That Syncing Feeling

We become so accustomed to the speed of sound moving through the air that when the audio signal is still moving through copper wire, it appears to be instantaneous. In analogue audio, it is rare to encounter timing errors in these electronic signals that cause us problems on a day-to-day basis. Yet, as recording and broadcast engineers know, when the signals are digitized the timing of the electronic signals becomes critical. With the advent of fully digital mixing consoles for live sound, the timing of all the input signals suddenly becomes an important issue. Signals that are not accurately synchronized can cause catastrophic signal failure, such as full volume noise or no sound at all.

Digital audio has been used routinely in sound reinforcement applications since the seventies, when digital delays became a practical method for aligning the acoustical arrival time of sound from multiple loudspeakers. Since then, the use of digital processing has crept into the signal chain, replacing large sections of the signal processing. However, this was usually done by converting the analogue signal to digital audio each time a new signal processor was placed in line. The audio moved between the equipment as an analogue signal. It is only in the past few years that we have seen sound reinforcement products with multiple digital audio input and output connections. As we enter the era when mainstream mixing console and audio sources become digitized, suddenly the timing of these signals becomes important. Non-synchronized digital audio signals, when mixed together, can produce full-volume clicks and pops or dead silence. Disastrous consequences in a live sound application.

Anyone that has worked with SMPTE timecode will be familiar with the concept of house sync. A common timing signal that is distributed to all the equipment that must be kept synchronized. A single highly accurate sample clock source is used as a master, and all other timing clocks within the signal chain synchronize to this signal. In digital audio, the timing is distributed within the digital audio signal and, in some cases, as a separate Word Clock signal that provides the exact timing of the audio samples within the digital signal stream. Not all devices have inputs for a word clock signal, but professional audio devices designed for multiple digital input signals usually do (check the back panel of the Yamaha PM1D, for instance). It is essential that the word clock signal be distributed carefully to all the audio mixing and processing in the signal chain. Degradation of the clock signal can also produce audible faults in the audio signal.

What about those remote signals or sources that provide digital outputs but cannot be externally synchronized. This might be a digital output from a synth, CD player or DAT machine. In low-cost digital mixers, these inputs may have to become the master clock source but, in more sophisticated digital mixers, the inputs are resampled (by a sample rate converter) and then locked to the mixer's sample clock. This is one of those compromises that force you to choose between an unreliable (or at least, unknown) work clock source and the horror of sample rate conversion. When this issue first arose, the very idea of converting the sample rate was associated with a reduction in audio quality. It was often preferred to convert the signal back to analogue rather than endure the change in sound quality caused by the sample rate conversion. Even though reclocking the digital audio signal is less invasive than wholesale changes to the sample rate (such as changing from the CD rate of 44.1 kHz to the pro-audio rate of 48 kHz) there are always decisions the converter must make in truncating or interpolating the audio to bring the timing into exact alignment. Thankfully, a lot of progress was made in the last decade to improve the quality of sample rate conversion and this is less of an issue now.

The quality of the sample clock and intermediate connecting cables are also important, both of which can add jitter to the timing of the clock. Jitter is the digital equivalent of wow-and-flutter in analogue audio. The audibility of the result will depend of the nature of the sound and the quality of the playback system. For many sound reinforcement situations, other signal distortions, such as reverberation and comb-filters from loudspeaker interaction, may reduce the significance of modest levels of jitter. However, like most audio issues, locking the digital audio signals to a reliable, high-quality sample rate clock is the most reliable solution. This includes using the appropriate cable to make word clock interconnections that preserve the waveform and enable the most accurate timing to be achieved within each digital audio device. Syncing to the lowest common denominator has never been the best way to achieve high quality sound reinforcement.

United Entertainment Media Inc.
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New York, NY, 10016
Ph. 212-378-0400
FAX 212-378-2160

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