Apple’s Best-Kept Secret About Audio I/O on Macs

I bought my first Apple computer on Thanksgiving night, 2006, at one of those “early Black Friday” sales at some now-defunct computer chain. It was a HUGE (by the standards of the day) 24″ iMac that was near the top-of-the-line in terms of their technology offerings back then.

One of the things I planned to use it for was audio recording and production.

Like most Macs, it had two 3.5mm jacks on the back, one for a mic/line input and one for headphone/speaker output.

Several years earlier, I had purchased a small portable A/D unit for recording stuff using “stealth mics”, and it has an optical digital (S/PDIF) output. I went to the Apple store and asked the “geniuses” there what I needed to plug it into my fancy new iMac.

I recall talking to a few different people, including their “top audio experts” (ie., their ‘mega-geniuses’), and about all they could tell me was that I probably needed some kind of external interface. They got online and did some searches and came up with a handful of external I/O units, most of which cost several hundred dollars.

I went home rather dejected that I’d have to spend even more money to hook my fancy digital A/D converter unit up to my iMac.

Then I got curious about what the exact technical specs were on the audio ports built into the iMac. So I started looking around the materials I got with my iMac, and I didn’t find anything useful. Next, I started searching around Apple’s website for the details. It took quite a while, but I finally found something. And it totally blew my mind!

WHAT BLEW MY MIND…

It turns out that every Mac computer made for as far back as I could find (at least since before 2000) has a super high-quality A/D and D/A converter built right into it — as well as integrated optical digital (S/PDIF) capability. WOW!

All I needed to connect my fancy external A/D unit to my iMac — or any other Mac computer I could find — is an optical S/PDIF cable with a 3.5mm (1/8″) plug on one end. YOWZA!

In case you’re wondering what I’m referring to, here’s one you can check out:

See that plug on the left? It goes into either the Mic or Headphone jack on your Mac computer or laptop. The one on the right is the older, more common TOSLINK style plug for optical S/PDIF connections. (These come in several varieties, with mini-plugs on both ends as well as TOSLINK plugs on both ends, in a variety of lengths.)

WHAT DOES APPLE HAVE TO SAY ABOUT THIS?

Quite a bit, actually. Only it’s freaking HARD TO FIND!

Here’s what I found about my 2006 iMac from their site today.

Audio In / Optical Audio in port

The Audio In port accommodates both digital optical input and analog audio input.

Analog audio line input is accepted through a 3.5mm mini phone jack. The sound input jack accepts line-level stereo signals up to 24-bit stereo 44.1-192kHz sampling rate. It also accepts a stereo miniplug-to-RCA cable adapter for connecting stereo equipment to the computer.

Optical audio input is SPDIF format and uses a standard Toslink cable with a Toslink mini-plug adapter, accepting up to 24-bit stereo and 44.1-96kHz sampling rate.

The iMac (Late 2006) also has a built-in microphone near the built-in iSight camera at the top of the computer screen bezel.

Headphone out / Optical audio out port

The 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 analog headphone / line output a standard audio cable with 3.5mm plastic or nylon optical plug should be used.

Click here to run a search on Apple’s site to return pages that document technical specs for most of their hardware. You want to include these terms in your search: “external features, ports, and connectors”.

It’s getting increasingly difficult to find the actual technical specs on the Audio I/O ports in their hardware. They’ve always kept this information somewhat of a secret for whatever reason. And I cannot tell you how many Apple Stores I’ve visited in different cities and asked their “geniuses” about this, and nobody can tell me!

Here are the tech specs I found for a “Late 2012 and later” iMac. I include this because these specs are fairly typical for their hardware going back at least a decade, if not further. As an aside, getting this high-quality audio I/O on a Windows computer has typically required a fairly expensive audio adapter card! (I’ve highlighted the relevant specs using italics.)

I/O ports

1. Headphone port

Line/headphone output 
The headphone output is automatically selected for audio output if no external S/PDIF optical digital output device is detected. The headphone output supports a stereo data stream at bit depths of 16, 20, or 24 bits per sample and at sample rates of 44.1 kHz, 48 kHz, or 96 kHz. The headphone output volume can be adjusted from 0.0 dB to -43.0 dB.

