MEASUREMENTS: Raspberry Pi 3 B+ as streamer - switching power supply, battery, WiFi, touchscreen noise??? (32GB RAM, MI: Fallout and The Beatles' 2018 "White Album")

Battery-powered Raspberry Pi 3 B+ with JustBoom Digi. Bohemian Rhapsody soundtrack.

Remember that within audiophile circles, there are often all kinds of claims about noise affecting streaming devices; especially computer-based devices. I already touched on this last week when referencing the videos by Innuos. Over the years, I had addressed software playback, different OS's, lossless formats, "need" for ethernet galvanic isolation, and explored the lack of difference between machines used for playback. In a similar way this post will also explore some "received wisdom" which seem so common among mainstream industry-supported audiophile websites.

Since I had my set-up running for last week's evaluation of the JustBoom Digi, I figure I might as well do a few more tests to demonstrate for myself (and you, fellow readers) whether these beliefs hold any truth.

MEASUREMENTS: JustBoom Digi S/PDIF HAT for Raspberry Pi. (Plus "Why isn’t digital audio 'just ones and zeroes'?" anxiety...)

I mentioned a few months back in September that I updated my Raspberry Pi 3 B+ Touch device with the JustBoom Digi HAT for S/PDIF coaxial and TosLink digital audio output. Over the last few months, I've been using it more regularly for my nightly music listening.

As I noted previously, this device is essentially a 1-chip solution featuring the Cirrus/Wolfson WM8804 transceiver with jitter attenuating PLL (rated intrinsic jitter of ~50ps RMS) released in 2009. It also uses the Murata DA101JC isolation transformer for the coaxial output.

The device is rather plain otherwise as you can see. It does sound good and is capable of up to 24/192 with both coaxial and TosLink outputs. And at <US$40, it's also very affordable. Let's see how well it measures!

NOS vs. Digital Filtering DACs: Exploring filtering turned off, implications, fidelity and subjective audibility. (Recent BorderPatrol DAC chatter...)

Hmmm... Non-OverSampled waveforms - "accurate", "high fidelity"?

The waveforms above were captured with my RME ADI-2 Pro FS ADC at 384kHz. As you can see, on the left side, we see the waveform from the TEAC UD-501 DAC with the digital filter turned off (Non-OverSampling - NOS mode). In the middle, we have the TEAC's "Sharp" oversampling filter engaged. And on the right, we see the same waveforms captured from an old 16-bit Philips TDA1543 x 4 DAC with chips in parallel (supposedly improves noise level), a multi-bit NOS chip, without any oversampling.

Here's the TDA1543 x 4 DAC board. Typically fed with a 12V DC power supply.

MUSINGS: Multidimensional Audio! (An old article... Subjective perceptual ability... And a rigged Atmos / DTS:X setup... :-)

Well folks, it has been a busy last number of weeks around here since I returned from the overseas work trip. No time to play with much audio or tech stuff as a result until earlier this week.

Today I want to just discuss/show a couple of things...

Consider the phrase "multidimensional audio". In today's world, I think if anyone were to use that phrase, one would be thinking about multichannel and surround sound as "multidimensional". Instead, that graphic above is taken from this article - "Multidimensional Audio" (1979) by Henning Møller of B&K.

MUSINGS: Raspberry Pi 3 B+ "Touch" Optimizations; CRAAP Settings, and the "Extremus" Filter Setting.

Okay, let's have some fun with the Raspberry Pi 3 B+ motherboard, piCorePlayer, and my little "Touch" device I recently updated. As discussed before and in the picture shown above, these days, I have the inexpensive JustBoom Digi HAT (~US$40) board attached out the back for simple S/PDIF connectivity as well as USB when I need.

Today, let's hit a few topics related to the Pi 3 B+ setup above. Some little tweaks, things to get it working well, and fooling around with a piCorePlayer/SoX filter you can try for yourself without unnecessary hype nor expense...

MUSINGS: On the RMAF 2018 MQA talk, pseudonyms, and the right to anonymity.

“A desire for privacy does not imply shameful secrets; Moglen argues, again and again, that without anonymity in discourse, free speech is impossible, and hence also democracy. The right to speak the truth to power does not shield the speaker from the consequences of doing so; only comparable power or anonymity can do that.”

--- Nick Harkaway, The Blind Giant

Boo!

Well, Halloween's coming up and it looks like we're about to talk about some scary, dramatic stuff involving emotional incontinence and anonymity :-).

MEASUREMENTS: RME ADI-2 Pro FS as DAC (Part II - frequency response, noise, distortion and jitter)

As we saw previously, the RME ADI-2 Pro FS is based on the AKM AK4490 DAC chip and provides a good assortment of features like filter settings. We've seen that the headphone output amplifier provides plenty of power and the beginnings of what looks like a highly accurate DAC; consistent with my subjective opinion.

Time for part 2 of the objective evaluation today as we continue to explore the DAC output quality.

MUSINGS / MEASUREMENTS: A look at Intersample Peaks and Overload Tolerance... (RME ADI-2 Pro FS, TEAC UD-501, Oppo UDP-205)

I suspect that many audiophiles were introduced to the topic of intersample peaks (ISP) when Benchmark first released their DAC2 HGC which advertised and discussed the provision of +3.5dB overhead for digital conversion back in late 2012. Over the years they've discussed this in their blog such as this post that came out to discuss the technical bits of the Sabre DAC and DAC2 in particular.

