Dr. AIX's POSTS — 28 March 2014

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I downloaded a few files from Pentatone Records a couple of weeks ago. The label, based in the Netherlands, has a great reputation for capturing and releasing classical music using DSD. Their titles are also available as downloads from their site. I’ll put up some spectrograms in a future post but I thought I would share some thoughts about audio levels and how relative and absolute amplitudes are handled.

The subject came up because the 96 kHz/24-bit and 44.1 kHz/24-bit downloads of a Mozart album were very different with regards to levels. standard resolution file (the 44.1 kHz one) was 6 dB hotter than the high-resolution file at 96 kHz. Additionally, neither one came close to using the entire bit depth available or dynamic range of the 24-bit PCM format. The original was captured using DSD 64 and then converted to PCM.

When questioned about the level difference and the process of production from DSD to PCM, the representative from Pentatone responded:

“We always take the original masters used to produce the SA-CD [NOTE: the DSD 64 files] versions and the CD layer. NOTHING is changed to retain the best possible quality. For the FLAC version we use the Weiss Saracon as sample rate converter, known to be the best around, to convert from DSD to 96kHz 24bit stereo and surround.

The 96kHz versions are converted to FLAC files to reduce server space and download time. This is a lossless process. When all files are available they get packed with extra metadata and MP3 files in a .ZIP container. I hope this answers your question and I hope you will enjoy the downloaded files.”

The sound of tracks was very nice and the spectra, as you would expect, didn’t extend past 22-23 kHz due to the noise shaping and filtering applied during the conversion. But the levels were obviously lower than I expected.

The maximum level of the files that I downloaded peaked at around -7 dB. Why would the mastering engineer adjust the levels to use the largest number of PCM bits? Could it be that the native DSD files were simply transferred without concern for the overall level? I could imagine that this was the case if they didn’t have the ability to adjust the levels of the native DSD stuff.

The response from Pentatone didn’t specify how or whether they did any editing or other processing to the original recording. Perhaps the tracks are kept in the DSD format without any level or EQ applied.

In the world the PCM, we can use a process called “normalize” to make sure that the highest amplitude sample just reaches the 24-bit limit. It’s a two-pass system that scans the multitude of samples looking for the max value. If that value is 3.85 dB from the 24-bit limit then every sample in the file will be increased by 3.85 dB. The relative amplitudes are maintained but the over level is raised. This is standard operating procedure for commercially released recordings.

But Pentatone didn’t do this AND the 44.1 version was louder than the 96 version, which is very curious. I still don’t know why.

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