6.13 A Brief Troubleshooting Guide

If a digital interface between two devices appears not to be working it could be due to one or more of the following conditions. The reader should refer to the main text for more detailed explanation of the conditions described.

Asynchronous Sample Rates

The two devices must normally operate at the same sampling frequency, preferably locked to a common reference. Ensure that the receiver is in external sync mode and that a synchronizing signal (common to the transmitter) is present at the receiver's sync input. If the incoming signal's transmitter cannot be locked to the reference it must be resynchronized or sample rate converted. Alternatively, set the receiver to genlock to the clock contained in the digital audio input (standard two-channel interfaces only).

The 'Sync' or 'Locked' indicator flashing in or out on the receiver normally means that no sync reference exists or that it is different from that of the signal at the digital input. Check that sync reference and input are at the correct rate and locked to the same source. Decide on whether to use internal or external sync reference, depending on application.

If problems with 'good lock' or drifting offset arise when locking to other machines or when editing, check that any timecode is synchronous with the video and sampling rate. If not, the tape must be restriped with timecode locked to the same reference as the recorder, or a synchronizer used which will lock a digital audio input to the rate dictated by a timecode input.

Sampling Frequency Mode

The transmitter may be operating in the AES3 single-channel-double-sampling-frequency mode in which case successive subframes will carry adjacent samples of a single channel at twice the normal sampling frequency. This might sound like audio pitch-shifted downwards if decoded and converted by a standard receiver incapable of recognizing this mode. Alternatively the devices may be operating at entirely different sampling frequencies and therefore not communicating.

Digital Input

It may be that the receiver is not switched to accept a digital input.

Data Format

Received data is in the wrong format. Both transmitter and receiver must operate to the same format. Conflicts may exist in such areas as channel status, and there may be a consumerprofessional conflict. Use a format convertor to set the necessary flags.

Non-Audio or 'Other Uses' Set

The data transmitted over the interface may be data-reduced audio, such as AC-3 or DTS format. It can only be decoded by receivers specially designed for the task. The data will sound like noise if it is decoded and converted by a standard linear PCM receiver, but in such receivers it will normally be muted because of the indication in channel status and/or the validity bit.

Cables and Connectors

Cables or connectors may be damaged or incorrectly wired. The cable may be too long, of the wrong impedance, or generally of poor quality. The digital signal may be of poor quality. Check eye height on the scope against specification and check for possible noise and interference sources. Alternatively make use of an interface analyser.

SCMS (Consumer Interface Only)

The copy protect or SCMS flag may be set by the transmitter. For professional purposes, use a format convertor to set the necessary flags or use the professional interface which is not subject to SCMS.

Receiver Mode

The receiver is not in record or input monitor mode. Some recorders must be at least in recordpause before they will give an audible and metered output derived from a digital input.