Clocking is an important aspect when you're looking to transmit digital data between different devices, for example, when expanding your channel count with S/PDIF or ADAT.

Digital audio equipment uses a clock to synchronise when each sample starts. In a digital system with multiple devices, all the devices must have the same common clock source. This ensures that each sample starts at the same time for each device and, along with matching sample rates, lets the devices synchronise correctly.

For example, when a device is set to 48 kHz, the audio transport in that device is passing samples 48000 times per second. In a device that includes ADCs (Analogue-to-Digital Converters) and DACs (Digital-to-Analogue Converters), this clock signals the start of each sample for conversion.

There must be only one leader clock device, with all other devices set as followers. The follower devices must listen to the leader's clock; this clock can be embedded in incoming digital audio signals (e.g. S/PDIF or ADAT) or via a dedicated connection such as Word Clock.

Word Clock

Word Clock (or Wordclock) is a digital synchronisation method that utilises BNC connectors, used to synchronise the clock(s) across an audio setup. No audio data is sent over a Word Clock connection, only the clock.

S/PDIF

S/PDIF stands for Sony/Philips Digital Interface. It's a digital audio format that allows for the transmission of two channels of uncompressed audio in a single direction over a single connection at sample rates up to 192 kHz at a bit depth of 24-bit. Along with audio, S/PDIF features an embedded clock, which can be used to synchronise different devices. 

S/PDIF signals can be carried over either via coaxial (RCA) connections or optical (TOSlink) connections. See this artcile for more information: How do I use RCA S/PDIF inputs and outputs?

ADAT

ADAT stands for Alesis Digital Audio Tape. It originated as a method of writing digital audio to physical media, however, it's now generally used as a shorthand to describe the ADAT Optical Interface (also known as ADAT Lightpipe). 

ADAT allows for the transmission of up to 8 channels at sample rates up to 48 kHz at a bit depth of 24-bit in a single direction over a single optical cable (sometimes called TOSlink cable). See this artcile for more information: How many channels can I send/receive using ADAT?

Like S/PDIF, ADAT features an embedded clock; this can be used to synchronise different devices.

Which device should I set as the clock leader?

It's best to select a clock leader that lets you perform all of your tasks with minimal switching around, depending on the connections you have available. For example, it's likely not convenient to set as leader a device that is not always turned on (e.g. a preamp that is rarely in use). Instead, you would probably find more convenient setting as leader a device that is always powered on in your setup (e.g. your audio interface).

What's the best clock source?

Generally, there isn't a noticeable difference between Word Clock and a clock sent over S/PDIF or ADAT, and most modern devices offer reliable clocks.

If you're in doubt regarding the performance of a given clock source, you can do a blind listening test comparing audio recorded with different devices set as leader, as well as with different clock sources (e.g. World Clock vs S/PDIF clock vs ADAT clock).

For more information on the different Sync/Clock Source options of Focusrite devices, see this article: What should my Sync/Clock Source be?

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