|Chromecast Audio with optical TosLink adaptor & cable.|
The first thing that needs to be addressed is whether the Chromecast Audio is capable of "bitperfect" output. Now, I do not have an easy way to capture the digital output, but one thing I can do is to make sure that a DTS signal encoded as 16/44 FLAC can be streamed from Plex to my digital receiver without audible issues... Indeed, in this regard the Chromecast succeeded.
|Chromecast Audio digital output to old Denon receiver - streaming DTS surround audio.|
There you go... A DTS stream of "Hotel California" (I think this was originally off my DVD-A copy of the album) streamed off the Chromecast Audio from the Plex server running on my Windows Server 2012 R2 machine in the basement. Controlled using the Plex app on my Galaxy Note 5 phone. Sounds great with no evidence of error (if there were, it would be obviously audible in the DTS stream).
Alright, with that out of the way, let's just measure the Chromecast Audio's digital output connected to my TEAC UD-501 DAC and make sure the digital output results in the same analogue quality off the DAC as expected.
A. RightMark 16/44:
As with any competent digital device these days, we should see little difference with 16/44 input. As expected, the Chromecast Audio TosLink output fed into the TEAC UD-501 measures essentially exactly the same as using a laptop and USB input (the USB measurement was done 3 years ago when I first got the DAC). Although the numeric results do not show much difference, we can see the difference between the TEAC output and the Chromecast's analogue output based on the different frequency response:
|Frequency Response - as you can see the two TEAC UD-501 graphs are a perfect overlay.|
B. RightMark 24/48:
The Chromecast TosLink to TEAC UD-501 results in a dynamic range of >18.5-bits versus the Chromecast's analogue output of 17.5-bits demonstrated last week.
Again, we can see the difference between the 2 DACs in the graphs:
C. RightMark 24/96:Finally, let's look at 24/96 - the highest resolution the Chromecast Audio is capable of at this time. I did these measurements in my sound room where the recently built HTPC was connected by USB and through the computer's TosLink to the TEAC DAC. As expected, essentially no difference in measured DAC output whether using TosLink or USB interface, and whether it was with the HTPC or Chromecast. Basically, the "sound" was that of the TEAC UD-501 DAC as measured with typical test tones using RightMark.
And the graphs:
|Frequency Response - notice all the TEAC UD-501 DAC outputs overlap exactly.|
|Noise Floor - TEAC UD-501 has 60Hz hum and harmonics.|
Though not shown, I tested 24/88 and am able to confirm no issues with the 88kHz samplerate using the Chromecast Audio TosLink output.
D. JitterSo far, there are no surprises. Digital bits faithfully transmitted down a cable results in the same analogue output, measured using typical test tones for frequency response, dynamic range, and distortion amounts. Furthermore, I trust nobody would be surprised then if I say it would be in the jitter domain where I expect to see some differences.
Remember, the Dunn Jitter Test was designed to demonstrate timing anomalies in S/PDIF interfaces like TosLink. Here's an excellent "Application Note" written by Julian Dunn from back in 2003 for further details. So, the question is just how jittery is the Chromecast's TosLink interface?
As you can see, there are some jitter sidebands in the 16-bit J-Test but it's the 24-bit J-Test that suggests this device is quite jittery compared to what I have seen in the past. In fact the strongest sidebands with the 24-bit J-Test had an amplitude about 95dB below the primary signal at 12kHz. This is one of the most "jittery" results I have seen. Nonetheless, realize that 95dB is still a very big number and this is not going to pose an issue despite claims by the most golden-eared audiophile!
It's hard to know without some comparison if this high jitter is mainly the Chromecast or could it be the TEAC UD-501's optical interface itself... So, for comparison here's the motherboard TosLink coming out of my recent HTPC build (based on the Gigabyte Z170X-Gaming 7), I'm using WASAPI drivers for the built-in Soundblaster Recon3Di audio chipset's TosLink interface.
