Spdif & adat

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chrisshennan

New member
Hey I'm trying to find a beginner friendly explanation on SPDIF and ADAT.

As far as I'm aware, both use the same TOSLINK cable and you could use SPDIF to connect one audio interface to another to create more channels. is this right?

Kind of lost!

This is for a college course I'm delivering and i need to get the students to understand what it can be used for

Thanks
 
Spdif uses an optical cable to digitally transfer 2 channels of audio.

ADAT uses an optical cable to digitally transfer 8 channels of audio.

Some pieces of equipment can do both with the same output, with some sort of software switch to choose between the two.
 
chrisshennan said:
Hey I'm trying to find a beginner friendly explanation on SPDIF and ADAT.

As far as I'm aware, both use the same TOSLINK cable and you could use SPDIF to connect one audio interface to another to create more channels. is this right?

Kind of lost!

This is for a college course I'm delivering and i need to get the students to understand what it can be used for

Thanks
Click to expand...

Hi,
I think the difference between digital audio protocol and physical connector is an important distinction to make.

Both can use the same interface, but the interfaces and protocols aren't necessarily inextricably linked.

S/PDIF protocol is capable of carrying two channels of digital uncompressed audio or six (maybe 8?) channels of compressed audio, which is why you see it on consumer 5.1 systems.
It can use coax cables or fibre cables.
Sometimes you'll see it terminated with rca connectors...sometimes with TS plugs, sometimes with lightpipe/toslink.

Adat is most commonly seen over fibre with toslink connectors but I'm pretty sure it can be carried over other types of physical connector/cable too.

Anyway...The point is if someone says they need an S/PDIF cable, you don't necessarily know what they mean. :)

Neither 'creates' anything, as such.
If an audio interface has, say, two mic preamps and spdif via coax, it is said to have four audio inputs. In discrete audio recording, we don't tend to consider compressed streams.
If that same interface had an additional adat/lightpipe input, we'd say it is a 12-input interface, although the adat protocol now provides the additional option of only carrying four channels at double the sample rate.

The same rate limitations might be worth noting too.
I think* (and please do google to be sure) adat protocol over lightpipe is limited to 8 channels at 48k or 4 channels at 96.
SPDIF, I believe can do the two channels at 192k over coax or toslink.

The connected hardware, in either case, may or may not be able to avail of the maximum sample rates.
 
Yes, it is a protocol thing. And, there's variations. I have s/pdif on XLR connectors, too, but it is common to associate ADAT with optical and s/pdif with rca.

Optical can be a benefit because the signal transmission over the cable length isn't affected by electro/magnetic fields. Suggesting that for digital to work optimally, rock solid sync has to be maintained. That's not always the case : )
 
oh...my...god, this has blown my mind :confused:

Okay, so, they both transfer digital audio-i got that!
But why would this be used? In what context? I find it a bit odd for this to be in the curriculum for this course as it seems like an advanced understanding.

Is there any way to boil this down a bit?
Maybe give some scenarios where it would be found?
Why would a student need to know this etc?

Thankyou for the answers so far though
 
Steenamaroo said:
That's a great point! :)
Click to expand...

haha Well, its supposed to be : ) My last optical transfer was full of clicks. Though I tested it first, maybe a pets tail compromised the optical connection. Now, I only have budget optical cables (3 for a dollar type) for that odd thing. I can't afford good co-ax cable either - mostly video type cable
 
A few years ago, ADAT and spdif were the main ways to connect digital gear. (Along with aes/ebu)

You had to know this to connect your stuff up. Now with everything in the computer, there are no physical connections to mess with.

But, a lot of interfaces use both ADAT and spdif for extra inputs. To use them, you need a set of mic preamps that have ADAT or spdif outputs, to connect into your interface.
 
AAC, AC3 and DTS can also be transmitted through common optical and coaxial connectors. Although those protocols usually aren't used for pro audio, it illustrates the difference between signal protocol and physical interface.
 
"Music Technology level 3"

Well, it is certainly a consumer thing - since they offered advanced degrees is setting the VCR clock. My reference DVD/CD player has analog and digital out. The setup menu allows me to disable the analog out circuit to reduce noise in the the digital out circuit.

