About that Apple Optical Output Port – It’s a TOS up
by Robert Pritchett, November 2006
I received a call from Micah Sheveloff about Thiel http://www.thielaudio.com/ wanting to get into the computer-based entertainment center business and with the Macs specifically. Their speaker systems require an amplifier between their units and the computers and I instantly thought of that little-used/known port on every Mac built after September 2005, including the Mac minis, MacBooks, iMacs, Mac Pros and even the Airport Express named the “Optical Audio Port”. http://developer.apple.com/documentation/Hardware/Conceptual/HWTech_Audio/Articles/Audio_concepts.html
So I had to do a little research and thought you might be interested in what I found.
Do you know what I’m even talking about? It’s the headphone jack!
But you see, Apple didn’t just take the port and make it just for headsets. They took it up a notch, literally. The audio port is a combo port, meaning that it takes copper and fiber type connectors. These jacks provide digital audio out as well as analog audio.
So first of all, why fiber? No electrical interference. Audiophiles like that, and so will you. No pops, hiss or jitter. Clear sound.
So let’s get geeky for a moment. We’ll discuss the mini-toslink and S/PIDF in a little bit, just stay with us.
Each Mac takes the technology just a little bit differently. The Apple Specs provide the following information:
The PowerBook G4 has both a combined optical digital input/audio line in (minijack) (17-inch model) and output/headphone out.
The Mac mini, MacBook and MacBook Pro duplicates that, but also the headphone/line output jack accommodates digital optical audio output, analog audio output with a 24-bit, 44.1-192 kHz D/A converter, digital audio output up to 24-bit stereo and 44.1-192 kHz sampling rate and supporting encoded digital audio output (AC3 and DTS standards). Analog audio line input is accepted through a 3.5mm mini phone jack. It also accepts a stereo miniplug-to-RCA cable adapter for connecting stereo equipment to the computer.
Optical audio input is S/PDIF format and uses a standard toslink cable with a toslink mini-plug adapter, accepting up to 24-bit stereo and 44.1-96 kHz sampling rate.
For analog headphone/line output a standard audio cable with 3.5mm metal plug should be used. For digital audio, a standard toslink cable with a toslink mini-plug adapter can be used.
The early 2006 iMac headphone / line output jack accommodates both digital optical audio output and analog audio output with a 24-bit, 16-96 kHz D/A converter. For digital audio, a fiber optic cable with 3.5mm plastic or nylon optical plug should be used. The late 2006 iMac sound input jack accepts line-level stereo signals up to 24-bit stereo 44.1-192kHz sampling rate. Notice the difference between those two?
The Power Mac G5 computers (and we can probably assume, so do the Mactels) have two optical audio ports of a type new to Macintosh. These optical audio ports allow the transfer of stereo audio or encoded 5.1 audio (commonly found on DVD movies). And the Power Mac G5 uses the Sony/Phillips Digital Interface (S/PDIF) protocol (IEC60958-3) for data and the Toslink optical (IEC60874-17) for the connector. Bits per sample are either 16-bit or 24-bit. For optical digital audio out, the playback sample rate of a 1 kHz, -1 dB FS 24-bit sine wave, 44.1 kHz output sample rate is usually around 32 kHz, 44.1 kHz or 48 kHz. The signal-to-noise ratio is greater than 130 dB. Total harmonic distortion + noise is less than -130 dB (0.00001 percent). For optical digital audio in, playback of a 1 kHz, -1 dB FS 24-bit sine wave, is usually Fsi-input sample rate (external clock mode) around 32 kHz, 44.1 kHz or 48 kHz Fsi-input sample rate (internal clock mode) of 16 kHz to 96 kHz signal-to-noise ratio (external clock mode). Greater than130 dB signal-to-noise ratio (internal clock mode, 16 kHz < Fsi < 96 kHz), is less than -112 dB. The total harmonic distortion + noise (external clock mode) is less than -130 dB (0.00001%) and total harmonic distortion + noise (internal clock mode, 16 kHz < Fsi < 96 kHz) is less than -112 dB (0.0003%). Feel all warm and fuzzy, now that you know that?
Now armed with that knowledge, how do we use it? We need to learn more:
Apple says;
“All current Macintosh computers include the circuitry to play high-quality audio content. Hardware features differ among Macintosh models, as appropriate for the expected use of that model.
Digital audio I/O conforms to the Sony/Philips Digital Interface (S/PDIF) model. S/PDIF technology results in a clean audio signal with no noise added as a result of transmission to or from the external audio device.
