Astell&Kern AK100 portable media player Measurements

Sidebar 3: Measurements

I examined the Astell&Kern AK100's electrical performance with Stereophile's loan sample of the top-of-the-line Audio Precision SYS2722 system (see www.ap.com and the January 2008 "As We See It"); for some tests, I also used my vintage Audio Precision System One Dual Domain. I made sure the AK100's battery was fully charged before performing the tests. For source signals, I both used WAV files stored in the player's flash memory and fed it S/PDIF datastreams via its TosLink input. The TosLink input successfully locked to data with sample rates of 44.1, 48, 88.2, and 96kHz, but not 176.4 or 192kHz. The maximum output level at 1kHz was very slightly higher than the specified 1.5V at 1.55V, and the player preserved absolute polarity for both internal and TosLink-sourced data. The output impedance was 22.5 ohms at all audio frequencies, including 6' of interconnect.

Fig.1 shows wideband spectra of the AK100's output while it decoded data sampled at 44.1kHz representing a full-scale 19.1kHz tone (cyan and red traces) and white noise at –4dBFS (blue and magenta). The white-noise spectrum rolls off rapidly above 22kHz, revealing that the AK100 uses a conventional reconstruction filter. An aliasing product of the 19.1kHz tone is visible at 25kHz, but this is suppressed by 84dB. These spectra were taken into the high 100k ohms load; the second and third harmonics of the 19.1kHz tone can be seen at –103dB (0.0007%) and –76dB (0.015%), respectively. The AK100's frequency responses with 44.1kHz data (fig.2, green and gray traces), 96kHz data (cyan and blue), and 192kHz data (blue and red) indicate that the output rolls offs rapidly just below half the sample rate and that the passband is free from ripples. Considering that this is a small, battery-powered device, the AK100's channel separation was superb, at >115dB in both directions below 1.5kHz (fig.3). The usual capacitive coupling between channels reduces the separation to 93dB at the top of the audioband, which is still excellent.

713AK100fig01.jpg

Fig.1 Astell&Kern AK100, wideband spectrum of white noise at –4dBFS (left channel blue, right magenta) and 19.1kHz tone at 0dBFS (left cyan, right red), with data sampled at 44.1kHz (10dB/vertical div.).

713AK100fig02.jpg

Fig.2 Astell&Kern AK100, frequency response at –12dBFS into 100k ohms with data sampled at: 44.1kHz (left channel green, right gray), 96kHz (left cyan, right magenta), 192kHz (left blue, right red) (0.25dB/vertical div.).

713AK100fig03.jpg

Fig.3 Astell&Kern AK100, channel separation, L–R (blue), R–L (red) (5dB/vertical div.).

For consistency with my tests of digital products going back to 1989, I estimate a DAC's resolution by feeding it dithered data representing a 1kHz tone at –90dBFS with 16- and 24-bit word lengths and sweeping a 1?3-octave bandpass filter from 20kHz down to 20Hz. The result is shown in fig.4, with the 16-bit spectrum the top pair of traces, the 24-bit spectrum the middle pair. The traces are free from harmonic products and the increase in bit depth drops the noise floor by up to 15dB, suggesting resolution between 18 and 19 bits, which is excellent considering the low maximum output level. This is readily enough resolution to allow the AK100 to resolve a dithered 24-bit tone at –120dBFS (bottom traces). Fig.5 repeats this analysis using a modern FFT technique, which unmasks some very low-level 120Hz components with the 24-bit data (blue and red traces). As the AK100 is battery-powered, I have no idea where these arise, but they were also present with FFT analysis of a dithered tone at –120dBFS (fig.6).

713AK100fig04.jpg

Fig.4 Astell&Kern AK100, 1?3-octave spectrum with noise and spuriae of dithered 1kHz tone at –90dBFS with 16-bit data (top) and 24-bit data (middle), and at –120dBFS with 24-bit data (bottom) (right channel dashed).

713AK100fig05.jpg

Fig.5 Astell&Kern AK100, spectrum with noise and spuriae of dithered 1kHz tone at –90dBFS with: 16-bit data (left channel cyan, right magenta), 24-bit data (left blue, right red) (10dB/vertical div.).