As you can see, jitter sidebands are evident, and in fact some of the motherboard's 16-bit J-Test sidebands are stronger than the Chromecast's. However the 24-bit J-Test is in fact substantially cleaner, adding to the evidence that the Chromecast's TosLink interface is indeed more jittery than most, including a modern computer motherboard.
For completeness, here's the HTPC connected to the TEAC UD-501 through the USB 2.0 interface using ASIO demonstrating the merits of an asynchronous digital interface for audio in terms of freedom from jitter-related anomalies. Notice especially the cleanliness of the 24-bit J-Test.
E. 24/96 Jitter Test!Well, since I had all the equipment hooked up, I thought this might be a good opportunity to try something "new" and different - a 24/96 J-Test!
Instead of a 12kHz primary signal as in the 48kHz 24-bit J-Test, let's stress the signal and push it to 24kHz while doubling sampling rate from 48kHz to 96kHz. This also means that the LSB 250Hz square wave jitter modulation tone is now pushed up to 500Hz. This should represent quite a "stress test" on timing accuracy and should exacerbate the anomalies seen above with the Chromecast Audio. Remember that timing effects becomes more critical with higher frequencies and the jitter anomalies should be more evident when we do this.
As you can see, indeed this "96kHz 24-bit J-Test" demonstrates the strain placed on the optical TosLink interface for both the Chromecast and the HTPC motherboard!
Again, we see how impressively jitter-free the asynchronous USB interface is in comparison. Although this is the first time I have shown this 24/96 test signal, I trust the result is expected and not a shock to anyone. For the longtime readers, you might recall that I had previously already demonstrated that the main problem with the TosLink interface is that of increased jitter - a good example among many over the years was when I looked at the use of a TosLink loopback with the Logitech Transporter in 2013.
Before anyone freaks out about the result and what it means about the Chromecast, do realize that the tallest sidebands are still lower than 70dB amplitude from the primary signal at 24kHz! Considering that we're looking at a high amplitude 24kHz ultrasonic primary signal (not the kind of thing one should ever see in real recorded music, nor hear except for intermodulation products), I doubt the jitter anomaly would ever pose any issue.
F. SubjectiveSo how does the Chromecast Audio's TosLink digital output "sound"... Well, it sounds like the TEAC UD-501 DAC :-).
Seriously, I did spend about an hour trying to hear the difference between the "jittery" TosLink input from the Chromecast compared to the computer's USB input. This was simply not possible for me switching between HTPC and the Chromecast TosLink inputs. Because USB is bidirectional and the computer senses when I switch away from it, unless I had two TEAC UD-501's side by side - one playing from the TosLink, the other from a USB connection and switching quickly between them, the time needed to start and stop USB playback is too long and I have no confidence in my ability to "remember" the sound in order to make comparisons at this subtle level.
Feel free to experiment though because the Chromecast TosLink interface provides an opportunity to experience what a relatively jittery playback device "sounds" like. You can then hear for yourself whether you still believe jitter is actually a problem when high-end companies and magazines talk about the "importance" of keeping jitter low. By the way, the measured jitter amount from this little US$35 Chromecast Audio appears to be significantly better than the results from this 16-bit US$10,000 CD/DAC combo, or this very recently reviewed CD player of the same price! High-fidelity indeed :-).
G. ConclusionsThis then concludes my 2-part look at the Chromecast Audio WiFi streaming device using Plex Media Server as the music source. In this segment, we discover that the device:
1. Is capable of "bitperfect" streaming - at least for 16/44 - by successfully passing on a DTS bitstream to a digital receiver.
2. As expected, streaming digitally to an outboard DAC results in analogue output measurements consistent with expectations for the DAC.
3. TosLink is prone to jitter and unfortunately, the Chromecast demonstrated more jitter with the 24-bit J-Test compared to most other devices I have tested. The severity of the jitter as expected worsened at the 96kHz sample rate with the novel 24/96 version of the J-Test. Remember though that the final jitter effect in the analogue output is an interplay between the sending source and the DAC. I was able to demonstrate that the Chromecast Audio was more jittery as a source than my PC with the Gigabyte motherboard's TosLink to my TEAC UD-501 DAC, but this might not be the same with every DAC.
