Use Correct Cables for AES3 (Balanced) Digital Audio
August 15, 2006
If you see a problem in the digital audio eye pattern, one thing to investigate is the interconnect cable.
Are you using ordinary audio cable? For digital audio, regular analog audio cable doesn't go the distance, literally. Even if everything is working optimally, the best usable distance you can expect to get with this cable is about 50 feet. The reason is that regular audio cable has too high cable capacitance and too low impedance for digital audio interconnects. The AES3-2003 digital audio standard specifies that the line driver (digital audio output from a piece of gear) have a source impedance of 110 ohms, and that the line receiver (digital audio input) provide a load impedance of 110 ohms. Tolerance on each of these specs is 20 percent. AES3-2003 also specifies that interconnecting cable be balanced and shielded and have a nominal impedance of 110 ohms over a frequency range from 100 kHz to 128 times maximum frame rate. At the common 48 kHz sampling rate, that comes out to 6.144 MHz. To meet this spec, a twisted pair cable for digital audio must have low capacitance. As an example of a digital audio cable, Belden 1800B, a 24 AWG twisted pair cable, is spec'd at 12 pF (pico-Farads) per foot of capacitance, with a nominal impedance of 110 ohms. At 6 MHz, signal attenuation is 1.81 dB per 100 feet, and maximum transmission distance is 1105 feet, assuming the minimum allowable output signal amplitude of 2V and the minimum allowable input signal amplitude of 200mV. Compare this with a common analog audio cable Belden 9451, a 22 AWG twisted pair cable with a capacitance of 35 pF per foot. (Note that there are many manufacturers of quality digital and analog audio cable. Belden is used as just one example.) While Belden doesn't spec the nominal impedance of 9451, it states that, in general, the nominal impedance for analog audio cable ranges from 40 to 70 ohms. If this cable was used for digital audio, there would be an impedance mismatch between the digital inputs and outputs and the cable. This means that the signal will reflect back up the cable resulting in a distorted digital audio voltage waveform where the eye pattern starts to close up. The high capacitance of analog audio cable further distorts the digital audio voltage waveform in that it increases its rise time. If these distortions are severe enough (with long analog audio cable lengths, for example), an AES3 receive circuit won't be able to detect the transition points that indicate ones and zeroes, and the audio could be muted or produce pops and clicks. So use the correct cable for balanced digital audio circuits, and save yourself a lot of troubleshooting.
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