There is somewhat of a Hi-Fi revival with computer and digital music. The Music Consumer is buying USB digital playback boxes for "improved" sound for the MP3 and Hi-Rate Audio files. That also exists without the computer and USB. So, a plam-sized box can be a headphone amplifier with USB, Optical In, spdif in and out and can play the highest rate digital files. Inputs on consumer digital ;
 

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A couple of points about S/PDIF if I may? The digital signal is conveyed over "75 Ohm" cable (and connectors but see *) . The common TV downlead cable IS 75R but a lot of cable is not, satellite cable for instance. Now, in the "studio" people tend to use whatever RCA to RAC cable comes under their hand for digital and for short, sub 3mtrs say, runs this usually matters mot a jot. However, if you want to run S/PDIF over a longer run it can be a problem unless the cable is of the correct "impedance" .

I think the suggested maximum cable length for S/PDIF is 5mtrs? (same as for USB) but a few years ago I did an experiment with the digital OP of a Mdisc to a pretty bog S PCI card and got perfect transmission over some 30mtrs of high quality "low loss" UHF TV cable. (I also successfully sent MIDI over 44mtrs of CAT 5 but that, as they say is another story!) . Bottom line...Unlike analogue audio, cable quality for S/PDIF CAN matter!

There are very cheap optical to co ax and V-V converters. Had a few and they work very well. They are little more than a hex inverter switch and a LED/P'CELL and so I wondered a few years ago if you could convert ADAT to co ax, send over many mtrs (long fibre links are expensive) then convert back to ADAT optical the other end? I never got a satisfactory answer and don't have the ADAT gear to try it myself.

*I once asked Neutrik what the impedance was of their big, "pro" RCA plugs. "Indeterminate" was the reply but they said they worked tolerably well if mated with decent co ax for digital. If you are making S/PDIF cables up it cannot hurt to use the big chunky gold plated jobbies! You will also find 75 R coax with a flexible core hard to source. No matter, use solid core because these are not cables you thrash about with!

Dave.
 
Meant to add...If you are doing anything at all serious with S/PDIF the proper connector is BNC but beware! They are also more common in 50 Ohm impedance than 75 so do check.

Dave.
 
chrisshennan said:
But why would this be used? In what context?

Maybe give some scenarios where it would be found?
Why would a student need to know this etc?
Click to expand...

There are various practical uses for s/pdif & ADAT to send & receive digital audio. And as said, there are different types of digital audio connections, the connection type itself, ie; what kind of physical connector used, and the protocol, ie; the electrical properties of a given type, how many channels it's cable of transmitting & receiving, etc.
That said, the s/pdif 'protocol' (2 channel/stereo digital audio) can be carried over coaxial cable with RCA connectors, BNC or other connectors which are typically 75 Ohm, or s/pdif can be carried over optical cable with Toslink connections.
ADAT can also be carried over the same optical cables & Toslink connectors as s/pdif, but the 'protocol' is different than s/pdif, because ADAT carries up to 8 channels of digital audio. This means you can have 8 mono channels, or 4 stereo channel pairs depending on how you assign them, pan etc.
Note;There's also ADAT SYNC (synchronization) cable connections using 9 pin D-sub connections, though they commonly transmit 'sync' and midi. I believe for digital audio, the D-subs transmit the AES protocol as far as I know. Using Toslink over optical is the standard for ADAT digital audio though.
Furthermore & typically, as I understand it, s/pdif over BNC is often associated with a more professional standard (+10 dB), while RCA's are typically associated with a consumer standard (-10dB).
You should research and verify what anyone say's on forums though, including from me.

The reason a student would need to know this, is to be able to understand how use it properly for a given scenario. To have a basic understanding (only what I have) just to be able to use it (what I do) is all that's needed. You don't have to be an expert (which I am not).

In my own practical use; I use mostly s/pdif over 75 Ohm coaxial cable with RCA connections. I use cables made specifically for digital audio (rather than standard audio cables). I buy HOSA brand myself, they're not expensive, but they're good.
What I use s/pdif for, is to send/receive stereo digital audio signals among music DAW (Cubase) computers, via their own audio interfaces (higher end external sound cards) installed.
I also use ADAT over optical cable/Toslink connections when I need to send 8 channels of digital audio among my DAW computer systems.
S/pdif & ADAT is used to send/receive digital audio among many other types of devices, not just DAW's of course....consumer & pro devices.