713AK100fig06.jpg

Fig.6 Astell&Kern AK100, spectrum with noise and spuriae of dithered 1kHz tone at –120dBFS with 24-bit data (left blue, right red) (10dB/vertical div.).

The AK100's reproduction of an undithered 16-bit tone at exactly –90.31dBFS is shown in fig.7. The three DC voltage levels described by these data are clearly resolved, with a symmetrical waveform. Increasing the bit depth to 24 gives a recognizable, if noisy, sinewave (fig.8).

713AK100fig07.jpg

Fig.7 Astell&Kern AK100, waveform of undithered 1kHz sinewave at –90.31dBFS, 16-bit data (left channel blue, right red).

713AK100fig08.jpg

Fig.8 Astell&Kern AK100, waveform of undithered 1kHz sinewave at –90.31dBFS, 24-bit data (left channel blue, right red).

Although fig.1 indicates that the third was the most prominent distortion harmonic at high frequencies into high impedances, the second was the highest in level at low frequencies into 600 ohms (fig.9). Fig.10 compares the spectra of the AK100's output while it reproduced a 1kHz tone into 100k ohms (blue trace) and a low 100 ohms (red), the latter more typical of headphones. Again, into the high impedance the third and fifth harmonics are the highest in level, with both the second harmonic dominating into the low impedance and all the harmonics higher in level. But even the second harmonic into the low impedance lay at –79dB (0.01%), which is low in absolute terms. The picture was similar with intermodulation distortion (fig.11). While the high-order intermodulation products around the 19 and 20kHz fundamentals are unchanged by the drop in load impedance, the difference product at 1kHz rises from –120dB (0.0001%) to a still-low –86dB (0.005%).

713AK100fig09.jpg

Fig.9 Astell&Kern AK100, spectrum of 50Hz sinewave, PCM data, DC–1kHz, at 0dBFS into 600 ohms (left channel blue, right red; linear frequency scale).

713AK100fig10.jpg

Fig.10 Astell&Kern AK100, spectrum of 1kHz sinewave, PCM data, DC–10kHz, at 0dBFS into: 100k ohms (blue), 100 ohms (red) (linear frequency scale).

713AK100fig11.jpg

Fig.11 Astell&Kern AK100, HF intermodulation spectrum, PCM data, DC–30kHz, 19+20kHz at 0dBFS into: 100k ohms (blue), 100 ohms (red) (linear frequency scale).

I assessed the AK100's rejection of jitter using both internally stored WAV versions of the 16- and 24-bit Miller-Dunn J-Test signal and the same data fed to the AK100's TosLink input. The results were identical for both operational modes and are shown in fig.12, the 16-bit spectrum represented by the blue and magenta traces, the 24-bit spectrum by the cyan and red traces. With 16-bit data, the odd-order harmonics of the Fs/192 LSB-level squarewave all lie at the correct levels and are not being accentuated by the AK100. These disappear with the 24-bit data, of course, but a single tone of unknown origin can be seen at 8.2kHz. Perhaps of most significance, with both 16- and 24-bit data, the spike that represents the high-level Fs/4 tone has a considerable degree of spectral spreading at its base, suggesting the presence of random low-frequency jitter.

713AK100fig12.jpg

Fig.12 Astell&Kern AK100, high-resolution jitter spectrum of analog output signal, 11.025kHz at –6dBFS, sampled at 44.1kHz with LSB toggled at 229Hz: 16-bit data (left channel cyan, right red), 24-bit data (left blue, right magenta). Center frequency of trace, 11.025kHz; frequency range, ±3.5kHz.

Overall, the Astell&Kern AK100 measures very well, not only as a standalone high-resolution player, but also as an affordable high-resolution D/A processor.—John Atkinson

COMPANY INFO
Astell&Kern, Korea
US distributor: iRiver Inc.
39 Peters Canyon Road
Irvine, CA 92606
(949) 336-4540
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COMMENTS
gld3gld3's picture

"Ten years later, while the current top-model iPod features a 160GB hard drive, it still can play only files with sample rates of 48kHz and below and a maximum bit depth of 16. Those of us with a growing library of high-resolution files are therefore restricted to playing them in our big rigs at home."

Not exactly. There is at least one option for those wanting to play higher sample rate/higher bit depth files on their ipods.

