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SPDIF vs. Optical – What Is SPDIF and What Is Optical?

When setting up your audio interface, such as the Focusrite Saffire Pro 24 DSP, you might come across two different ports labeled “Optical In” and “SPDIF In”. This might lead you to wonder about the differences between these two types of connectors. Why does the device have separate interfaces for both? What are the unique capabilities of each? This article aims to answer these questions and more, providing a comprehensive comparison of SPDIF vs Optical connections.

What is SPDIF?

The Sony/Philips Digital Interconnect Format, also known as S/PDIF or SPDIF, is a digital audio interconnect used in consumer audio equipment like the Focusrite Saffire Pro 24 DSP. It’s designed to output audio over short distances. The signal is transmitted over fiber-optic cables with Toslink connectors or coaxial cables with RCA connectors.

SPDIF can be used with both optical and coaxial cables, while the optical port can only be used with optical cables. This is because SPDIF is the encoding or data link protocol, not the wire or cable that runs it over.

SPDIF is a format that interconnects components in home entertainment systems or home theaters with digital high-fidelity systems for sound and music. It’s based on the AES3 interconnect standard and carries two channels of uncompressed PCM audio or compressed 5.1/7.1 surround sound such as the DTS audio codec. However, it’s unable to support lossless surround formats that use up a larger amount of bandwidth.

What is Optical?

Toslink or optical is a format that transmits signals through light and fiber optics. Specifically, optical or fiber-optic cables transfer information through the light that’s beamed via a plastic or glass fiber optic medium. The optical format usually refers to the ADAT protocol running over a fiber-optic cable, since optical cables themselves can be used in other formats such as SPDIF.

With the optical format, the output signal that travels through the cable must be converted from an electrical one to an optical or light-based one. After the signal has reached the optical receiver or input, it undergoes conversion again from optical to electric. Optical was partly developed to combat Radio Frequency Interference (RFI) and Electromagnetic Interference (EMI) that coaxial cables are susceptible to. It also doesn’t lose signal over distances because light doesn’t suffer from attenuation or resistance found in copper cables.

The Main Differences Between the Formats

Optical (the format, not the cable) and SPDIF are both digital connection protocols. The ADAT protocol for optical travels across Toslink or fiber-optics while SPDIF is usually transmitted over a coaxial cable with an RCA jack but can also make use of Toslink. However, SPDIF works with only 2 channels of audio or in stereo while optical is capable of carrying 8 channels instead at 44.1 or 48 Kilohertz (kHz). In certain situations, it’s 4 channels at 88.2 or 96 kHz instead.

Why Not Use Both?

It’s beneficial to have both outputs present for sound systems like Focusrite Octopre, Audient ASP800, Behringer ADA8200, and so forth. These multi-channel preamp boxes are capable of outputting 8 channels through the ADAT protocol. This is great for adding channels for full-band tracking, live-recording, or drums and other instruments. Meanwhile, single or dual channel preamps with a digital output will usually output the SPDIF signal over a coaxial or optical cable.

Why Even Bother with Using SPDIF?

Some beginners might feel like the presence of SPDIF is redundant since the “superior” Toslink or optical format with ADAT protocol uses more channels than the “inferior” stereo-channel digital audio interconnect. However, don’t be blinded by the specs and raw numbers. You don’t always need to use all 4-8 channels. Having SPDIF around also means you can use the external DAC or monitors with the digital output if you so choose.

SPDIF is an important output option because you can use it with an external Digital Analog Converter (DAC) or digital-output monitors to make interfacing multiple units as effortless as possible. This is because digital connections like SPDIF carry the clock signal along with other signals. This way, you won’t need to use a different USB DAC every time and try to make it work with your Focusrite Saffire audio interface and other units.

Which Should You Use?

Either is fine as long as you know what you’re in for with the SPDIF or optical format. Optical carries 8 channels at 44.1/48 kHz while SPDIF carries only 2 channels or is in stereo. It depends on the usage and the circumstance since SPDIF-enabled equipment will output SPDIF anyway if you don’t need those extra channels. SPDIF is essentially an alternative communication protocol to ADAT that also makes use of coaxial and optical cables. What’s more, a coaxial has a more secure and stable RCA jack.

You can link up your optical and SPDIF ports on your preamp box with either two optical cables or one optical cable and one coaxial cable respectively. Coaxial with SPDIF might also be the way to go since the more expensive optical cables can’t be bent or pinched tightly. Fiber-optics tends to be more sensitive to damage versus copper wires even though the latter is, in turn, susceptible to attenuation and interferences from RFI or EMI. With that said, at least with SPDIF you can switch from optical to coaxial cables when an optical cable is damaged. This isn’t the case with the standard optical format.

In conclusion, understanding the differences between SPDIF and optical connections can help you make the most of your audio equipment. Whether you choose SPDIF or optical largely depends on your specific needs and the capabilities of your equipment. By understanding these differences, you can ensure that you’re using the right connection for your audio setup.