Description
QED DAV FLX1 solid-core oxygen-free copper cable 75 Ohms SPDIF with 75 Ohm RCA plug, gold plated contacts and a high-end Hicon BNC, used by Nordost for years on their cables. Use to connect your CD player, streamer, or Blu-Ray player to DAC. This bargain-priced high-quality SPDIF RCA to BNC cable can be made at any length. (Up to a recommended maximum of 5m).This cable is unusual in that most RCAs plugs are not 75 Ohm, so no matter how good the cable, the plugs will cause reflections of the high-frequency digital signal and cause an increased risk of timing errors (jitter). These plugs designed originally for 75 Ohm use in component video and RF applications are a perfect fit for the QED cable and ironically perform better than any of the QED plugs on this cable.The Hicon BNC is also 75 Ohms high end cable (used by Nordost in their cables), reducing reflections and thus jittering and improving sound quality. Background Q: Why do digital cables make a difference – isnt digital perfect sound forever?A: Because years ago, the designers of the digital audio interfaces decided that the audio signals should be sent imperfectly in real-time, rather than perfectly but late!Our day-to-day experiences of sending digital signals are that they arrive perfectly, so what is different about audio? I dont get errors when I save my Word document to my hard drive or send an email to my cousin in the US; how is it so hard to send a signal 1m between two hi-fi components?The critical difference between Hi-Fi and digital documents being sent is that the audio signals are sent IN REAL TIME WITH NO BUFFERING OR ERROR CORRECTIONIn the case of a document sent across the word or to the printer, the data is transmitted in packets and assembled by the receiving machine; in the event of an error, there is time to ask for the signal to be re-sent it, is error corrected, so the result is 100% perfect. This all takes time. The audio signal has no time for any of this. It is sent as a continuous stream (Hence the phrase Streamer) in real-time, so there is no time to process it. If there are errors, then they affect the sound. Why In real-time? - this was decided years ago in the audio industry to allow video and sound to be synchronised - otherwise, lip-sync issues will be caused when playing a DVD or watching TV.The SPDIF interface is applied not only for CD players but also for DVD, Blu-Ray, Streamers etc., not just audio. How do better cables help? Jitter The phrase digital cables is a misnomer. All cables are lengths of wire or glass fibre, through which ANALOGUE voltages or pulses of light are sent. In the case of a wire, the analogue signal is a so-called square wave representing the 1s and 0s of the digital signal. In theory, this should be perfect; however, in practice, this square wave is rarely square - instead, it has rounded edges. The rounder they are, the more timing errors are introduced, called jitter. (How does the receiving machine know where the transition from 1 to 0 is if the edge of the wave is not a sharp vertical transition but a curve or angled line?) ReflectionsIn addition, as the signal hits the end of the cable, it is partially reflected, overlaying an out-of-phase rounded square wave on top of the original signal. This again contributes to errors. Longer cables reduce this issue; short cables are not a good idea. Interference Finally, Radio Frequency interference and Electromagnetic Interference can also introduce errors in the signal and affect the receiving equipment. This emphasises the need for good shielding; in some cases, using Ferrite beads can help with some special equipment. (They can also hinder if incorrectly specified). The better the cable, the squarer the wave, the less reflection, and the less spurious signals from interference. Length of cable – why 1.5m? Summary There are only two occasions in audio where a longer cable – or an optimum length cable is better than a short one. Digital cables have 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 the 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 determines how accurately the source can interpret the signal in value 1 or 0 and also timing which is not so easy. The signal reflects back off the ends of the cable, the plugs and connected equipment (echoing back and forth). It 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 transition time frame 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 is that a longer cable eliminates the false readings from the ghost images and thus reduces timing errors, called jitter, and therefore sounds better. Measurements and experimentation have determined the optimum size to be 1.5m or more. Very detailed explanation- for the curious, accompanies the diagram in the photos. Why SPDIF cables should be 1.5m long, detailed explanation. When the SPDIF signal is launched into the cable from the Transport, it is essentially a voltage square wave, consisting of rising and falling edges. These edges are no more than voltage transitions 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 one 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 and make the interface reliable. When the regulatory testing is done, they attach inexpensive, inferior cables and measure the emissions. To ensure that the manufacturer passes these tests, they take several precautions. One is designing in the slower than necessary 25 nanosecond rise-time. Another is inserting 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 two 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, mainly when using the best digital cables, which are usually low-loss. So, how does this affect the jitter? When the first reflection returns to the DAC, if the transition already in process at the receiver has not been 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. Lets 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.Unfortunately, better (usually more expensive) cables produce better digital sound. Blame the people who decided on the digital interface decades ago for not separating audio-only from the need to send audio with moving pictures.
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