Remember however that ISPs and the potential for intersample overloading have been discussed in the technical literature for years. For a good review of the topic, have a look at this paper from Nielsen and Lund "Overload in Signal Conversion" presented at the AES Conference in 2003. Because almost all modern DACs utilize oversampling filters, the internal interpolation creates extra "points" between samples. For loud music (common since the late 1990's!) with samples approaching the 0dBFS level if not already clipping, it's likely that many of these intermediate points created will be above 0dBFS. Even in 2003, the paper listed a number of albums and showed that quite a few have "hot spots" where the "true peak amplitude", after going through an interpolator would be above 0dBFS. (A NOS DAC doesn't do digital intersample interpolation but could still overload with analogue filters [as per Mans' comment].)

MEASUREMENTS: RME ADI-2 Pro FS as DAC (Part I - output levels & digital filter settings)

After having explored a couple weeks back the ADC capabilities of the RME ADI-2 Pro FS, for this installment, let's start with evaluating the DAC output quality of the device and in the process examine the objective fidelity with audio playback.

Remember, with that previous article, I had already discussed my subjective opinions based on listening sessions. My opinion has not changed in the last number of weeks as I'm still very much enjoying the sound I hear hooked up to my main system. Subjectively, the output sounds very clean, has a neutral tonality and low noise level very much like the Oppo UDP-205 and other high quality DACs. As usual, it will be interesting to compare measured results among the different devices I put through the test bench.

But before we do that... Let's start with the basics and some of the "microscopic" measurements I usually begin with - things like output levels, filter settings, etc...

MUSINGS / MEASUREMENTS: About THD(+N) and standardizing testing here... (With a taste of the RME ADI-2 Pro FS as DAC...)

Last week, when I published my look into the RME ADI-2 Pro FS ADC performance, I started using SpectraPLUS for measurements in high resolution for THD and THD+N. I know, it's an old measurement and everyone does it. I realized shortly after publication of the article that while I was using the same technique to measure the RME vs. Focusrite ADCs, I had the ADCs running at 192kHz and so issued an addendum that the results included the rising ultrasonic noise from the ADC and not just a reflection of the DAC. Not unreasonable as a comparison between the two ADCs I think, but this would not be fair to the DAC or other component measurements since much of the noise would be arising from the ADC stretching out to a 96kHz bandwidth. It's worth taking some time to think, going forward, how I could improve the usefulness of this test and in a standardized way here when focusing on whatever device is being tested...

If we look around, we see that the THD+N spec is probably the most used objective "number" for audio equipment as a quick snapshot of fidelity. The THD (page on calculation, how it's done) component tells us whether harmonics are being added to the sine wave at the integer multiples of the fundamental frequency (these are the results of "nonlinearities" in the equipment), and the +N piece adds the noise component found in the signal being tested which of course is also subject to noise limitations of the measurement device and the computational limits of the FFT technique used. In essence, the THD+N ratio is a representation of everything that's being added to a simple single-tone test which of course has its limitations as a test paradigm as well when real music is far from static.

MEASUREMENTS: RME ADI-2 Pro FS ADC performance and as measurement hardware.

I discussed the features and some listening impressions of the RME ADI-2 Pro FS ADC/DAC a couple weeks ago. As I noted at that time, my main aim in owning this device is for the purpose of using it as an ADC for measurements here on the blog going forward.

For the purpose of measurements, we want a tool that can allow us to obtain reproducible results and good accuracy. Over the years, I've achieved reproducible results with consistently minimal inter-test variation by standardizing the way I run most measurements with the digital sources, cables, standard procedures, and types of tests I run. What I want is better accuracy - an ADC that has lower noise floor for improved resolution, doesn't add as much of its own distortions, have higher timing accuracy (eg. for jitter tests), and perhaps more features to expand the measurement quality (eg. higher sample rate to capture impulse responses more accurately, and can handle wider input levels for the range of devices tested).

Given that this is the priority, instead of measurements of DAC performance first, let's get straight to describing the characteristics of and using the ADI-2 Pro as a measurement tool which means we'll need to get a taste of how well the analogue-to-digital capability performs. Let's compare the results I'm getting from the RME to my previous measurement ADC device over the last couple years, the Focusrite Forte for a sense of the changes in resolution I can expect from the new upgrade.

Update: Raspberry Pi 3 B+ "Touch" Streamer, JustBoom Digi HAT, 3.0A power supply... And did they mess up Coltrane's "Both Directions At Once" in hi-res!?

Back in early 2017, I documented on the building of my little Pi "Touch" music streamer that I've been using over the last while for playback through my Logitech Media Server system at home. Over the last year, I've used this quite a bit for music playback and also some of the testing I've done on this blog.

As some of you may know, there has been an update of the Raspberry Pi 3 to the B+ model (~US$40). The upgrade isn't a major change to the existing Pi 3 board. However the new SOC, based on the Broadcom BCM2837B0 quad-core Cortex A53 (ARMv8, 64-bits) is slightly faster at 1.4GHz (1.2GHz previously, about 15-20% speed gain), and another upgrade is better ethernet speed which supports gigabit link but since it's still communicating through the USB2.0 port, will max out at ~300Mbps. In some benchmarks I've seen, though not true gigabit speed, the throughput is about 2x that of the previous Pi 3. For those interested in wireless connectivity, there are upgrades in the WiFi and Bluetooth departments as well.