MUSINGS: The Squeezebox Family...

Well, after finishing the last couple of posts, I sent the Squeezebox Receiver back to fordgtlover. Thanks for the opportunity to test out another member of the SB family! I very much appreciate the generosity!

Before the Receiver left on the long trip back to Australia; I decided to take a family portrait:

I remember patiently awaiting the arrival of the Slim Devices Squeezebox 3 back in late 2005 after putting in the pre-order. It's thanks to this little device that my audio listening habits changed forever away from physical media. Despite the ups and downs with various versions of the server software, these little boxes have certainly made access to my own music remarkably easy in a way almost unimaginable previously. My father has access to my music library across the city through his Touch and wherever I go in my travels, music streams to the PC/Mac as well. The stability of my server these days with >5000 albums is fantastic with uptimes of months (this is with Windows, and the only reason it goes down these days is because I install Windows updates every few months).

Sadly, the Squeezebox line was discontinued in August 2012 by Logitech.

Looking back, I'm still impressed by the technology put together by the Slim Devices team to get the infrastructure in place, and later with the Logitech team and especially the Touch. Even today, the objective analogue audio output performance of the Transporter remains superb (it came out in September 2006) and as I look at all that can be done with the Touch (came out April 2010) including the ability to handle 24/192 (EDO kernel), communicate with a number of USB DACs, transport DSD64 via DoP, it's amazing the power of what can be done with an Open Source architecture where the "community" is empowered to maximize the machine's capabilities. Have a look at the used prices of the Touch these days, it's a reminder of just how much value it offered audiophiles. The computer audiophile community needs more devices of this caliber; devices that can "raise the bar" and do it without need for valuation as a luxury or "artisan" item when it's just "good sound" and "good functionality" that I believe many of us are after.

Looking ahead, it's great to see ongoing development in the Squeezebox community towards replacement hardware and software - check out the communitysqueeze.org FAQ. Wonderful to see the DIY machines being put together out there (check out the picture gallery!). With the availability of small, low powered, but reasonably fast computers like the Wandboard and the already ubiquitous ports of client software like Softsqueeze/Squeezeplay/Squeezeslave, Squeeze Player (Android), Squeezecast (iPod/Phone), I have a feeling that I'll be running my Logitech Media Server (or some equivalent) for the forseeable future...

... Now, if they could support multichannel FLAC/WAV and native stereo/multichannel DSD (I still think there's a need for better file formats than .dsf and .dff), I think we'd have all of audio covered. :-)

Vive le Squeezebox!

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Well, it's summer time in the northern hemisphere and that means the kids are out of school. The heat wave finally arriving here in Vancouver. Time for some vacation, BBQ's, camping, lazy afternoons, general R&R, and of course good summer tunes (Katrina & The Waves I'm Walking On Sunshine is playing in the back currently). :-)

I'm also starting to go through Ethan Winer's recent 2012 book "The Audio Expert: Everything You Need To Know About Audio". Only started and already it's a good read written in an accessible manner for those interested in the science and technical aspects of audio without going deeply into the math. The Kindle edition (IMO a bit pricey) along with some mindless fiction like Dan Brown's Inferno will likely accompany me on the summer trips coming up.

As always, enjoy the music everyone!

MEASUREMENTS: Do bit-perfect digital S/PDIF transports sound the same?

Using suggestions from this page, the Touch can be used to transport DSD to the TEAC as DoP wrapped around a 24/176 FLAC file through Triode's USB kernel. Neat hack - proof of bit perfect transmission. Unfortunately the files are huge, so I likely will await an efficient solution. Note that this is NOT the test setup described below, just something cool. :-)

Let's talk about digital transports for a bit.

Most of us I'm sure remember those days when the only way to get digital data to an outboard DAC was through a CD transport. Although we can still resort to a CD reader, I suspect that many of us here have gone mostly into the computer audio realm with data on hard drives or flash/SSD devices. Thankfully gone are the days when the data being read off the CD could be inaccurate and interpolation may be needed in realtime, or be susceptible to mechanical failures of CD drive mechanisms (though hard drive failures and need for backups present another challenge).

I have shown that bit-perfect data can be transferred and played back without any concern off a USB asynchronous interface (eg. the various Mac and Windows software players and laptops). While I have the Squeezebox Receiver still on hand, let us have a look at the effect of using different transport devices with S/PDIF interfaces objectively.

