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Audio Connections between components -- Digital and Analogue

I regularly encounter people who don’t know whether a connection between two components is analogue or digital, let alone the differences between them, so I thought I’d write up a quick guide. 



Click on the picture for a larger version.


A connection is either analogue or digital, not both. Different analogue connections can be connected with an adaptor cable, but different digital connections cannot. You cannot use a cable to connect a digital socket to an analogue input. That requires a DAC (Digital to Analogue Converter).


Confusingly, some 3.5mm (1/8") analogue headphone sockets also have an optical digital transmitter built in. When a 3.5mm (1/8") headphone plug is connected, they are analogue. When a mini-optical cable is connected they are digital.



3.5mm (1/8”) TRS

6.5mm (1/4”) TRS

These are the same plugs and socketsyour headphones and IEMs use, because they carry the same signal essentially.  TRS stands for “tip, ring, sleeve”. The tip carries the left signal, the middle ring the right, and the bottom and sleeve are the combined ground and signal return.

RCA — “Single-ended"

This is the most common plug for connecting hi-fi components. Left and right each have their own plug, where the centre pin is the positive and the outside shell is the negative and ground. The difference between RCA and TRS is just that the plugs are different, so you can buy an adaptor cable to go from one to the other.


XLR — “Balanced"

Like RCA, left and right each have their own plug. The difference is that the ground is separate from the signal return, which is a mirror-image signal of the positive. It is most commonly used for long cable runs in pro audio as this set-up cancels out noise. It is also popular in high-end audio as many components use differential (balanced) topologies to cancel out noise and/or increase output power, depending on the intent of the designer.



If you are connecting devices using a digital connection, the device that receives the connection and outputs music to your headphones is doing the digital to analogue conversion. The transmission will be usually in the PCM format. A digital connection doesn't send an MP3, FLAC, AAC or ALAC file, but it is converted (decoded) to PCM first and sent using a specific method of digital audio transmission, usually either USB or S/PDIF*.



A balanced version of S/PDIF that uses a 3-pin XLR plug, similar to analogue XLR, but carrying a digital signal instead.



Some computers and phones now support APTX audio transmission, which gives almost CD quality over wireless.



Sony/Phillips Digital Interface. The most common digital connection available other than USB. Technically, it should use 75 Ohm coaxial BNC cables, but for convenience most manufacturers use RCA sockets and cables which look similar to the RCA cables used for analogue connections.



Toshiba Link. This has become the standard optical transmission method of S/PDIF. The almost square-shaped plug and socket is known as Toslink, and the 3.5mm plug is known as mini-optical. Some headphone sockets, such as on Apple's computers and Astell&Kern DAPs, have a headphone socket that is also a mini-optical port.


USB digital audio has a specific transmission standard that is different to how digital data is transmitted to and from regular computer components.


*Note: As this is a basic guide, I haven't mentioned DSD.

Comments (4)

Thanks for making this Currawong! I was certainly confused between the different inputs/outputs of audio interfaces and this guide is a quick and easy-to-understand reference.
Although I had a grasp of the basic terminology, it was nice of you to spell everything out here. I used to get a little confused seeing both analog and digital cables that have XLR connectors.
Here's a good question: which devices should use analog power cables, and which should use digital ones?
"Audiophile" power cables supposedly work by rejecting high-frequency noise and interference that can affect audio circuits. I'd guess that different types of noise affect different types of circuits to greater degrees. The main thing I avoid is cables with gold-plated pins on the plugs, as gold isn't a good conductor. The cables I use aren't so exotic as they are mostly stiffer with better shielding.
Well, what I was referring to was power cables that are labeled as, for example, "analog AC power cord" and "digital AC power cord" within the same product line, and which devices each type are compatible with. For one, I think all headphone amplifiers would use the analog one, but I could be wrong.
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