My 5th gen ipod is currently playing "Happlessness Blues" by Fleet Foxes at  sample rates of 96kHz  and bit depth of 24 (FLAC). How? Rockbox. 

John Atkinson's picture

gld3gld3 wrote:
My 5th gen ipod is currently playing "Happlessness Blues" by Fleet Foxes at  sample rates of 96kHz  and bit depth of 24 (FLAC). How? Rockbox.

Rockbox does indeed add a lot of functionality to iPods (and other portable players). However, it can't overcome the hardware limitations of the player. Even if you can play a 24/96 files, as you report, the iPod's DAC chip is limited to sample rates of 48kHz and below, so the file will be downsampled.

In additon, while Apple's AAC and Lossless codes will preserve 24-bit resolution, the iPod's DAC is a 16-bit part, so will truncate the word length by the 8 LSBs. For example, if you look at the spectrum at www.stereophile.com/content/ipod-classic-160gb-1khz-90dbfs-16-bit, this shows my 2010 iPod Classic playing a dithered 16-bit tone at -90dBFS. Everything is correct, and the noisefloor is free from harmonic spuriae.

By contrast, the spectrum at www.stereophile.com/content/ipod-classic-160gb-1khz-90dbfs-24-bit shows what happens when the iPod plays back a 24-bit dithered tone at the same level encoded with the ALAC codec. The appearance of the odd-order harmonics is due to truncation of the 24 bits to 16.

So yes, it may be possible to play 24/96 files on this iPod running the Rockbox software, but the file will be downsampled and truncated. The AK100 doesn't suffer from these hardware limitations.

John Atkinson
Editor, Stereophile

jcborges's picture

How were your measurements of the iPod taken? I ask because looking at the specs of the iPod DAC at http://cirrus.com/en/products/cs42l55.html it is definitely capable of 24-bit, although it is limited to 48khz sampling frequency.

Why would Apple put a 24-bit capable DAC in the iPod and have it truncate to 16? And how could it be done? And couldn't Rockbox bypass that to access the full bit depth available on the chipset?

Thanks and regards

John Atkinson's picture

jcborges wrote:
How were your measurements of the iPod taken? I ask because looking at the specs of the iPod DAC at http://cirrus.com/en/products/cs42l55.html it is definitely capable of 24-bit, although it is limited to 48khz sampling frequency.

I took a 24-bit AIFF file representing a dithered 1kHz tone at -90dBFS and prepared from it WAV, ALAC, and AAC at 320kbps files. The WAV file wouldn't play in either my iPod Classic 160GB or my iPhone 3GS, but from the other three versions I expected to get a spectrum that looked like the red and blue traces in fig.5 at www.stereophile.com/content/astellampkern-ak100-portable-media-player-measurements.

Instead, from both players and with AIFF, ALAC, and AAC versions of the data, I got the spectrum shown at www.stereophile.com/content/ipod-classic-160gb-1khz-90dbfs-24-bit. The characteristic pattern of odd-order harmonics indicates truncation of the 24-bit data to 16. By contrast, the AK100 playing both the AIFF and ALAC versions of the data gave the correct spectrum shown in the review.

jcborges wrote:
Why would Apple put a 24-bit capable DAC in the iPod and have it truncate to 16?

I have no idea. This was not what I was expecting, as the new "Mastered for iPod" protocol recommends using a 24-bit master to prepare the AAC version, which in turn implies a 24-bit datapath..

jcborges wrote:
And how could it be done? And couldn't Rockbox bypass that to access the full bit depth available on the chipset?

That is possible, but as you point out, the chipset is still restricted to sample rates of 48kHz and below, contrary to what the original poster claimed.

John Atkinson
Editor, Stereophile

jcborges's picture

First, thanks a lot for your answers. But you left me wondering about something. 

If understood correctly, your measurements were made in the analog domain. What about digital?

If you play a 24bit/48khz ALAC file through a digital dock, what is the format of the PCM stream that comes out? 24-bit or 16-bit truncated? Have you ever checked or know someone who has, for example by connecting it do a DAC that shows the input format?

In the other words, if you have 24/96 files that you want to put in an iPod to playback digitally, should you just resample them to 48khz or should you apply TPDF dither to avoid truncation? Does it matter? 