Remember to keep in mind that the S/PDIF interface, unlike packetized asynchronous USB (or ethernet) conducts its data transfer in a unidirectional serial fashion formalized around 1985 when the AES3 (AES/EBU) standard was also laid down. As I think many of us have read, S/PDIF combines the data and clock signals using "biphasic mark code" and it is this "feature" of the interface that has resulted in many an audiophile nightmare regarding timing issues - jitter concerns especially well publicized. This is to a large part the basis for the well known paper by Dunn and Hawksford in 1992 asking "Is The AESEBU/SPDIF Digital Audio Interface Flawed?". (Of course the topic of jitter can be very complex as described in the paper going far beyond what we need to concern ourselves with here.)

With that said, let's be practical and see what the results looks like with the different devices using TosLink and coaxial interfaces (with comparison to asynchronous USB)...

Setup:

For this round of testing, I decided to take a break from the TEAC UD-501 DAC and go back to the ASUS XONAR Essence One for a bit. Although I have been listening to the TEAC a lot in the last couple months, on a daily basis, I still use the ASUS Essence One at my computer workstation and it was just more practical to run these tests there. Despite my concerns around the upsampling feature of the ASUS, it measures well and sounds excellent.

Here's the hook-up:
* Transport device * -> Coaxial/Toslink/USB cable -> ASUS Essence One -> Shielded 3' RCA -> E-MU 0404USB -> 6' Belkin Gold USB -> Win8 laptop

Coaxial cable = 6' Acoustic Research
TosLink cable = 6' Acoustic Research
USB cable =  6' Belkin Gold

Transport devices tested:
1. Squeezebox 3 -> Coax / TosLink
2. Transporter -> Coax / TosLink
3. Receiver -> Coax / TosLink
4. Touch -> Coax / TosLink
5. Laptop -> CM6631A Async USB to S/PDIF -> Coax / TosLink
6. Laptop -> Async USB direct to ASUS Essence One

As you can see I've got the host of Squeezebox devices on the test bench along with the usual two ways to connect the computer's USB port to DACs (direct or through USB-S/PDIF converter).

I. RightMark Analysis:

Since there are so many devices/combinations, I'll show the results a few at a time to demonstrate what was found. Let us start with the results of the four Squeezebox devices. I decided to "max out" the capabilities of the SB3 and Receiver by using 24/48 sampling rate:

Numerically, not much difference...  From my subjective listening during the tests, I would agree with these numbers in saying that "it sounds like the Essence One DAC"; there are more similarities in the sound than subjective differences.

Let's have a look at the frequency response graph because there does appear to be some difference - here's coaxial interface only:

Hmmm, interesting! Small differences at the top end. Let's zoom into that top end and have a good look:

Notice the shape of the curves suggest slightly earlier roll-off with some devices. The flattest, most extended frequency response (and possibly most "accurate") is the Transporter, followed by SB3, then Touch, and Receiver. Remember, we are talking only about 0.15dB difference between the Transporter and Receiver at 20kHz; not perceptible IMO but since we're looking for evidence of a difference, useful to note.

Let's now include the TosLink measurements:

Even though we ran out of colors, it doesn't matter because there are still only 4 curves. There is no difference between coaxial and TosLink; they overlay on top of each other essentially perfectly.

Let us now add the computer-USB interfaces (take away the TosLink since no difference):

As you can see, the flattest response curves come from USB direct and Transporter. Here's the ranking: Transporter, ASUS USB direct, SB3, Touch, CM6631A, and Receiver. We'll talk more about this later, just keep in mind then that frequency responses are *slightly* different between transports despite bit-perfect settings...