Most models support analog audio in and typically have a built-in microphone. Some models support S/PDIF digital audio in through an optical digital interface.
All models support analog audio out, through an internal speaker or speakers, a headphone jack, a line-out jack, or some combination of the three. Computers described in this note also provide digital audio out, through both an electrical interface and an optical digital interface.
3.5 mm (1/8”) Combination Audio Jack
Both electrical and optical audio signals are provided by the computer through the combination audio jack, commonly called the combo jack. The combo jack accepts standard electrical audio cables with a 3.5 mm (1/8”) stereo plug and standard TOSLINK optical cables with a 3.5 mm optical plug. Adapters are available that attach to the friction-lock type F-05 plug to convert it to a 3.5 mm optical plug. TOSLINK cables are available from pro-audio, musician’s supply, hi-fi and other retailers.
The electrical signals for the combo jack are connected as they are with the standard 3.5 mm electrical jack, described in the previous section.
Note: The terms 3.5 mm and 1/8-inch are interchangeable in defining the jack.
Digital audio signals can be provided by a 7.5 mm
optical digital jack. The physical connector, shown in Figure 1, is commonly
referred to as a TOSLINK connector. The jack is for both input and output and
conforms to IEC 60874-17.
Figure 1 Optical digital S/PDIF connector
Cables with TOSLINK friction-lock type F-05 connectors are available from pro-audio, musician’s supply, hi-fi and other retailers.
So digging a little deeper I found these links on jacks and S/PIDF:
http://en.wikipedia.org/wiki/Jack_plug
http://en.wikipedia.org/wiki/S/PDIF
http://www.andrewkilpatrick.org/mind/spdif/
Many of the pics found here were taken from Ram Electronics. http://www.ramelectronics.net You might want to visit them for more information.
The Apple Optical Audio port takes the fiber optic Toslink mini jack. So what the heck is a Toslink connector?
TOSLINK® is a registered trademark of Toshiba and comes from TOShiba-Link which is a variation on the EIAJ optical “Red Book” standard for digital audio (stereo or multi-channel) that uses light from an LED "transmitter" as the source, instead of laser light. TOSLINK was established to create digital streams between MiniDisc, CD players and DAT recorders using either 1 mm plastic optical or glass fiber. Signal cable has to be less than 10 meters in length and usually are less than 5 meters in length. The connector itself is usually a JIS F05 (JIS C5974-1993 F05).
For more information look here:
http://en.wikipedia.org/wiki/TOSLINK
http://www.audioholics.com/techtips/audioprinciples/interconnects/toslink.php
Okay, now what? We can either use the headset jack or get fancy and interface with Audio-Visual gear that uses the Toslink technology directly or the S/PID copper solution. But with the Apple jack, we need to use a mini-toslink first to get from here to there, assuming we want to do the fiber thing. (And we do!)
Apple sells the Belkin PureAV Digital Optical Audio Cable with mini-toslink adapter
http://store.apple.com/1-800-MY-APPLE/WebObjects/AppleStore?productLearnMore=TA968LL/A for about $30 USD.
And Apple also sells the Monster iCable Fiber kit for the about same price
or you can get it here - http://www.monstercable.com/productPage.asp?pin=2809
And there is a Monster Cable $40 kit for the AirPort Express on Apple’s website as well.
In most cases, if you don’t have Audio/Visual receivers that have a direct toslink port, a converter to 75 ohm coax (S/PDIF) may be needed and Ram Electronics provides those.
Converters are required to move from toslink to S/PDIF and can range anywhere from $25 USD to $40 USD
http://www.ramelectronics.net/html/audio_toslink_adapters.html
So how do we hook these up? Ram Electronics provided a somewhat skewed diagram: http://www.ramelectronics.net/assets/images/av-connect/Home-Theater-connection.pdf
If you can go fiber-to-fiber (toslink-wise), you are better off, but your distance is limited due to the LED source characteristics. Ram Electronics provides a toslink extender for another $30 that can take the fiber cable out another 30 meters. They also provide toslink splitters ($10 USD) and switches ($30 USD).
Coax is cool, but fiber is better, believe me. And Apple has had this technology available for 2 years and you probably didn’t even know you had it on your machine. Now you do!
See how Dave Nagel put it to use:
http://www.creativemac.com/articles/viewarticle.jsp?id=69934
and how he used the Belden mini-Toslink kit - http://www.creativemac.com/articles/viewarticle.jsp?id=47126
In the future, hopefully we will see how Thiel puts it to use too.