Another common use for ADAT is when using 2 audio interfaces in the same computer, one ran into the other via their ADAT, typically to to add more I/O's.

If sending digital signals via s/pdif or ADAT devices outputs, directly to receiving devices via their s/pdif or ADAT inputs, you don't typically need anything in between those devices except cables.
If you're using an s/pdif or ADAT input for non s/pdif or ADAT devices, you'll need (what was said by someone in this thread) a Mic pre with s/pdif or ADAT out. I myself do not require anything but digital cables in my own scenario, as I send & receive only pure digital audio with s/pdif or ADAT among my audio interfaces My interfaces take care of any digital to analog/analog to digital conversions as needed, to my monitoring equipment etc.

Digital audio is not only cable of carrying 'digital audio' but also can carry clock information, synchronization, midi information and transport information. In fact with my own use, I do just that with my DAW PC's, running Stenberg Cubase's own proprietary VST System Link, which works via any type of digital audio connections among any number of computers. This allows one to run several computers in perfect synchronization, linking the transports together.
 
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.." The setup menu allows me to disable the analog out circuit to reduce noise in the the digital out circuit".

No one seems to have caught which circuit is, actually, the Noise King
 
Bit late, but I can add a little more to the info here. I was one of the few involved in starting that qualification up, many years ago - and you are quite right that in Level 3 there is no requirement for in-depth understanding of digital protocols, BUT - students are expected to understand important differences and critical areas in their dealing with connecting kit up. So History is pretty important.

Important things for them to remember - sp/dif = Sony & Philips Digital InterFace - a protocol designed to be used between domestic and 'prosumer' equipment, although remember that prosumer is a fairly modern contraction. It can be carried by optitical fibre, using Toslink connectors, and short domestic grade fibre, or can be carried as an unbalanced connection on ordinary phono connector fitted cables. Initially designed as 110 Ohm impedance, it was capable of functioning on typical hi-fi type cables with indeterminate impedance as long as lengths were 1.2m or less - which was always the length of cables supplied when new (4ft). One of the important factors of the equipment connected by sp/dif was the use of copy protection bits in the data stream. A domestic/consumer Sony DAT machine, connected to a Philips one would add a flag to identify the recording being played as either an original, which would then be able to be recorded onto the Philips (or any other machine fitted with the copy protection, which was ALL domestic and semi-pro machines). However, this copy of the original could not then be copied again - attempting this either simply would not engage record, or worse still, the record light came on, the tape ran, and nothing got recorded. The system was also fitted to some CD players, again making copying a CD to a DAT, for example - impossible, unless you went analogue - which of course is what people did.

The AES/UBU data format is very similar, balanced at 110 Ohms, and does not have copy protection fitted.

Optical to ordinary cable adaptors were available too.

Some DAT recorders were virtually Identical. Panasonic and Technics, both identical, bar colour (white or black) had price tickets where the Panasonic 3700/3800 was double the price of the Technics SV-DA10 - but the Panasonic had audio ins and outs on XLR, with AES/EBU on XLR, while the technics had sp/dif and phonos! AND Copy Protection. A thousand pounds for making a copy of a CD!

Worth also noting that students need to know that you cannot connect an ADAT machine to a Toslink equipped stereo machine - same connector, different protocol.
I remember one year a student asserting this worked, but only the first two channels were recorded. Total rubbish.

The kids for Level 3 need to know the components that would help them make recordings given unfamiliar gear and a pile of cables. Although I have no involvement now, I'd seriously doubt if they need to know about impedance. Fibre is a good subject to show they understand the benefits of fibre vs copper.

ADAT as a connector style is of course very current - Behringer's X32 system for example uses the protocol to throw chunks of 8 tracks around between equipment. Probably also worth spending five minutes on what exactly ADAT recorder were, and why they were pivotal in the emergence of affordable digital recordings.
 
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