The all-or-nothing nature of digital audio can be illustrated with the eye pattern test (Figure 4). A special analyzer overlays thousands of bits. The effects of noise and jitter reduce the size of the “eye” but as long as it is larger than the AES3 minimum, the signal integrity will be preserved. Since cable capacitance is cumulative, at some length the high frequency roll-off affects the integrity of both analog and digital signals.

Figure 4 – A rounded digital pulse looks like an eye. If the eye collapses too much, data integrity is lost.

Audio industry-specific digital audio formats, such as AES3 and SPDIF, were actually designed to work with the cables used for analog interfaces.

A “good old 25-foot mic cable” can pass digital audio without degradation, and the digital signal does not require a special cable (Figure 5).

If a sound system has a mix of analog and AES3 digital devices, one can use the same cable for each, as long as they route analog outputs to analog inputs, and digital outputs to digital inputs.

Simple is good, and it’s what the pioneers had in mind when they invented both professional and consumer digital audio interfaces and formats.

Figure 5 – A 25 ft. analog-grade mic cable (top) performs as well on the eye test as a digital-grade STP (bottom).

Ironically, the effects of excessive length can be worse for a digital signal than an analog signal. Let’s stretch the rules for both and consider a cable run of 1,000 feet (300 meters).

The analog signal will exhibit some high frequency loss, which depending on the program material may result in a slightly “muddier” sound.

The digital signal may sputter or stop completely if the distinction between the one and zero is lost.

We’ve all experienced this when comparing digital television with analog. A degraded analog signal is still “watchable” while a degraded digital signal produces lock-ups, blotches, and other kinds of chaos. “Digital,” whether audio or video, is pretty much an all or nothing proposition.

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