The rest of the RightMark graphs - no significant difference:

II. Jitter Analysis:

Dunn J-Test stimulation of jitter. To keep it more manageable, I'll group them into 16-bit and 24-bit side-by-side first, let's just look at the coaxial interface here:

A. Squeezebox 3 (16-bit / 24-bit):

B. Transporter (16-bit / 24-bit):

C. Receiver (16-bit / 24-bit):

D. Touch (16-bit / 24-bit):

E. Laptop -> CM6631A USB to coaxial (16-bit / 24-bit):

F. Laptop -> USB direct (16-bit / 24-bit):

Objectively, it looks like the CM6631A USB-to-S/PDIF and USB direct 24-bit graphs are cleaner, and of the Squeezebox devices, the Transporter on the whole seems to have the least data-correlated jitter. Even at its worst, the sidebands for the SB3 around the primary signal is down around -120dB. Is this a problem? I doubt it since auditory masking will easily make this inaudible (assuming one could even hear down that low around 12kHz pitch). Furthermore, in theory, the J-Test should create a "worst case scenario" for jitter which is unrealistic in real music.

Here's the difference between coaxial vs. TosLink:

A. Squeezebox 3 24-bit, Coaxial vs. TosLink:

B. Transporter 24-bit, Coaxial vs. TosLink:

C. Receiver 24-bit, Coaxial vs. TosLink:

D. Touch 24-bit, Coaxial vs. TosLink:

E. CM6631A USB to S/PDIF 24-bit, Coaxial vs. TosLink:

In general, we can say that indeed TosLink is worse (remember however TosLink is immune to electrical noise with galvanic isolation so there are some positives in this regard). Interestingly this is very clear with the Transporter! However, increased jitter with TosLink is not a given because the SB3 and Receiver seem to behave in the opposite fashion and show less jitter artifacts with the TosLink interface.

Remember that all of these jitter graphs are indicative of the interface between the transport device connected to the Essence One DAC. The graphs could be different with another DAC since much of the result will depend on the accuracy of the DAC in extracting the clock information and what other steps it might take (eg. data buffering, reclocking) to further stabilize the timing - it's not just about the transport device.

III. DMAC Protocol

So, up to now we can see differences between bit-perfect devices with RightMark, and obvious differences with the J-Test. How do they sound? Let's see what the computer "hears". For this test, I am using the Transporter playback as reference against which all the others are being compared.

First, I must admit that I'm not as confident about these numbers as I am of the graphs and plots above simply because it was really tough getting this done properly! From previous experience with the Audio DiffMaker program, results can vary depending on environmental factors like temperature of the equipment and subtle "sample rate drift" over time. With each transport measured, cables needed to be reconnected, settings needed to be changed, and for each condition, I ran the test 3 times to get a sense of the "range" of results. Admittedly, I made an error with the 'USB direct' measurements and did not realize this until after the fact so did not include the results here (foobar was accidentally set to output 16-bit instead of 24-bit).

The bottom line is that the results suggest that each device "sounded" different according to the computer. Instead of the usual high "correlated null depth" like in my previous tests with player software around 80-90dB (similar to the Transporter tested against itself above), we're seeing numbers in the 60-80dB range between transports. The computer thought the Squeezebox 3 sounded the most different from the Transporter. Good to see that it was able to detect the Receiver playing 320kbps MP3 as "most different" (ie. lowest correlation) to provide a point of reference. A reminder, this measurement is logarithmic so the actual mathematical difference between the MP3 sample compared to the others is larger than what it might look like on the graph.

Remember that this is a measurement of the difference between each device and the Transporter connected to the Essence One. There is no implication here of whether one sounds "better" than another since that would of course be the listener's subjective judgment call.

IV. Summary

Let me see if I can summarize this based on the results here along with what I know/believe over the months of testing as applicable... Q&A format:

Q: Do all bit-perfect transports sound the same?

A: Based on the results, not exactly. Even though bit-perfect (I have verified this with the Touch, Transporter, SB3, CM6631A, ASUS USB direct with ASIO), small differences in frequency response can be measured. Furthermore, jitter analysis clearly looks different between devices and this also varies between coaxial and TosLink interfaces (with TosLink generally worse than coaxial for jitter). Likewise, the DMAC test also suggests the level of audio correlation when playing musical passages is not as high as previous tests with bit-perfect software or decoding lossless compression. Within the Squeezebox family, not surprisingly the Transporter performed the most accurately with flattest frequency response and lowest coaxial S/PDIF jitter, although I was quite surprised by the stronger TosLink jitter.

Q: Why do you think the frequency response varies?