A couple of weeks ago in the post about MQA, in the comments, I mentioned that this blog is about "perfectionist" audio and objective testing to tease out subtle effects and the minutiae. To the perfectionist, we can of course pick apart performance in all kinds of ways. With that mindset, I suppose I could express a dissatisfaction with the level of jitter found. But the truth is that there's just no need to apply that level of scrutiny to a device like this given the level of quality achieved at this price point. Other than the audiophiles, nobody cares. And other than objective audiophiles where we can actually demonstrate the imperfection and reliably "grade" quality of gear based on these results, practically, it's just inaudible.
I'd be absolutely happy with the Chromecast Audio's analogue output in my secondary stereo system. And I don't believe anyone is really going to notice the jitter in day-to-day use. Heck, it's only US$35.00; probably less than the amount one pays for coffee a month!
Well done Google! Now if you could tighten up that TosLink timing accuracy a bit in the next version, you'd really make the final 0.1% of audio lovers (the "perfectionist") really quite happy :-).
Have a great week ahead! Enjoy the music...
Addendum / Final Thoughts:
It has come to my attention that a certain website/blog claims that there is some kind of "silent majority" out there presumable referring to some kind of purely subjectivist audiophile philosophical leaning. As far as I am aware, there hasn't been any kind of census that would allow anyone to know what is or is not the viewpoint of music lovers out there. Besides, perpetuation of that kind of "us vs. them" split is rather nonsensical as I had written previously.
Consider this. What is the value of presumptuously claiming that one's views resonate with the "majority" if this "majority" (ie. purely subjective leaning hobbyists on a website) is itself a "fetish minority" (ie. number of listeners who desire very expensive DACs, expensive computer servers, mystical tweaks, magic cables) of another "audiophile minority" (ie. compared to all music lovers out there)? Any way you slice it, that's a small number.
Needless to say, I believe having objective results make a review more complete. In my conclusion above, I make assumptions that most music listeners and music lovers probably care little about jitter. This is a post about the Chromecast Audio after all and other than "audiophiles", how many people actually care or even know that jitter exists in a digital stream? In my opinion, objective testing allows us to appreciate the quality of engineering in the sound and in a case like this, a more complete review sheds light on this subject; something I feel that pure subjective reviews would be unable to discuss or at best simply guess that maybe a device is "more jittery". Also, here is an example of what I mean by "accuracy" and why it is not a vague notion. I hope that even the most ardent "subjectivist" can accept that an accurate DAC should be jitter-free, right? The magic number is ZERO jitter. If I were reviewing a $5,000+ DAC, should I not at least try to verify that the engineers went to the trouble of ensuring timing precision? If I were reviewing something with a $7,500 clock peripheral (on top of say a $24,000 base DAC), should I not at least check to see if that actually improved the measurements even a little to make it worthwhile (don't neglect the possibility that timing accuracy could worsen)!? So please stop with the idea that the term accuracy is somehow meaningless - yes, there are "clear targets" for an "ideal", we do know what audio engineering should be striving for. To claim otherwise without a logical, internally consistent, reasonably comprehensive, and non-circular thought process is disingenuous and denies reality.
Finally to close off, I had the pleasure of attending a nice lecture this morning at the Canadian TRIUMF particle physics labs as a celebration of sorts as this is the centennial year of Einstein's theory of General Relativity (and recent LIGO discovery). It truly is wondrous to think about the mysteries of the universe. It was also a great reminder of how in science, for the sake of progress, theory and beliefs go hand-in-hand with measurements and observations. Despite his beliefs in an "elegant" universe, remember that even Einstein acknowledged his mistakes when objective measurements showed otherwise (eg. he eventually acknowledged that the universe was not a static structure but that it expanded despite his earlier assumptions). Arguments in the audiophile world is but minutiae in the large scheme of things when equipment quality including a little $35 streamer can be so good! But even here, truth I believe must be derived from that combination of subjectivity/belief/theory and objectivity/empiricism.