 Thanks and regards

John Atkinson's picture

jcborges wrote:
If you play a 24bit/48khz ALAC file through a digital dock, what is the format of the PCM stream that comes out? 24-bit or 16-bit truncated?

If you play a 24-bit AIF or WAV file, the AK-100's digital port outputs 24-bit data. But I will check with ALAC data.

John Atkinson

Editor, Stereophile

John Atkinson's picture

John Atkinson wrote:
jcborges wrote:
If you play a 24bit/48khz ALAC file through a digital dock, what is the format of the PCM stream that comes out? 24-bit or 16-bit truncated?
If you play a 24-bit AIF or WAV file, the AK-100's digital port outputs 24-bit data. But I will check with ALAC data.

Okay, played back a dithered 24-bit 1kHz tone at -90dBFS, encoded both as AIFF data and as ALAC data. In both cases, all 24 bits were active in the AK100's optical output and the digital-domain spectra of the two codecs were identical, with a noisefloor at the 24-bit level.

So the answer to your question is yes, when the AK100 plays a 24-bit ALAC file, the digital output is true 24-bit as it should be.

John Atkinson

Editor, Stereophile

JohnnyR's picture

*eyeroll*

So having a player that has crappy high output impedance and doesn't have gapless playback is a good thing? For the money I would expect a lot better. What's wrong with using FLAC in other players that cost less and have gapless playback to boot?

Another laughable product that Atkinson likes.cheeky

John Atkinson's picture

JohnnyR wrote:
So having a player that has crappy high output impedance and doesn't have gapless playback is a good thing?

As I wrote, the most recent firmware upgrade allows gapless playback. I examined the effect of the 22 ohm output impedance at length in my review - "crappy" is a mischaracterization of the relatively small changes in frequency response that result from this impedance, especially when you consider that many headphones and in-ear monitors have boosted low frequencies.

John Atkinson

Editor, Stereophile

JohnnyR's picture

For $699 you should get a lot better response and features. Sorry but changes from 1 to 2 dB would be scandalous in a preamp or power amp but since it's this THING then it's "okay". oh brother.blush Seriously, lower priced players don't have this problem so it's a FLAW easily avoided if they had designed it right.

ChrisS's picture

Comrade,

There must be something about your Stalinist upbringing that makes you believe that the perfect audio product exists somewhere out there, when really that kind of perfection only exists in your imagination.

Di Keller's picture

I bought one of these today on impulse. It just sounded sooo good. Got it home and it is not that mac friendly. The big problem is being able to upgrade the version so it will be gapless. It is done via Iriver Plus 4 for pc. But I haven't been able to find a firmware update for mac. Iriver really don't seem to like us :(

John Atkinson's picture
Di Keller wrote:
The big problem is being able to upgrade the version so it will be gapless. It is done via Iriver Plus 4 for pc. But I haven't been able to find a firmware update for mac.

You don't need the iRiver PC app. You connect the AK100 to the Mac using the USB drive option. After downloading the new firmware from the A&K site - it will be called AK100.hex - you copy it into the AK100's root/system directory. (Make sure that your AK100’s battery level is at least over 80% and make sure of your AK100’s memory size is at least 50MB.) When you reboot the AK100, it will automatically install the new firmware.

John Atkinson
Editor, Stereophile

Di Keller's picture

^^ Firstly thanks for answering so promptly. I found the A&K site I have figured out how to do it. But the problem is finding a link for the firmware that works. I keep getting zip files which won't unzip. Or are incomplete.

Di Keller's picture

Just tried again and finally got it. Thank you for your help. !!

John Atkinson's picture
Di Keller wrote:
Just tried again and finally got it.

I remember having to try downloading the new firmware several times before it worked.

Di Keller wrote:
Thank you for your help. !!

You're welcome. Enjoy your music on the AK100.

John Atkinson
Editor, Stereophile

Di Keller's picture

Finally all set up.!! Had all sorts of problems installing the upgrade. Followed everything to the letter and it just didn't work. I went back to the place I bought it from. They tried it and it didn't work. The answer was really simple in the end. We were all putting the hex file into the system file , if you just dropped into the AK100 folder it worked out the rest it self. Good news, while I was there they put a $6500 pair of headphones on me, If I was religious I would have said it was a religious experience . Hahahaha!! The sound was amazing !! I could feel my face crumpling. :)

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