A: My belief is that this is not a jitter issue. The reason I say this is that there appears to be no difference between coaxial and TosLink even though jitter varies between the two interfaces as demonstrated by the Dunn J-Test. I believe that this is the result of mild clock speed / data rate differences of the transport devices. Since the word clock has to be recovered from the S/PDIF signal, clock accuracy is dependent on the transport's internal clock - some transports may be timed a little quicker, some a littler slower and the DAC has to adjust to this (of course the E-MU 0404USB ADC measuring the audio has a part to play in setting where it believes the roll-off should be). This frequency roll-off variability is not seen with laptops connected to an asynchronous USB device for example (that's of course the point of being asynchronous; not time-coupled to the data sender by having the recipient working off its own clock and telling the sender to speed up or slow down if necessary).

Q: But surely different/better/more expensive digital S/PDIF cables can help?

A: No. I don't think so. As I have measured and discussed before, digital cables make no substantial difference to timing/jitter as far as I can tell. Even though very long or poorly constructed cables may add to the jitter, the difference IMO is much less than what I'm showing here and as far as I can tell is irrelevant for a reasonable length of decently constructed coaxial/TosLink.

Q: Those jitter plots look nasty... I bet I can easily tell the transports apart!

A: Of course, anyone can claim anything over the Internet or in print since there are rarely if ever any actual "double checking" with sound methodology or formal peer review in the case of print magazines (obviously these are not scientific journals). Although I have shown these measurable differences, as a (currently) 41 year old male who works in an office environment, have generally avoided very loud concerts, and have a hearing frequency threshold around 16kHz, I do not believe I would be able to differentiate any of these bit-perfect transports in controlled testing with the same ASUS Essence One DAC.

Q: Surely you just need better gear to hear it!

A: The data correlated jitter with any of these devices would be >100dB below the primary signal. The frequency response difference is less than 0.15dB at 16kHz (my frequency threshold). Unless there's some significant interaction that causes anomalies in the output significantly beyond what I measure here, these difference would be inaudible to me irrespective of the quality of the sound system. Of course if you have younger ears and better hearing, this could be different. I believe speakers and headphones would introduce much more distortion and change to the frequency response than what I'm measuring here with a good modern DAC.

Q: Well, if that's the case, then I might as well go for the cheapest digital transport/streamer I can find, right?

A: Well, maybe, maybe not. When it comes to sound quality, I think a digital transport would have to be quite incompetent to sound poor (eg. non-bit-perfect, horrific jitter or imagine if the frequency response rolled off way too early because of severe S/PDIF timing inaccuracies). Therefore, spending more on a digital transport is IMO not primarily about sound quality but rather features and the aesthetic "look and feel" you're after (eg. better remote, can handle higher sampling rates, more reliable, fits into the decor...). Sound quality IMO is better served by putting the money into good speakers/room treatments/amp/DAC. Back in the "old days" of CD spinners, better mechanics with higher reliability and accuracy just cost more money. Even then it's not a given; I remember spending five times more on a higher model Harmon/Kardon CD player 20 years ago and that failed within three years whereas an inexpensive JVC from Costco with digital out still runs fine today. I have not had occasion to try the "low end" devices like the <$100 media streamers (eg. WD TV)  to see how those compare to the Squeezebox dedicated audio units.

BTW: If you're not aware, the Squeezebox devices by nature are asynchronous since they receive the data through WiFi or ethernet from Logitech Media Server and buffered with a decent amount of internal memory. You can see that the TosLink and coaxial connections have worse jitter than what's measured directly off the analogue outputs (eg. look at SB3, Touch, Transporter, Receiver jitter measurements).

As usual, feel free to comment or link to any good data you may have come across regarding this topic especially if conclusions are different from what I've presented.

Musical selection this evening:
Philippe Jaroussky - Carestini (The Story of a Castrato) (Virgin Classics, 2007) - amazing vocals and fascinating musical history. ("It's a man, baby!" -- Austin Powers)

Happy listening! ;-)

MEASUREMENTS: Squeezebox Duet - Receiver & Controller (Analogue Output).

A number of moons ago (December 2012), this blog was started to obtain data on ~320kbps MP3 vs. lossless audio. Near the end of data collection for that study, I started performing some tests and began posting them on the Squeezebox forum for discussion. 

Over the ensuing months, I started posting data on the family of Squeezebox devices which I collected over the years as the foundation of my sound system at home. Up to this point, I have had the opportunity to measure ALMOST the whole family of products from the SB3 onwards...

