- The DIGI-COUPLER 75 Ohm features a very constant and precise 75 Ohm impedance: 75 Ohm ± 0.5 Ohm up to 3 GHz.
This is important, since otherwise the signal will be reflected up and down the cable (i.e. standing waves). Such signal reflectionsdeteriorate signal integrity by causing the digital audio signal as seen at the receiving end to be overlapped with time delayed replicas of itself (i.e. signal reflection induced inter-symbol interference).At the receiving side this negatively influences the D-to-A converter clock retrieval circuitry’s (PLL’s) ability to maintain a stable sample rate (i.e. fluctuations in the timing interval betweenthe audio signal’s samples occur).
The resulting distortion of the audio signal is officially called “jitter distortion” and is particularly perceivable as reduced resolutionand imaging.
Note: To maintain a precise 75 Ohm impedance and avoid signal reflections, the advised minimum value of bending radius for The DIGI-COUPLER 75 Ohm is 8.0 cm.
- The DIGI-COUPLER 75 Ohm features a very high frequency bandwidth.
As an example: The -3 dB bandwidth of a 10 m. run is 770 MHz. (For more data see Attenuation graph in the sidebar)
A high bandwidth is important: An SPDIF digital audio signal contains frequencies that extend far beyond the usually specified minimum required bandwidth (~6 MHz). Even though this minimum bandwidth provides useable transmission, accurate data transmission and recovery of low-jitter clocks in real-world noisy environments will benefit from greatly increased bandwidth:
The digital audio signal consists of elementary data units (bits) in the form of zeros and ones. These are square waves (merlon shaped elements) which vertical edges serve as a demarcation for the signal receiving circuitry (e.g. D-to-A converter) to be able to retrieve the signal clock and to recognise the contained bit values.
Cable bandwidth decreases with cable length used. Lower bandwidths increasingly cause the digital audio signal’s square waves edges to get rounded off and become smeared over time (inter-symbol interference). This impairs the receiving circuitry’s ability to correctly determine the individual bit values and their timing. The so-called “jitter distortion” and “bit error rate” increase and the audio quality decreases.
A short note regarding optical fiber cables for digital audio and bandwidth:
Although these exhibit a very high bandwidth and near perfect noise immunity by themselves, the bandwidth of the unavoidably extra involved optical transmitter and receiver parts (especially those used in common consumer devices) is one of the most important performance limiting factors. Others are: The optical power available, so-called modal dispersion, attenuation and connector transition losses.
Depending on the situation, though in many cases, coaxial SPDIF transfer is preferable if low jitter distortion replay is aimed at. This provided a good cable like our The DIGI-COUPLER 75 Ohm is used...
- The DIGI-COUPLER 75 Ohm features a very low signal attenuation per unit length. As an example: At 100 MHz the attenuation is a mere 0.11 dB/m. (For more data see HF Performance Graphs in the sidebar).
Low signal attenuation (i.e. low signal loss) is especially important when long cable runs are used (e.g. in professional / studio applications).To maintain optimum signal quality, a maximum signal to noise ratio at the signal receiving side is required. The signal therefore needs to undergo as little as possible attenuation during transport. This of course equally applies to digital audio as to video and HF signals.A low signal attenuation also positively adds to immunity against external noise, though latter starts with a good shielding:
- The DIGI-COUPLER 75 Ohm is a so-called “Superscreened” or “Low Transfer Impedance type” cable.
To maximize performance in keeping the signal in and noise (RFI and EMI) out, the highest possible shielding factor is required. Without making compromises, The DIGI-COUPLER 75 Ohm’s design therefore features a so-called “triaxial construction” which makes use of multiple highest quality shields.
(Triaxial construction: essentially a coaxial cable surrounded by an extra separately insulated shield).