Clocking Explained

In A to D and D to A conversion, the digital signal needs to be clocked accurately to prevent distortion. Digital audio is made up of amplitude values of a signal at different points in time (known as samples). Samples are taken many times a second, and need to occur at regular intervals (44.1 kHz is the sample rate used in commercial CD audio). If this does not happen, the amplitude of the signal will not be recorded or played back at the correct time and so distortion will be introduced into the signal. When the clock is not consistent in its timing, the resulting distortion is known as “Jitter”. If the clock frequency changes, it is known as “Drift”.

Single and Multiple Devices

When you are only looking to use one unit, such as an audio interface, you don’t really have to worry too much about clocking, however when using multiple units it becomes a little more complex. Multiple devices need to be synced so the digital audio being transferred between them is read correctly. A simple analogy explaining the need for syncing two devices with a clock signal is by imagining two spinning cogs. They have to be spinning at the same rate for the cogs to mesh together. If one is spinning faster than the other, the teeth will not interlock and the system won’t work.

When syncing two or more devices, there needs to be one unit that is providing the clock signal to all the rest of the units. The device which is providing the clock signal is known as the master, while all the units accepting this clock signal are referred to as slave devices. The master clock signal can either come from the internal clock on the master device, or you can get a separate, dedicated unit.

Choosing a Master Device

Choosing the master device is usually very flexible. Quite often the best device to set as the master is the one that is connected to the computer (where possible) such as a ProTools Rig or an audio interface. This way, when you open a project in a DAW, the audio interface should change to match the project sample rate, and therefore change the sample rate of all the slave devices. However this depends on the devices being used.

Connections

Clocking between devices can be carried out in a few different ways. Devices can be connected using a BNC cable connected to the “Word Clock In/Out” of the device. Other connector types can be used such as RCA in this way as well. Devices can be connected in daisy chain configuration where a BNC T piece is used to split the clock signal.

When using AES, S/PDIF or ADAT, the clock signal is encoded into the data stream so there is not necessarily a need for a BNC cable to provide a clock signal. The type of connectors used to clock depends on the equipment being used in the setup (some may only have certain connector types available on the unit) and how many devices are needing to be connected together.

Example Setups

Example 1 - Interface as Slave

ASP880 mic pre (Master) and an iD22 audio interface (Slave) connected via ADAT:

The iD22 has an ADAT optical out which can be used for slaving devices, but it does not have a word clock output (BNC connector). The ASP880 has a word clock (BNC) input, but not an optical input so there is no way for the iD22 to be the master in this set up. In this case the ASP880 would need to be the master device by using ADAT which includes clock data in its data stream. The only problem with this setup (more of an inconvenience than anything) is that you have to manually change the sample rate on the ASP880 to match the sample rate of the project in your chosen DAW.

Example 2 - Interface as Master with Daisy Chain

Two ASP880 mic pre (Slaves) and an RME Fireface 800 audio interface (Master) connected via ADAT along with BNC word clock outputs.

The Fireface 800 has a word clock output on the unit and so can work with the word

clock input of the ASP880s. By connecting a BNC cable with a T piece on the end to

the first ASP880 making sure to set the clock mode to external on the back of the unit. The other side of the T piece should be connected to the second ASP880 (again with the clock mode set to external), however with this device it needs to be terminated properly with a 75Ω resistance across the cable, which in the case of the ASP880 means pressing a button on the back of the unit (to prevent reflection of the clock signal). Connecting the devices in this way allow the Fireface to change its sample rate to match a DAW project automatically along with the sample rates of the connected slave devices. There would be no need to manually change the sample rates on any of the devices.

Example 3 - External Master Clock with Multiple Slaves

External master clock with an ASP880 mic pre (Slave), RME Fireface 800 audio interface (Slave) and Digital Console (Slave).

This example depends on the outputs available on the master clock. Some master clocks have multiple buffered word clock outputs (BNC) in which case you can plug each device into a separate output of the master clock, making sure to terminate each device individually. Alternatively if the master clock only has one word clock output then you simply daisy chain the devices like in example 2, terminating the last device in the chain. When working with an external clock you have to make sure that the master clock rate is the same as your DAW session because it won’t change it automatically.

Example 4 - Location Recorder Example

Two ASP880s and two external recorders, an Alesis HD24 and a Joeco Black Box Recorder.

In this example it is not immediately obvious which device should be the master. However looking into the options available to you in the hardware, this is a case where there is only one option. Out of all this hardware, only the Joeco recorder actually has a word clock output, so therefore this device must be the master, daisy chaining the clock signal to the ASP880s and the HD24. This example shows you how easy it would be to arrive for a location recording with some gear and suddenly realise that you are unable to sync the devices to each other. If instead of the Joeco recorder, you had a recorder without a word clock output, you would have needed an external master clock device or risk not being able to use the setup.

Symptoms of Clocking Problems

• Pops and Clicks in the output
• Caused when a device is looking for a sample, but because it is not synchronised with the device sending the audio, it is looking for the sample at the wrong point in time.
• The severity of the clicks can be varied and can sometimes go unnoticed. Sound on Sound recommends playing a reliable sine wave of 4kHz through your system and listening carefully to see whether there is any clicking anytime you change anything in your setup. Not checking could come back and bite you later when you realise some recorded audio has some clicks and pops in it.
• No Audio Output
• Some devices simply will not allow any audio through them when clocked incorrectly.

Key Points to Think About

• Check that the right devices are clocking internally or externally.
• There can only ever be one device clock set to internal clocking mode when using multiple devices without a master clock.
• Make sure that the device at the end of a BNC word clock cable has a 75 ohm termination.
• If the device that is interfacing with your DAW is set as master, it should automatically set the correct project sample rate for all slave devices.

Further Information 

Sound on Sound

Digital Clocking - http://www.soundonsound.com/sos/apr03/articles/digitalclocking.asp

Digital Interfacing - http://www.soundonsound.com/sos/feb07/articles/interfacing.htm

Understanding Digital Audio Theory:

A great book for understanding the theory behind sampling and processing digital audio

http://www.amazon.co.uk/Digital-Audio-Explained-For-Engineer/dp/141960001X

Sampling and Why We Only Allow Up To 96kHz Operation:

http://lavryengineering.com/pdfs/lavry-sampling-theory.pdf