Squeezebox 3 / Classic

Transporter

Boom

Radio

Touch

Obviously missing from this lineup is the Duet - a combination of the Receiver and the LCD-endowed Controller which I have always wanted to try but never saw a unit locally to buy back in the day. I want to send a big thank-you to 'fordgtlover' from the Squeezebox forum for taking the time and expense to send me the Squeezebox Duet for testing - all the way from Australia no less!

Here she is:

The Controller comes with a nice weighted metal stand which also plugs in as a recharger. The LCD screen is reasonably bright and easy to read. Buttons have a nice click and selection is made with the turn wheel. One could ask for a smoother feel to the wheel, but as is, it's reasonably functional. Even though you can connect the Receiver via ethernet, the Controller obviously needs a functioning WiFi network.

The Receiver is a relatively non-descript black box with just a lit wireless icon on the front to indicate power & device status. Getting it connected up was quite simple and quick using the Controller - see here for the details.

A look at the back ports on these devices. From left to right of the Receiver: analogue stereo, TosLink digital, coaxial digital, Ethernet (10/100), and wallwart power (9V, 0.56A). You can make out on the top of the Controller a phono plug as well for audio output to headphones on the go.

I: Receiver Measurements

Internally, the Receiver uses the Wolfson WM8501 which is spec'ed at 100dB SNR and although it can go to 24/192, the Receiver is limited at 24/48 (as are all the other Squeezeboxes except Transporter and Touch).

Okay, lets get into it. First I wanted to have a look at the analogue outputs of the Receiver. Since this is a loaner unit, I'll try to extract as much as I can before sending it on the long journey home to Australia...

Lets pull up an oscilloscope reading* - 1kHz 0dBFS square wave (24/44):

The yellow channel is the right, and blue left. Notice there is some channel imbalance on this unit with the left channel a bit louder. Left channel peak voltage measures at 2.47V, right channel 2.40V.

The square wave itself looks nice. Minimum noise, good contours with minimal ringing (compare with the Controller below). From this, one would suspect that the usual RightMark analysis would show good results.

Here's the impulse response (as usual, 16/44 impulse recorded with the E-MU 0404USB at 24/192):

A standard linear phase filter response. Absolute polarity is maintained. We can therefore expect a sharp roll-off at Nyquist.

A. RightMark results:

The usual setup:

SB Receiver --> Shielded RCA --> E-MU 0404USB --> Shielded USB --> Win8 laptop

Firmware: 77

Receiver volume 100%.

Cables were the same for all these measurements (a 3' shielded RCA cable from Radio Shack, shielded 6' USB cable).

I tried both ethernet and WiFi. Since I could find no difference, I'll just present the WiFi data here. I noticed that the WiFi in the Receiver was able to achieve better signal strength than the Touch at the same location. 2 floors up from the ASUS RT-N56U router, my Touch got ~65% strength, while the Receiver was up at ~75% (Controller also about 75%).

Where I did the measurements, the Receiver was up around 85% WiFi signal strength.

As you can see, I measured the Receiver against the Touch all the way up to the max sampling rate of 24/48. Also, I included the 24/44 data from the Squeezebox 3/Classic as comparison. The graphs below are derived from the 24/44 data. Notice that we gained 4dB from 16 to 24-bits using the Receiver...  Not much.

Frequency Response:

Noise:

THD:

Relatively stronger odd-order harmonics up at 3, 5, and 7kHz compared to the Touch or SB3.

Stereo Crosstalk:

As you can see, a mixed bag. The Receiver's frequency response is similar to the Touch; both are flatter than the SB3 which tends to roll off the deep bass a bit more. The Touch clearly beats the other two in terms of noise performance (SB3 and Receiver about equivalent here), but scores lower with stereo crosstalk (not really an issue IMO). Total harmonic distortion was higher with the Receiver.

B. Dunn Jitter Test (16 & 24-bits):

16-bits:

24-bits:

Low jitter using this test in the analogue output.

II: Controller Measurements

The controller is a neat device. As shown above, there is a metal charging base and it's motion sensitive so will automatically wake when picked up (a few seconds wait time to wake up if not in the charger). It's pretty good at doing what it's meant to - as controller for the various Squeezebox units (not just the Receiver). The scroll wheel allows accelerated item selection, but these days, I mainly use a tablet to control the Squeezebox devices and it's of course quicker to type in search items than scrolling through letters.

