Q Can I feed 16-bit digital audio into a 24-bit digital device via S/PDIF?

Sound Advice : Mixing



Hugh Robjohns



Can I link the S/PDIF out of my Boss DR-880 to the S/PDIF input of my Focusrite Scarlett 8i6 audio interface and get good results? The DR-880 output is at 16-bit resolution but the Focusrite’s S/PDIF input operates at 24-bit. Do I have to match the bit rate [or ‘word length’] as well as the sample rate? Both units are set to 44.1kHz. When I make this connection, I have nothing at all showing in my Focusrite Scarlett’s Mix Control software: nothing shows in the metering at all, no matter what the output level is.



If a device, such as this Boss DR-880, can only output 16-bit digital audio over S/PDIF, you can still hook it up to a modern 24-bit audio interface — but the interface must slave to the other device’s clock signal.If a device, such as this Boss DR-880, can only output 16-bit digital audio over S/PDIF, you can still hook it up to a modern 24-bit audio interface — but the interface must slave to the other device’s clock signal.



SOS Forum post



SOS Technical Editor Hugh Robjohns replies: Yes, you can. The S/PDIF interface format always carries 24 bits of audio data and the 16-bit signal from the DR-880 is automatically padded out with zeros (effectively) in the bottom eight bits. That’s definitely not the problem.



Having the same sample rate is important, but digital equipment can only have one clock source. Everything else must slave to that, so a common cause for an apparent lack of input signal is when the interface is running from its internal clock rather than slaved to the incoming signal. In this situation, the external input is non-synchronous with the interface’s internal clock and thus unusable. Make sure that the 8i6 is configured to slave to the external input from the DR-880, which becomes the wordclock master for your interface/computer. Other possibilities include a ground loop between the two devices, or insufficient signal level from the source. Digital ground loops don’t cause audible hums but often result in glitchy or non-functioning connections, while a poor-quality or overly long S/PDIF cable can degrade the signal level substantially, rendering it unusable. If available, an optical S/PDIF connection would avoid ground loops completely, as would an in-line digital transformer in a coaxial connection. Another reason could be incompatible channel-status bits, but although that is much harder to resolve it is also extremely rare!  

Published in SOS October 2015