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QEDs award winning Qunex P75 digital SPDIF cable to connect CD / digital player or streamer to DAC. Extremely rare 1.5m RCA to BNC as QED dont make this combination! I have fitted a new Sommer Cable of Germany Hicon BNC to a New Old Stock QED 75 Ohm cable. These BNC cables are designed for Audiophile use and cost me over £9 to buy from Germany, making this cable a bargain at under £30 delivered.Triple shielded and true 75 Ohm construction for reduced jitter.Burned in using the Tara Labs Cascade file for 48 hours producing a much smoother sound as a result.This sounds excellent with Chord, NAIM, Linn etc DACS with BNC as no adaptor is required which compromises performance by generating reflections and spurious signals causing jitter.Please see the excellent 100% feedback I have received for hundreds of QED digital and analogue cables.Length of cable – why 1.5m? SummaryThere are only two occasions in audio where a longer cable – or an optimum length cable is better than a short one. With Digital cables there is an optimum length of 1.5m or more. (The other occasion is for MM phono cartridges, which need a specific capacitance).The reason for this requires an explanation. Please refer to diagram in the photos. The signal travelling down a SPDIF (so called digital cable) is actually a square wave ANALOGUE voltage signal, however in reality this square does not have instantaneous changes - the squares are sloped and somewhat rounded off too as it takes some time to change state from 0 to 1 or 1 to 0. The accuracy of the pulses at the end of the cable determine how accurately the source can interpret the signal in value 1 or 0 and also timing which not so easy.The signal reflects back off the ends of the cable, the plugs and connected equipment (echoing back and forth) and produces ghost images of itself, which can fool the receiver into thinking that the “ghost” signals are the original signals. With short cables, under 1m, the ghost signals arrive close to the originals within the time frame of transition from 0 to 1 or 1 to 0, before the transition occurs. A 1m cable means the reflection arrives at about the same time as the transition is to be recorded. With longer cables the reflection arrives too late to influence the receiver (The transition has already been recorded). Longer cables also mean lower amplitude or signal reflection thus receiver can more easily determine between the correct signal and the spurious reflections. The bottom line as that a longer cable eliminates the false readings from the ghost images, and thus reduces timing errors, called jitter and thus sounds better.The optimum size has been determined by measurements and experimentation to be 1.5m or more. Detailed Technical Explanation for those with an enquiring mind.Why SPDIF cables should be at least 1.5m long. When the SPDIF signal is launched into the cable from the Transport, it is essentially a voltage square-wave, consisting of rising edges and falling edges. These edges are no more than transitions of voltage from about –250mV to +250mV, the rising edge transitioning from minus voltage to plus voltage and the falling edge transitioning from plus voltage to minus voltage. When an edge transitions, it can be described as having a rise-time or fall-time. This is the time it takes for the signal to transition from 10% to 90% of the entire voltage swing. (note that this DOES Not happen instantaneously) The rise-time is important because this is what causes reflections on the transmission-line. If the rise-time were very, very slow, say 50 nanoseconds, then there would be no reflections on the transmission-line unless it was extremely long. Alternately, if the rise-time were less than 1 nanosecond, reflections would occur at every boundary, such as the connection from the circuit board to the wires that go to the connector.Typical stock Transports have around 25 nanosecond rise-times. The primary concern for the manufacturer is to pass FCC regulations for emissions and electromagnetic interference, as well as making the interface reliable. When the regulatory testing is done, they attach very inexpensive, inferior cables and measure the emissions. To insure that the manufacturer passes these tests, they take a number of precautions. One is designing-in the slower than necessary 25 nanosecond rise-time. Another is the insertion of various filters in the Transport to eliminate high frequencies from the signal. As a result of these choices, there is a hazard created in using too short a digital cable.It is a result of the slow rise-time. When a transition is launched into the cable, it takes a period of time to propagate or transit to the other end. This propagation time is somewhat slower than the speed of light, usually around 2 nanoseconds per foot, but can be longer depending on the dielectrics used in the digital cable. When the transition reaches the end of the transmission line (in the DAC), a reflection can occur that propagates back to the driver in the Transport. Small reflections can occur in even well matched systems. When the reflection reaches the driver, it can again be reflected back towards the DAC. This ping-pong effect can sustain itself for several bounces depending on the losses in the cable. It is not unusual to see 3-5 of these reflections before they finally decay away, particularly when using the best digital cables, which are usually low-loss.So, how does this affect the jitter? When the first reflection comes back to the DAC, if the transition already in process at the receiver has not completed, the reflection voltage will superimpose itself on the transition voltage, causing the transition to shift in time. The DAC will sample the transition in this time-shifted state and there you have jitter. Let’s look at a numerical example:If the rise-time is 25 nanoseconds and the cable length is 3 feet, then the propagation time is about 6 nanoseconds. Once the transition has arrived at the receiver, the reflection propagates back to the driver (6 nanoseconds) and then the driver reflects this back to the receiver (6 nanoseconds) = 12 nanoseconds. So, as seen at the receiver, 12 nanoseconds after the 25 nanosecond transition started, we have a reflection superimposing on the transition. This is right about the time that the receiver will try to sample the transition, right around 0 volts DC. Not good. Now if the cable had been 1.5 meters, the reflection would have arrived 18 nanoseconds after the 25 nanosecond transition started at the receiver. This is much better because the receiver has likely already sampled the transition by this time.
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