MEASUREMENTS: Apple iPhone 4 & iPhone 6 audio output.

In terms of general look-n-feel, or usability of the product, there's nothing I can say here that has not been said about Apple's most popular devices - the iPhones. Though I'm not an Apple fanboy, my wife loves the Apple "ecosystem" and has been using an iPhone and Mac combination since the release of the iPhone 3G in June 2008.

There is no question that Apple produces some amazing devices focused on usability and pitched as lifestyle products. With enough financial resources, they can of course fund research and compete in the specifications arena as well. In the last few years they did fall behind on screen real-estate but it's good to see that with the iPhone 6, they're making headway in this area as well... This should really give the Android makers like Samsung some good competition in the Asian arena where logosyllabic writing systems predominate and a larger screen size is almost a must.

Since my wife hadn't upgraded her iPhone 4 in quite a while, out came the credit card for this thin "little" guy:

As you can see, this is the "gold" colored version of the regular 4.7" 64GB model (versus the 5.5" 'plus' model with the much-publicized bending tendency). I think the gold color looks nice - at least a little different from the usual silver or white and I'm sure this will sell well in Asia also. Not that it really matters much because if you use a case, the back will be covered anyhow (although the gold trim around the home button looks nice). Notice the camera lens protrusion. Some have commented that this looks bad. Indeed, it will prevent the unit from lying flat and it will "tip" somewhat. Again, it doesn't really matter if you use a back cover. I'd consider this a small cosmetic price to pay for better focusing mechanism, larger aperture, and optical image stabilization (the DxO folks rated the camera function very well).

As you can see, my almost-1-year-old LG/Nexus 5 phone on the right is a little larger with a 5" screen. But check out how thin the iPhone 6 (middle) is compared to the iPhone 4 (left) and Nexus 5. The iPhone 6 feels great in the hand. The rounded corners make it comfortable to hold and it's light but still feels solid. No, I did not try bending this thing :-).

One other thing to notice is that the headphone jack is now at the bottom of the phone. I find this less intuitive than the previous top-left placement.

In use, well, it's an iPhone :-). Runs all the usual apps, nice bright "Retina" screen (all modern 'premium' phones have excellent resolution these days anyhow), very snappy with the A8 1.38 GHz dual-core processor, 1GB of DDR3 RAM (modern Android phones have 2-3GB already). I'm curious what this M8 "motion coprocessor" will bring to the table in terms of future apps (funky 3-axis gyro, accelerometer, proximity sensor).

Let's Talk Sound...

It's interesting after all these years, I have seen few measurements of the sound quality out of these ubiquitous devices which I suspect has taken over the role of the iPod for music playback for many if not most Apple consumers. So without further ado, let's have a look at the output of both the iPhone 4 and 6 to compare and contrast what has happened in the objective measurements of these devices two "generations" apart.

Fist, let's look at the headphone out through the digital oscilloscope. Here's a 1kHz square wave played at full volume (0dBFS):

iPhone 4 - 1kHz square wave 0dBFS.

iPhone 6 - 1kHz square wave 0dBFS.

As you can see, the "square" wave tracings are very similar. Neither phones show any clipping (I confirmed with sine wave as well - not shown). The iPhone 6 is marginally "louder", putting out 1.4V versus the 1.3V from the iPhone 4. Channel balance is excellent on both machines.

Notice the "ringing" in the waveform with both phones... The reason why is readily apparent when we look at the impulse response:

iPhone 4 - 16/44 impulse response

iPhone 6 - 16/44 impulse response

They both maintain absolute phase but as you can see, both phones use a minimum phase filter with no pre-ringing. Interesting! Didn't know Apple has been doing this all these years...

Okay, let's now get to the usual RightMark 6.4.0 measurements.

Setup (the usual):
iPhone 4/6 [100% volume] --> shielded 3' phono-to-RCA cable --> E-MU 0404USB --> shielded USB --> Windows 7 PC

iPhone 4 firmware - iOS 7.1.2
iPhone 6 firmware - iOS 8.0.2

Screen brightness ~50%. These measurements were made with the iPhone 4 connected to my home WiFi router (no SIM card inside) and the iPhone 6 has my wife's SIM card inside and with cell phone and HSDPA data activated. I made no concessions for "better sound" since I don't believe people listen to these devices in a crippled fashion without data connection.

I used the latest ONKYO HF Player (1.2.1, $10 for the hi-res features and FLAC playback) to play the test files. No EQ or any other DSP process like upsampling applied for the test signals of course. I promised folks that I would try measuring the effect of various loads from the headphone output... Alas, I haven't found the time to get this together. Therefore, I'll just give you the results off the E-MU and will update with another post when I get some 30/100/300-ohm load measurements done.

16/44:
The summary result comparing the iPhones with Nexus 5, AudioEngine D3, Dragonfly 1.2, and TEAC UD-501 for a desktop "reference" DAC:

As you can see, the little iPhones hold their own in terms of 16-bit accuracy. As with most devices, 16-bit, 44kHz audio is not an issue these days and all competent devices would have no problem decoding this most basic bit depth and samplerate.