The Controller has a standard 3.5mm phono plug up top for headphones. Although it can also play back audio, as far as I know, this feature was implemented at a "beta" level only. You have to go into Settings -> Advanced -> Beta Features -> Audio Playback to turn this on. Because the unit has a little speaker inside, I also needed to download the "Headphone Switcher" app from Applet Installer which adds an item in the "Extras" menu to turn the headphone on. Eventually I was able to get the unit to automatically switch between the speaker and headphone after some fiddling and rebooting the device (honestly I don't know which step was the key - the nature of 'beta' I guess). I did notice some slowdown to the UI responsiveness when streaming audio to the Controller.

According to the SB Hardware Wiki, the Controller has a Wolfson WM8750 inside. Understandably, this is a low power chip meant for portable devices. Data sheet lists DAC SNR up to 98dB. Maximum sample rate goes up to 24/48.

Setup:

Controller --> phono-to-RCA shielded Radio Shack cable (3') --> E-MU 0404USB --> shielded USB --> Win8 laptop

Firmware: 7.8.0-r16739

Controller volume 100%.

Here's what a 1kHz square wave, 0dBFS, 24/44 looks like under the oscilloscope playing off the phono plug:

In comparison to the Receiver above, it's clear that this isn't going to be "hi-fidelity". There's a bit of ringing and noise evident. Peak voltage is around 800mV and the two channels are reasonably balanced.

Here's the impulse response:

Upside down - the Controller inverts polarity. Standard linear phase digital filter.

A. RightMark results:

Comparison is made with the Receiver and Touch. Numerically, the thing that really sticks out is that massive intermodulation distortion up at 4%! I'm a bit surprised how the program did not calculate a higher THD as well given how nasty the graph looks - this is why I post up the graphs as well... I checked and double checked the testing to make sure settings were not clipping the signal. Made no difference - it is what it is!

No meaningful improvement with going to 24-bits.

Frequency Response:

Odd noise spikes from 5kHz up. Have not seen this before in my other tests. I did the test 3 times and this appears to be a real finding. Also, earlier roll-off down to -3dB by 20kHz.

Noise:

THD:

This looks bad but the score was about the same as the Receiver. RightMark is probably just looking at the odd and even harmonics for the calculations.

Intermodulation Distortion + Noise:

Ouch. Non-linearity is an issue.

Stereo Crosstalk:

As I said earlier - the Controller isn't high fidelity :-).

B. Dunn J-Test:

16-bit

24-bit

Very strong jitter! Many many nanoseconds of jitter :-). Again, this is not distortion from clipping.

III: Summary (Analogue Output)

Receiver: Reasonable analogue output from the device. It performs similar to the old SB3 in terms of dynamic range and noise floor. I hooked up the Receiver to my bookshelf system upstairs ('vintage' Sony MHC-1600 from university days powering some Tannoy mX2's) for a few days to listen. Very enjoyable - got a chance to listen to the Into Darkness soundtrack, the recent HDTracks release of Cole Español, and stroll down pop memory lane with Samantha Fox's Greatest Hits :-). I pulled out the Squeezebox 3 to compare and subjectively, I agree that there's a bit more bass with the Receiver and Touch than the SB3. While that channel imbalance was measurable, I didn't find any gross anomaly with the speaker system but I could hear the difference with the AKG Q701 headphones for example listening to where Ella Fitzgerald's voice was centered on Ella Sings Gershwin, I'd say it's subtle so would not affect my listening pleasure (could also be placebo since I was specifically listening for this!).

Controller: Firstly, how does it sound? Well, perhaps surprisingly OK :-). It's low powered so I listened with a pair of JVC HAFXC80 IEM headphones. I would compare the sound output to what I hear off my Samsung Galaxy S2. If you have a Controller, have a good listen - that's what a high jitter device sounds like (I have not ever seen this many side bands in all the testing so far!). For me, the best I can describe is that the Controller sounds "distant" even with volume pushed up and slightly "hollow" compared to listening through the headphone out from the Touch. The percussion at the start of Star Trek Main Theme from the "Star Trek: Into Darkness" soundtrack for example sounded less defined and spatially smeared, getting worse as the orchestral dynamics build up as if there's some low-level static in the background. How much of this can be definitively attributed to the high jitter is hard to say since the unusual frequency response, noise levels, intermodulation distortion and headphone output limitations are all significant factors in the sound. I highly recommend just streaming 192kbps MP3 as this will improve the responsiveness of the Controller and you're not going to hear a difference.