During playback of a 1 kHz sine wave at -3 dBFS voltage level, 24-bit sample depth, 44.1 kHz output sample rate, 100 k load (unless otherwise specified), the audio output has the following nominal specifications:

  • Jack type: 3.5 mm (1/8-inch) stereo combo
  • Maximum output voltage: 1.4 VRMS (+5.15 dBu)
  • Output impedance: <24 ohms
  • Frequency response: 20 Hz to 20 kHz, +0.5 dB/-3 dB
  • Signal-to-noise ratio (SNR): >90 dB 
  • Total harmonic distortion + noise (THD+N): <-80 dB (0.007%)
  • Channel separation: >85 dB

S/PDIF optical digital output 
The S/PDIF (Sony/Philips Digital Interface Format) optical digital output is automatically selected when an S/PDIF optical digital output device is detected on the external combination audio port. The S/PDIF optical digital output supports pulse-code modulation (PCM) and Arc Consistency Algorithm #3 (AC-3) audio formats with the following stereo data stream characteristics:

  • PCM: 16, 20, or 24 bits per sample at sample rates of 44.1 kHz, 48 kHz, or 96 kHz
  • AC-3: 16 bits per sample at sample rates of 44.1 kHz, 48 kHz, 88.2 kHz, 96 kHz

The S/PDIF optical output channel status conforms to International Electrotechnical Commission (IEC) 60958-3 consumer mode digital audio.

During playback of a 1 kHz sine wave (S/PDIF output format at 0 dBFS output level, 44.1 kHz sample rate, 24-bit sample depth, unless otherwise specified), the digital audio output has the following nominal specifications:

  • Jack type: 3.5 mm (1/8-inch) stereo combo
  • Digital audio signal-to-noise ratio (SNR): >130 dB
  • Digital audio total harmonic distortion + noise (THD+N): <-130 dB (0.00003%)

Honestly, the only thing you could improve upon with an external A/D+D/A converter is 192 kHz sampling rate and a better S/N ratio on the input. But for 24-bit sampling rates, your S/N ratio should be well over 104 dB on both sides, with over 110 dB dynamic range.

Again, if you own any Apple computer that runs OS X built since 2000 or so (even the older G-series machines), your hardware has similar audio specs to these! The biggest difference is going to be lower sampling rates on the older hardware — so maybe just 44.1 and 48 kHz and 16- and 20-bit sampling depths. The newer hardware goes up to 24-bit 96 kHz.

WHAT ABOUT MOBILE DEVICES?

If your Apple hardware runs OS X, then it probably has specs similar to what I’ve shown above.

For whatever reason, Apple’s mobile devices are not as well-endowed. Let’s just leave it at that.

2 thoughts on “Apple’s Best-Kept Secret About Audio I/O on Macs

  1. Doccus

    Seriously?? If it sees a digital device on the other end the headphone jack is S/PDIF? That’s freakin wild! Except I am now CHOKED I got rid of my old DAT player/recorder, wnowing I could have directly hooked it up!. Well, I’ll look for another!!!! I actually was searching because I wanted an interface to hook it up to my analog cassette player. Doesn’t look like it’s a typical speaker output, though, now. Thanks for doing the legwork..
    I hae a lot of *extremely* rare and collectible cassettes that need repair, some including audio. There’s just no way I’ll ever find another copy to dub over to the new tape, so has to be off of my imac..
    Thanks . One less interface I have to buy 😉

    Reply
  2. escocesrojo

    I was about to buy a $150 DAC to connect to one of the iMac’s USB plugs, when the idea of using toslink cable in the audio output came to my attention. The other end of the toslink cable will be connected to an Orei DA21 Optical SPDIF/Coaxial Digital to RCA L/R Analog Audio Converter with 3.5mm Jack Support Headphone/Speaker Outputs. I think I should be able to plug a Sony MDR V6 into the headphone out on the Orei DA21. I could also use a short stereo RCA cable from the Orei DA21 to a stereo receiver, then connect the headphones to the receiver’s headphone jack for greater volume control.

    Reply

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