Slightly more high frequency roll-off with the iPhone 6. -0.5dB at 20kHz.

Noise level: All pretty close, Nexus 5 slightly noisier.

THD Graph

Stereo crosstalk: other than the TEAC using stereo RCA connectors, the others all using the same shielded 3' phono-to-RCA cable. The Dragonfly v1.2 has notably higher crosstalk.

24/44:
Okay, let us now go one step up into high-resolution territory. Can the iPhones manage better than 16-bits?

The answer is YES, the iPhone 6 is clearly capable of better noise floor and dynamic range when fed with 24-bit data. The measured performance is between 17-18 bits of dynamic range... Not bad for such a compact device and about on par with the AudioQuest Dragonfly 1.2 previously measured. Since I don't normally measure 24/44 with my other gear, I don't have comparisons in the table to other devices.

I measured the audio output using either the Onkyo HF player (FLAC files) or the Apple iTunes built-in "Music" app (AIFF). As you can see, the iPhone 4 functions as a 16-bit device in terms of noise floor and dynamic range performance when fed 24-bits. It benefits very slightly with 24-bit audio - at best 3dB improvement. Notice that there isn't any real difference between the Onkyo app and standard 'Music' app. Just remember to turn off any EQ feature to make sure it's bit-perfect (I noticed the iPhones Music app had set the EQ to "Classic" or something like that by default; I don't know if this was a setting my wife had previously set).

Some graphs:

Frequency Response: minimal difference.

iPhone 6 benefiting from the 24-bit data compared to the iPhone 4.

THD

Stereo crosstalk. Inter-test variation evident. (The -104.9dB iPhone 6 reading was atypically low; usually around -90-100dB.)

24/48:
Let us now raise the sample rate to 48kHz and see:

Click on the table to enlarge. 

Again, we see that there is essentially no difference between different music player apps on the iPhone (Onkyo HF Player vs. standard "Music" app linked to iTunes). This time I've included 24/48 results from the recently measured Microsoft Surface 3 laptop, the Squeezebox Touch, and a couple of USB DACs - the AudioQuest Dragonfly 1.2, and AudioEngine D3. Note that the Dragonfly and AudioEngine D3 were measured at 24/96 and I mainly wanted to demonstrate the noise floor performance. In terms of harmonic and intermodulation distortion, the iPhones perform well; the only "atypical" performer in terms of distortion is the Dragonfly.

In short, I am impressed by the low noise level and high dynamic range achieved by the iPhone 6's internal DAC! It's getting really close to the AudioEngine D3 in terms of low noise which is superb.

A few more graphs:

Frequency response: Squeezebox Touch seems to have more of a bass roll-off.

Noise floor. iPhone 4 unable to benefit significantly from 24-bit audio.

THD

Stereo crosstalk: Microsoft Surface 3 worst of the bunch here.

24/96:
The chart looks OK (using Onkyo HF Player with FLAC files):

But in reality, it's a no-go:

Neither iPhones are capable of native 96kHz samplerate and the data has been re-sampled down to either 44kHz or 48kHz.

Jitter:
The J-Test audio track was played off the iPhones with Onkyo HF Player for this measurement.

A bit more jitter noted with 24-bit data for the iPhone 4 (sidebands and wider "skirting"). Also notice the iPhone 4 has higher noise level so the jitter modulation pattern in the 16-bit test is not as evident compared to the iPhone 6.

Like essentially all decent hardware measured over the last while, it's hard to make a case for jitter being an audible issue given how low the distortion is; typically way below -100dB off the primary signal. The jitter spectra look the same whether I used the "Music" app or Onkyo HF Player; again, a reminder that software does not affect jitter performance as far as I can tell whether with these portable devices or with computer audio. Remember that this is even with either WiFi or HSDPA wireless data turned on.

Subjective:
iPhones are everywhere, go listen for yourself :-). Heck, bring in your favourite headphones and have a listen at the local Apple Store. Be that the case, I did of course listen to both the iPhone 4 and 6 with my headphones here at home. I really could not put a finger on any significant sonic difference between the two phones so spent most of the time listening to the iPhone 6.

What is most obvious using less efficient headphones like the AKG Q701 is that the headphone amp just isn't strong enough - no surprise there. 100% volume is merely 'loudish' with both the AKG and Sennheiser HD800. One effect of this is that bass frequencies requiring a bit more "oomph" just does not sound impressive using either headphone.

The iPhone 6 comes with the newer EarPod headphones with volume control. These sound better than the old earbuds from previous generations before the iPhone 5. They do feel more comfortable as well in the ear. But there's no denying that these don't sound that good with mediocre treble definition and muddy bass obvious within a few seconds after listening with the Sennheiser HD800.

