2 Digital Video Systems Test and Measurement
AES/EBU digital audio has raised some interesting questions because of its characteristics. In professional applications, balanced audio has traditionally been deemed necessary in order to avoid hum and other artefacts. Usually twisted, shielded, multi-conductor audio cable is used. The XLR connector was selected as the connector of choice and is used universally in almost all professional applications. When AES/EBU digital audio evolved, it was natural to assume that the traditional analogue audio transmission cable and connectors could still be used. The scope of AES3 covers digital audio transmission of up to 100 metres, which can be handled adequately with shielded, balanced twisted pair interconnection. Because AES/EBU audio has a much wider bandwidth than
analogue audio, cable must be selected with care. The impedance, in order to meet the AES3 specification, requires 110-ohm source and load impedances. The standard has no definition for a bridging load, although loop-through inputs, as used in analogue audio, are theoretically possible. Improperly terminated cables may cause signal reflections and subsequent data errors. The relatively high frequencies of the AES/EBU signals cannot travel over twisted pair cables as easily as analogue audio. Capacitance and high-frequency losses cause highfrequency roll-off. Eventually, signal edges become so rounded and amplitude so low that the receivers can no longer tell the ‘1’s from the ‘0’s which renders the signals undetectable.