Synchronizing Digital Audio
December 30, 2005
Problems can occur in dubbing digital audio from one device to another (including digital VTRs). Among the symptoms are pops or clicks in the dubbed material. Sometimes these are at a low level and may not occur at regular intervals. Another symptom is that the recording device may not record the incoming signal at all.
If that happens, check that all devices are operating at the same sample rate. Normally for broadcast plants, a digital audio sample rate of 48 kHz is standard, as that’s the rate used for most digital VTRs. For recording, editing and post-production, higher sample rates like 96 kHz or 192 kHz are now often employed.
Note that CD players use 44.1 kHz sampling rate. While some players designed for broadcast operations perform an internal sample rate conversion to bring the output up to 48 kHz, many do not. In that case use an external sample rate converter, or one supplied in a digital audio mixer. (Many digital audio mixers today come equipped with sample rate converters on all or some of their input channels.)
In addition to operating on a common sample rate, devices with digital audio should be locked together with a common digital audio synchronization reference--word clock or AES reference. In a video facility, that digital audio reference should also be locked to house video reference (color black) . Many devices designed for video production or broadcast include both types of reference inputs--video color black and AES reference.
Most manufacturers of video sync generators offer AES reference (and often AES tone as well) as either a standard feature or as an option. Also available are stand-alone digital audio reference generators that can be locked to video reference.
Make sure that not only players and recorders are locked, but also digital audio mixers and editors, sample rate converters, analog to digital converters, digital to analog converters, as well as other processing gear.