Introduction

I was listening on Qobuz to this recording of Ivan Fischer conducting the Budapest Festival Orchestra playing Brahms Symphony 3. It happens to be a 192k – 24 bit recording. This recording sounded a bit “off” but I couldn’t place exactly why. So I captured the bitstream and here’s what I found.

The Capture

First: this capture consists of a single brief section of the recording purely for educational use and is not distributed. This places it under “fair use” for copyright.

I captured the opening of the 1st movement, which starts with about 1 second of room silence then the orchestra starts at medium volume. When I opened the audio file in Audacity, I noticed the wave didn’t show any obvious change in amplitude when the orchestra started. It looked like this:

Perceptually, it was quiet for the first second before the orchestra played. But this audio file shows the room silence at only 30 dB below peak levels, which is much too loud.

The Solution

Aha! I thought, perhaps this is ultrasonic noise! A spectrum analysis showed this to be the case:

This is an obvious glaring flaw in this recording. We have supersonic noise peaking at 70-80 kHz, just below the 192 kHz Nyquist limit. It’s so loud, it’s at the same level as easily audible 4 kHz audio content!

I could easily fix this in 2 ways: apply a low pass filter or resample it at a lower rate. The first step of down-sampling would apply a low pass filter, and this recording didn’t need to be at 192 kHz anyway. So I resampled it to 96 kHz. The half-rate integer multiple keeps the resampling method computationally simpler and cleaner. I could have down-sampled 4:1 to 48 kHz, but 96 kHz would be sufficient, as most of the noise was above 48 kHz.

The Result

Here’s the resulting wave file:

The room silence opening is now at least 50 dB below peak, which is typical.

The spectrum analysis:

The supersonic noise has been eliminated.

It is a bit unnerving that a professional recording can get released with such a serious flaw. Yet I noticed that this recording on Presto Classical is available at most 96 kHz, not 192. Could it be this problem was introduced by Qobuz processing it for streaming? Probably not, as Qobuz says they stream whatever the studios give them, without changing it. Supersonic noise in that spectrum almost certainly didn’t come from the room or the mics. It was most likely introduced by an improper format conversion from the original DSD to 192 kHz PCM. I don’t know why it sounded “off” since the high frequency noise should have been inaudible. Perhaps it was interacting with passband frequencies, causing audible intermodulation distortion. Or perhaps it was the result of improper low pass filtering, which also caused aliasing in the passband. Whatever the root cause, it was fun and educational to explore, and shows that recording studios sometimes make mistakes.