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.