"fordgtlover" wanted to know how well the Receiver serves as a digital transport - good question! This then will be the topic for the next installment along with comparisons with the Touch... Stay tuned... Something tells me this is going to be complicated and hopefully quite interesting!

MEASUREMENTS: Logitech Squeezebox Boom & Radio.

Well, the Radio is (sadly) the only Squeezebox device still in production these days. DAC is the TI TLV320AIC3104.

I calibrated the headphone output to my E-MU 0404USB, WiFi streaming, measured with the usual i7 workstation.



Not expecting hi-fi performance out of this unit of course and pretty well got what was expected... Overall, an OK performer in the 16-bit domain for music streaming duties and incapable of taking advantage of 24-bit audio with only a measly 1dB improvement in dynamic range.

It's a "radio" after all...

------------------

For posterity and completeness, here is the SB Boom.

Internal DAC is the TI TAS3204.

Like with the Radio, this was connected from the headphone out in the back --> E-MU 0404USB, WiFi streaming.



Again, not a hi-fi unit by any means. Good dynamic range but stereo crosstalk is a bit on the high side. Like the Radio, you can see a slight improvement with 24-bit data but not much of an improvement to call it worthwhile.

It sounds great as a stand-alone system which is all I really use it for in the living room. 

MEASUREMENTS: Logitech Squeezebox Transporter [Updated June 26, 2013]

It's Squeezebox Transporter time!

In order to do the measurements, I brought the gear downstairs to the basement which is an electrically quieter environment. There's a Belkin PureAV PF60 power center there for all the equipment. Also, the measuring computer is now the AMD Phenom X4 laptop with Win 8 usually used by my kids :-). The laptop was running on battery - I could detect a 0.5dB difference with the AC adaptor plugged in down at the -110dB noise level.

One note about the XLR measurements you'll notice - the THD levels are a bit higher than RCA. I believe this is a result of the fact that the E-Mu 0404USB could not handle the XLR voltage from the Transporter and I had to use the analogue attenuators to (just barely!) avoid clipping.

Lets start with the 44kHz signal:


Undoubtedly, the XLR output is significantly better than RCA. One observation is that through the RCA's, the stereo crosstalk remained around -90dB whether the signal was 16 or 24-bits. I'm not sure if this is the limit of the Transporter itself, or has to do with the cables I used - a pair of AudioQuest 6' interconnects. I don't remember which model of AudioQuests these were (bought a few years ago in a moment of weakness :-) but they are longer than the 3' I had been using to measure upstairs.

Now for 24/96:


Again, we see the -90dB stereo crosstalk limit with the RCA output. XLR's measure is fantastic! Likely hitting the performance limits of the E-MU 0404USB and ~3dB better than the Essence One (of course the E1 did not have the benefits of a power filter or low noise environment of the laptop running on battery).

The WiFi router was in the same room as the Transporter hence the 90+% wireless strength. No difference in the measurements whether WiFi or ethernet.

To show in graph form the difference in noise floor between the RCA and XLR:

Notice the noise spikes like at 60Hz using the RCA cable (the AQ construction seems to be shielded but can't confirm unless I cut it open!).

Conclusion:
1. Overall the Transporter measures well! Phenomenal XLR performance - best I have been able to measure so far; >19-bit dynamic range.
2. A bit unclear about that stereo crosstalk measurement with RCA however. A bit higher than I'd have expected. Might need to try a different cable and see... (In case anyone wondering, I have not opened up the Transporter to change the unbalanced volume attenuation.) --- SEE ADDENDUM: I believe it's the cable!

