Thomann sssnake Cat Snake 3FC.Photo: Thomann.de
I see four‑channel XLR breakout boxes connected by Ethernet RJ45 cable, such as Thomann’s Cat Snakes, becoming increasingly popular. As a decidedly low‑tech option for live sound, four sets would route 16 channels of audio over four runs of cheap Cat5 cable, with the huge advantage that if somebody chews up one of the cables while rigging, or whatever, I could just whip out a reel of Cat5, cut to length, quickly crimp connectors on it, and be good to go. It’s a much cheaper option than a Dante stagebox — but does this really work, at least for fairly short runs, at mic level?
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SOS Technical Editor Hugh Robjohns replies: Does it work at mic level? Yes — but there are very important caveats. First, though, although Ethernet cable is used, it’s not carrying digital data, so it’s not comparable with Dante or other Ethernet‑based interfaces. The passive Cat5 snake systems, which are available from several manufacturers, simply pass four balanced analogue signals over four twisted pairs of wires; the Ethernet cable is just a convenient and cost‑effective product.
I mentioned caveats. As a bare minimum, this system needs screened cable with screened connectors — so definitely not the cheapest Cat5 UTP (Unscreened Twisted Pair) cable. The screening prevents interference getting into the cable, and is vital for allowing phantom power to work. No screen means no phantom power, as some disappointed users have discovered the hard way! In practical terms, Cat6 FTP or STP (Foil Screened or Shielded Twisted Pair) cable is specified, usually with robust EtherCon connectors (an RJ45 plug within an XLR‑like shell).
Ethernet cable is designed to pass symmetrically balanced digital signals of identical strength... In audio applications, symmetrically balanced signals aren’t guaranteed.
Ethernet cable is designed to pass symmetrically balanced digital signals of identical strength, and in that scenario there’s negligible crosstalk between the four channels (and negligible interference). Hence the success of UTP cable generally but, with faster connection speeds, an overall screen helps to reduce interference (outwards, as well as inwards) so it’s more commonly employed today, hence the wide availability of inexpensive STP cable.
Analogue multicore cables used in live sound, though, must convey signals at vastly different levels, and not always symmetrically balanced ones. This increases the risk of crosstalk dramatically, so in mitigation analogue multicores typically employ individual shields around each pair of signal wires, rather than an overall screen. Usefully, Cat6 Ethernet cables are also available with individually screened pairs. These are usually denoted as S/FTP, F/FTP or sometimes SSTP (Shielded Foiled/Screened Twisted pair).
I didn’t have any Cat6 STP and SSTP cables to hand, but did have two‑channel individually screened multicore to simulate SSTP cable, and star‑quad cable (which can be wired as two unscreened pairs with an overall screen) to simulate STP cable. So I tested the crosstalk issues and generated a plot using my Audio Precision test system.
The green line (bottom) is the inter‑channel crosstalk over 25m of two‑channel multicore cable (Van Damme Studio Grade Blue Series) with individually screened pairs. The test signal was symmetrically balanced at +4dBu, sweeping from 20Hz to 20kHz. The marginally higher blue line is the same symmetrically balanced signal running down 25m of Canford Audio’s star‑quad cable, with two unscreened pairs tightly twisted together within an overall screen. This demonstrates that with symmetrically balanced signals at similar levels, crosstalk is negligible regardless of whether the pairs are individually screened or not. The worst‑case crosstalk is ‑90dB at 20kHz, while at mid‑frequencies it’s around ‑120dB. The gentle rise in crosstalk level with frequency is normal, and due to capacitance between the wires. Symmetrically balanced signals largely self‑cancel their magnetic and electrostatic fields, thus hugely reducing induction into adjacent wires, minimising crosstalk.
In audio applications, symmetrically balanced signals aren’t guaranteed, and sometimes we work with unbalanced signals. ‘Impedance‑balanced’ outputs are common on budget mixers and other equipment, including many transformerless capacitor mics from respected manufacturers. The impedance‑balanced format conveys the signal over one wire in the pair, with the other grounded, much like an unbalanced signal — as there’s no self‑cancellation of the magnetic/electrostatic field, there’s more crosstalk.
The khaki line across the middle of the chart shows the crosstalk arising from an unbalanced source feeding a balanced input over the individually screened multicore cable. The crosstalk is about 40dB worse through the midrange than for the symmetrically balanced test, but, at around ‑70dB, it’s still practically inaudible. The red line (top) is the same unbalanced source feeding the overall‑screened star‑quad cable and, as predicted, crosstalk is 20dB worse than using individually screened pairs. At ‑50dB at 2kHz it’s audible too.
Although I’ve not tested Cat6 STP/SSTP cables, the constructions of the multicore and star‑quad cables are similar, and it’s safe to assume that equivalent performance differences would be obtained when conveying impedance‑balanced or unbalanced signals. So while STP (overall shielded) cables are fine where symmetrically balanced signals can be guaranteed, individually screened (SSTP) cable has very significant advantages if there is any possibility of conveying impedance‑balanced or unbalanced signals.