I listened to both lossy 320kbps MP3 encoded with LAME and AIFF lossless music. The music sounds great using my Sennheiser HD800. Typical audiophile female vocals like Jane Monheit's Come Dream With Me sounded wonderfully detailed with vocal nuances intact. With the iPhone 6 plugged into my AudioEngine A2 speakers for "near-field" listening, the soundstage was excellent and voice well focused. The classic Miles Davis Kind Of Blue sounded nice and warm as it should - nothing was missing, including the elevated background noise and ambient sounds (is that someone clearing his throat 9 seconds into "So What"?). Mark Knopfler's Privateering sounds excellent with multi-layered strings, percussion and vocals on the title track. Loud tracks like Joe Satriani's "Crowd Chant" from Super Colossal (a dynamically compressed DR8 track) sounded fine with the phones capable to rendering details through the "mass" of vocals and playful guitar-voice interchange. More efficient headphones like the closed back Sony MDR-V6 or Audio-Technica ATH-M50 provide plenty of volume and as typical for headphones of this nature, the bass also takes on a more visceral property despite giving up a bit of detail compared to something like the HD800.

Summary:
Apple is without question the "800lb gorilla" of the music industry whether we're talking about music downloads or potentially the future of streaming audio. As a family of audio devices, the iPhone is arguably the most important music player in the world at this point in time. iPhones continue to be a point of entry in portable audio given the popularity. As I publish this post, I see the iPhone 6 is up for pre-order in China today; no doubt Apple will sell massive numbers there despite the higher price point compared to competitors.

While I cannot speak of the iPhone 5's audio performance, there has obviously been an improvement between the iPhone 4 to 6 with the ability to play 24-bit audio. The iPhone 6's DAC measures very well up to the maximum 48kHz samplerate. Furthermore, I was surprised that Apple has been using a minimum phase digital filter at least since the iPhone 4. Not that I really feel it makes much of any difference, but the iPhone can claim "no-pre-ringing" just like the Pono/Ayre folks might claim. Of course, with a typical minimum phase filter like we see here, there is quite a bit of post-ringing. The Ayre folks dampened that with a -6dB at 22kHz slow roll-off filter which IMO isn't necessarily a good thing if you want flat frequency response all the way to 20kHz.

As I mentioned at the start, I unfortunately have not had the time to start measuring using various simulated headphone loads to demonstrate frequency response anomalies especially with low impedance headphones. Ken Rockwell's review of the iPhone 5 puts the output impedance at 4.5 ohms which is excellent! Hopefully the iPhone 6 will be similar in this respect.

Given the measurements I'm seeing, the iPhone's DAC is excellent and can produce very accurate output. Given the small form factor and the need to balance power usage with other phone functions, headphone amplifier power is limited out of necessity. For sonic quality, therefore, the most important factor would be how well the headphone matches the amplifier. Assuming the iPhone 6 has an output impedance similar to the iPhone 5 around 5 ohms, a good ~40+ ohm set of high quality, high sensitivity headphones should provide excellent neutral sound. Not that lower impedance headphones would sound bad of course (most IEM's have low impedance for example), just potentially not as flat frequency response.

I must say that I am impressed by how smooth the iPhone 6 is in use speedwise and the thinner profile with softer curves definitely feels very comfortable in the hand compared to the more chunky iPhone 4. It reminds me of the curved iPhone 3G but with a more "premium" feeling metal case. My wife loves it already. Finally... There is one significant feature I wish the iPhone 6 had - inductive charging. Both my Nexus 5 and 7 have this feature and has wide compatibility with Qi chargers. For the better part of a year now, I have not had to plug the Nexus phone/tablet into anything at all. Even if the iPhone 6 were a couple of millimeters thicker, I think this feature would be worth it!

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Epilogue:

Over the past year, there has been speculation about when Apple might go "high-res" with iTunes. Considering that iTunes does not even offer lossless downloads currently, I certainly would not hold my breath! Furthermore, with the iPhone 6 capable of 24-bit but not higher samplerates beyond 48kHz, the idea of 88kHz+ music doesn't even seem to be on the horizon.

Although I think lossless 16/44 iTunes would be a great idea, I believe Apple is smart not wasting their time in the high-resolution space for the masses. As I expressed previously, I believe "high resolution" audio will be a disappointment to most people after the novelty wears off; it just doesn't sound much better if at all given the same mastering. That's one issues. Another issue is that with devices like the iPod/iPhone/iPad, where there is limited storage space, you can already upload music in ALAC (I don't see why anyone should waste space with AIFF these days) if you really want. 24/96 is approximately 250% the size of the equivalent 16/44 lossless files, and 24/192 would be around 500% the size. Considering how little audible difference there is between high bitrate lossy (256-320kbps MP3 or AAC) and lossless already, it makes no sense to load up a portable device with all these  huge lossless high-resolution files when there are so many other things the storage space could be used for (eg. movies, documents, apps, photos, books/magazines, videos...). When you factor in that the typical listener is likely to be enjoying music in suboptimal conditions like the subway, bus, car, walking around the streets, or exercising - having audio files taking up so much space just doesn't make sense.

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To all the Canadians out there. Have a wonderful Thanksgiving weekend! Enjoy some turkey and of course make sure to take in some lovely tunes...

Addendum (January 29, 2016):
Using a 1kHz signal, measured open vs. 20-ohm load, the iPhone 6 has an estimated output impedance of 3.2-ohms.