ADDENDUM - (Cables do make a difference :-)

Further exploration of the stereo crosstalk issue shows that it likely was the cable afterall! I switched the analogue out from the AudioQuest (can't confirm but the cable looks like the current "Yosemite" model on their web page) to a pair of old 6' Tributaries interconnects (probably 50% the cost). As seen below the Tributaries improved stereo crosstalk by ~2.5dB:


Here's how the graph looks (AQ [white] vs. Tributaries):

Furthermore we're starting to see that 24-bit audio is starting to show better separation compared to the 16-bit data with the Tributaries suggesting that the cable was the limiting factor! I expect a good 3' cable would improvement the measurement even further. At this point then, I believe the Transporter's RCA stereo crosstalk is just fine.

Interesting that the more expensive AudioQuest measured worse than the Tributaries in this setup. As a result of this, I'm going to demote the AQ's to my CD player - something I would never know to do unless I ran this objective test.

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Before I leave the Transporter alone... I wanted to see if turning on the TosLink effects loop affected measurements. Normally, I have the Transporter --> TosLink --> Behringer DEQ2496 (room EQ) --> TosLink --> Transporter as DAC, so it'd be nice to know that the DEQ in digital mode doesn't affect the final sound (tested at 24/96).

With the DEQ2496 on bypass mode (ie. no room EQ processing):


Using the Tributaries RCA cable as output, no difference whether the DEQ2496 was digitally in line or not. Note that I used rather generic TosLinks - well constructed relatively thick plastic optical cables bought on sale for $10 each. Alas, by this time I had disconnected the XLR's and I didn't care to disrupt the Transporter setup again :-)

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Transporter Jitter measurements (Dunn J-Test 24/48, XLR):

Direct from the analogue output, this looks good!  The usual beautiful Transporter jitter plot. The 6 tiny spikes/sidebands are estimated as <300ps total (I think Stereophile measured 230-270ps) - nice corroboration!

Now what happens with the DEQ2496 in line (remember, the digital data is now going through 2 TosLink cables as described above)?

Obviously quite a bit more jitter has been injected by the Behringer! My estimate looking at the top 8 sidebands suggests that the jitter level now is 2ns using some measurements in WaveSpectra!

Since I can just turn the digital loop on and off, I can do instantaneous A-B'ing of the sound. Even in this condition, I cannot say the clearly increased jitter is at all noticeable. What can I say, even knowing this I'm just fine with keeping the DEQ2496 in line and use the room EQ function since *that* is audible! At least now I can say I've done an A-B test examining the effect of 2ns jitter for myself.

Addendum: Feb 27, 2013
Thanks to slimdevices forum member "tpaxadpom" who measured the digital output with the AP2722 unit:
AES/EBU 377.3-424.5 ps
SPDIF RCA 566 ps
SPDIF BNC 283-330.2 ps (rca cable with 2 bnc adapters yielded the same results)
Toslink 1.462 - 4.103 ns depending on the cable used. I've tested 4 or 5 different toslink cables.

Looks like the Transporter's BNC connection is the winner followed by AES/EBU. TosLink worst for jitter not surprisingly.

Addendum: March 6, 2013
Got some AES/EBU digital cables - here are the measurements.

Addendum: June 26, 2013
As part of the transport measurements, I decided to have a closer look at the Transporter. Here's some more data to consider:

Lovely 24/44 square wave at 0dBFS off the RCA output. Peak voltage of 2.95V. Very nice channel balance.

Impulse response (16/44):

This is the standard "sharp" filter. Linear phase. Absolute phase maintained.

When you set to "slow" roll-off, look what happens:

Wow... Barely any pre and post-ringing! However, clearly to achieve this, roll-off is expected to be very significant.

The "big board" RightMark summary - all based on RCA analogue output:

Look at the 16/44 frequency response; clearly roll-off is quite significant. Here's the graph:

You see the response deviating significantly by about 8kHz and more than -1dB by 15kHz. For good 'youngish' ears, that's significant.

This is what 24-bit buys you in terms of noise floor - remember this is with RCA output, expected to be even better with the XLR:

At 24/96, the filters are still quite different, but inaudible difference for anyone but cats, dogs and machines :-):

RCA analogue output Dunn J-Test:
16-bit (16/44):

24-bit (24/48):

Summary: I remain very impressed with the Transporter. Kudos to Sean Adams and the Slimdevices team back in the day. Technically a beautifully designed machine and it remains my primary digital front end in the listen room. I was a bit surprised by the slope of the slow roll-off - much more than I thought!