One digital input counts as two - how?

[nrand]

I am looking at some AES EBU mixers and switchers for my outboard effects and I have noticed that some might have two outputs for example, designated ! & 2 and 3 & 4 - speaking of four separate signals into two XLR inputs. I have looked around on google, but could not find the right question I guess.
Can someone her educate me about this, how it works, and maybe caution me about any possible pitfalls with such systems?
Thanks!

 

[Steenamaroo]

Have a read through on wiki. There should be articles for all the buzzwords and acronyms.
SPdif/aes/ebu/optical/coaxial/lightpipe/ etc.

For example, "Aes/Ebu" wiki says
"It is able to carry two channels of PCM audio over several different transmission mediums including balanced and unbalanced lines and optical fiber. A consumer variant of the standard, S/PDIF, is also available."

With digital formats the connector type goes out the window. TRS or XLR doesn't mean balanced or stereo like it can in the analog world.

SPDIF is often carried over a single coax cable (RCA to you and me), but it's stereo digital.
I assume they used that connector because everyone has that cable left over from their telly installation.

In your example, it sounds like there are two separate stereo digital ports.
Hope that's useful.

 

[Mo Facta]

First thing to remember is that digital signals are just data so multiple channels, depending on the protocol, can be carried over one data stream on a single transport medium. MADI, for instance, can send 64 channels over a single BNC (coaxial) cable. The channel capacity is determined by the protocol. An AES/EBU signal uses a stereo-interleaved PCM (wav) stream to send both channels over a single balanced cable. This is how it does this (brace yourself for tech jargon overload):

The bit stream consists of the PCM audio data broken down into small samples and inserted into a larger structure that also carries various status and information data. The highest level organization is the audio block, which roughly corresponds to a number of samples of the PCM data. Each block is broken into 192 frames numbered 0 to 191. Each frame is further divided in 2 subframes (or channels): A (left) and B (right). Each subframe contains the information for one single sample of the PCM audio, or more simply, one channel of audio. Each subframe is organized into 32 time slots numbered 0 to 31, each of which corresponds roughly to a single bit. Not all of the time slots send actual audio data: a number of them are set aside for signaling use, and others for transmitting data about the channels. In normal use only 20 time slots are used for audio, providing a 20-bit sound quality (compare with a CD at 16 bits per sample). So a complete audio block basically contains 192 samples from two channels of audio and other data, containing 12288 bits in total.

Hope that clears it up for you.

Cheers

 

[Farview]

spdif and aes/ebu are stereo digital protocols. The left channel is one input (or output) and the right channel is the other.

Think of it as a headphone jack, one cable-two signals