A look at the music library: How much DAC intersample overhead is enough? True Peak analysis of genres in an eclectic digital collection.

In a recent article about Linn DS streamers, I touched again on intersample peaks ("True Peaks") as I measured the DAC hardware. In the comments section, there was further talk about this topic so this got me thinking over the last few weeks to spend some more time on this but from a different angle.

For those who are not aware of what "intersample peaks" are, recall that DACs implement filters to ensure accurate reconstruction of the waveforms (unlike the jagged-looking NOS DACs which some audiophiles still subjectively appear to prefer). Typically this is done through oversampling whereby 44.1kHz (ie. CD samplerate) might be increased by something like 8x (352.8kHz) while applying a low-pass filter that cuts off content beyond the 22.05kHz Nyquist frequency. The DAC will interpolate to create those extra upsampled datapoints. In this DSP process, some of the reconstructed "intersample" points might have >0dBFS values. If these "True Peak" values are too high beyond the limits of the digital filter or output level of our DACs, the samples will "clip", resulting in extra distortion in the playback.

For this post, instead of considering from a hardware measurement viewpoint what kind of overhead a DAC can handle, let's consider from the music content side by using a relatively large music library - my library of thousands of albums - and consider just how much overhead peak we might find and "need" based on the commonly-owned music genres in an audiophile's collection.

Linn DS streamers: Majik DS(/1), Klimax DS/2 and Klimax DSM/2 measurements. (Examples of "high end" audio.)

The Linn Klimax DS/2 here at home for auditioning back in 2024.

Readers might recall about a year ago, we ran a blind listening test using hi-res captures of the analog output from different DACs to see if there was a clearly audible difference between a very cheap DAC (the Apple USB-C headphone dongle) versus much more expensive "audiophile high-end" streamer DACs which were the Linn Majik DS(/1) and Klimax DSM/2 Katalyst. If you don't know the results of that online study, here they are, and the subjective comments around what was heard.

While I showed 0dBFS 1kHz THD+N results for the Majik DS and Klimax DSM/2 back then, there was quite a bit more data collected as part of the preparation for that Internet Blind Listening Test that I did not publish. Even though my friend linnrd (who has contributed to these pages), as suggested by his screen name, happened to be quite the Linn fan, I appreciate that he is not blinded to the complexities of human preferences. He's well aware that both subjective impressions and objective performance play their part and he was totally open to me running measurements on his Linn digital gear to examine for technical accuracy.

For this post, let's dive deeper into the technical performance of some Linn streaming DAC gear. Here are the devices I'll be showing results of...

Further Explorations into "Intersample Overs" - Resampling/Downsampling & De-Clip by Charles King

Greetings everyone. It's great to interact with some of you over the years around contents I've posted in this blog. As you perhaps know, recently, I talked about resampling hi-res audio files in my article on "Post-Hi-Res" with the idea that the vast majority of albums we download as supposedly high-resolution content simply do not warrant the file size or bitrate. As such... I routinely just bring them back to 16/48 or 16/44.1.

Here's an interesting comment by Charles King on this and his explorations of the topic:

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Hi Archimago,

I was a bit taken aback on reading your 25 July post in which you talked about a need to guard against intersample overs when downsampling hires files. I've collected quite a few albums in hires over the years, often to check if I could hear any difference (I can't, and have given up on that) or to see if they provided better mastering (occasionally true, though in some notable cases the mastering is audibly worse). Since I don't want to litter my long-term storage with gigabytes of useless data I end up downsampling these to 16/48 in Adobe Audition (which is rated as having one of the better resamplers) and then compressing to variable-rate AAC (which is transparent to me).

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].)