Know-how

Shielded audio cables are the basis of every professional signal connection, right? Well, not necessarily. In fact, STAGETEC has found that an unshielded analog connection in a NEXUS-based system is less noise susceptible

There was a time when to be “digital” meant, without question, to be “good”. Nowadays, most users know that – even in the digital domain – the quality of the components, intelligent strategies, and clever circuitry layout are still important. And, in those places where analog signals are converted to digital data, and vice versa, such excellence becomes essential. Today, high-quality analog input and output stages are a key feature of NEXUS. When it comes to noise rejection, two of its features, extremely high common mode rejection and a ground coupling capacity of a few picofarads, are especially important, since conventional circuits produce significantly higher values. The result of this tiny capacity is a virtual absence of grounding, even for audio frequencies rather than just DC. The quality of NEXUS inputs and outputs is such that line cables without shielding can actually produce better audio specifications.

Various Kinds of Noise

So how is it possible to dispense with shielding or a ground connection? Most users believe that grounding is necessary as a screen against external noise. It is important to give consideration to noise produced by induction and capacitance as well as high-frequency noise. Inductive noise normally affects both signal lines within the cable and cannot be eliminated by a screen, so that shielding has no effect. The only way to minimize the effect of inductive noise is by reducing the distance between the signal lines – and to compensate the noise by twisting the conductors. In the case of capacitive noise, however, a shield can be useful. The external noise is then diverted towards the ground, thus keeping it from interfering with the signal. The drawback of this approach is that the ground differs at various points along the cable. Shielding provides less protection against high-frequency interference than might be expected. On the contrary, in some cases, the shield may actually act as an antenna with a capacitive connection to the signal conductors.

Capacitive Noise

Without shielding, the capacitive noise can go directly into the signal lines. Here again, keeping the signal lines close together – or working with twisted pairs – can cause both conductors to pick up the noise. In that case, the noise is summed on both wires and therefore becomes a common-mode signal. The desired signal, that is, the difference between balanced signal lines, however, is not affected, provided that the inputs and outputs have a sufficiently high common mode noise rejection. Should an external signal, nevertheless, get only into one of the signal lines, it is important to offer analog audio outputs with the lowest possible output impedance. That ensures that the noise is immediately coupled to the other signal line. The noise is then the same for both signal lines, and so cancels out again. This finding is not new: Stephen H. Lampen (Belden) discussed it in his presentation „Transporting Audio Signals on Category 5 UTP”, at the 109th AES Convention in September 2000.

Tests with NEXUS

But so much for the background theory: what about the practice? Unfortunately, Belden had no audio devices with a common mode rejection ratio in excess of 80 dB, so verification of this claim has proved impossible. For Stage Tec, however, this is no problem: NEXUS modules have a common mode rejection of sometimes more than 150 dB, with an output impedance of only 8 ohm, while the parasitic ground capacity only amounts to a few 10 pF, so that all specifications by far exceed the official requirements. To substantiate STAGETEC’s findings about the redundancy of shielded cable under such conditions, the company tested the noise sensitivity of various cable types connected to a NEXUS module: a 120 m long Cat. 5 S-STP cable1, and a 300 m Cat. 5 UTP cable run2.

Expect the Unexpected

As far as output voltage unbalance measurements go, UTP cable provided values in excess of 30 dB better than S-STP cable – even though the UTP cable was more than twice as long. This came as no surprise, as this is due to the significantly lower ground capacity of UTP cable. What were interesting, however, were the crosstalk values between adjacent channels. There, unshielded UTP produced values that were up to 20 dB better than with S-STP cable! That finding prompted Stage Tec to examine the drawbacks of working with UTP cable. To this end, Stage Tec simulated an unbalanced noise source by placing a slightly unsymmetrical signal on both conductors. Again, the UTP cable’s crosstalk values were a lot better – by a sensational 35 dB. In this experimental setup, the balance was „worsened“ to the minimum permissible according to the German Performance Requirements (Pflichtenheft) 3/5. Doing so made no difference for S-STP cable thanks to its shield. The UTP cable, however, exhibited a dramatic change: its crosstalk rejection ratio dropped to that of S-STP cable.

Unshielded Cable is a Winner

With high-quality inputs and outputs exclusively, unshielded UTP cable for analog signal lines yields much better crosstalk values than its S-STP counterpart, even though the S-STP cable used for the measurements was already far superior to other cable types commonly used in audio. Working with UTP cable is a better idea also when the quality of the audio inputs or outputs lies below that of the input and output stages found on a NEXUS – provided such I/Os still conform to the German Performance Requirements 3/5. Working with shielded cable, on the other hand, is necessary in cases where the balance of the connected devices is below par, where the parasitic ground capacity is high, or if absence of ground cannot be guaranteed.

The Future Belongs to RJ45

Nothing, then, should deter us from using unshielded audio cable, provided the digital system used is fitted with high-quality analog interfaces. Phantom-powered condenser microphones obviously cannot do without shielding. Nevertheless, all analog STAGETEC modules are now available with the usual XLR sockets, or alternatively, with the RJ45 connectors used in IT environments. Because of their mechanical characteristics, RJ45 sockets should only be used in fixed installations. Over all the advantages of the RJ45 cabels are obvious: making new cables becomes a breeze; the cable itself is less expensive; while the audio quality of RJ45 connections is actually better.

On the seminar at the Banz monastery, Dr. Jahne made a presentation to prove this theory. With a setup with a 300 m run of unshielded UTP cable, he showed the effect on unshielded cables in an extreme and dramatic way: On one of the lines he connected a dynamic microphone while the adjacent conductor pair carried a 1 KHz test signal with a level of 22 dBu. Even at maximum microphone preamplification, the test signal could not be detected, while there was no hum either. Our measurement devices allowed us to detect a crosstalk attenuation of 140 dB at 1 KHz.