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Is Hi-Res music worth what it offers? (1/2)

Digital recordings exist in two main encoding formats. The PCM (Pulse Code Modulation) format which encodes the signal from n-bit samples (e.g. 16 bits for CD) taken at regular intervals (number of samples per second, e.g. 44100 times per second for CD) and the DSD (Direct Stream Digital) format which encodes the signal on a single bit but at very high speed (Delta-Sigma modulation).

Recordings using the PCM are available in several resolutions defined by the number of quantization bits and the sampling rate:

  • 16 bits, 44.1 kHz (Audio CD quality)
  • 16-bit, 48 kHz (DVD standard PCM)
  • 24-bit, 48 kHz (DVD Extended PCM)
  • 24 bits, 44.1 kHz
  • 24 bits, 88 kHz
  • 24 bits, 96 kHz
  • 24 bits, 176 kHz
  • 24 bits, 192 kHz
  • 24 bits, 352 kHz

There are other resolutions including 32-bit floating but not available on the market.

Obviously the higher the resolution used, the better the sound should be with a gain in the high frequencies and in the signal to noise ratio (in theory 98 dB in 16 bits, 144 dB in 24 bits). But as we will see this is very theoretical and in practice the values obtained can be very different. The reasons may be technological, depending on the quality of the equipment used for recording or playback of the musical material, but they may also depend on the care taken in audio processing and external disturbances or other errors by technicians during recording, mixing or mastering.

The number of encoding bits in the PCM gives the dynamic range of the signal and its signal-to-noise ratio. The sampling frequency gives the maximum frequency of the signal that can be reproduced (this frequency must be at least twice higher than the highest frequency to be reproduced according to the Nyquist-Shannon Theorem). The 44.1 kHz frequency of the CD therefore theoretically allows to reproduce a signal up to 20 kHz.

Recordings encoded in DSD are available in DSD64 (sampling 64 times higher than the PCM sampling of the Compact Disc - thus 44100 Hz × 64 = 2.822 4 MHz), DSD128 (sampling 128 times higher than the PCM sampling of the Compact Disc) or DSD256. Other data rates are possible but very rare for the consumer market. Files in DSD are very hard to process and analyze, it is often necessary to convert them previously in PCM. As we will see this conversion is not without serious problems.

What are we going to check to evaluate the quality of the Hi-Res recording?

We noted earlier that two values of a PCM signal determine the technical quality of a recording: its sampling frequency and therefore its bandwidth and the number of quantization bits and therefore its dynamic range and signal to noise ratio.

Except in the case of serious processing errors (e.g. spectrum aliasing), the sampling frequency directly determines the bandwidth of the recorded signal, assuming that it is used correctly. We will discover that this is unfortunately not always the case. It happens too often that the bandwidth is truncated or more rarely I hope extended by armonic generation artifices.

The number of quantization bits is much more difficult to determine. If it defines the dynamics and the signal-to-noise ratio of the recording, these characteristics can simply be modified with digital filters (noise reduction, reduction or increase of dynamics, etc.)

Which source of music in Hi-Res?

There is more and more music in Hi-Res. Without SACDs, which were not as successful as expected, music lovers can now buy files in FLAC, ALAC, AIFF or WAV formats on Qobuzz.com or even HDtrack.com, HighResAudio.com, Highdeftapetransfers.com or even HDMusicStore.com. It is also possible to listen to Hi-Res streaming music on Qobuzz.com but also on Amazon Music HD. For my tests I used Qobuz.com with a Hi-Res subscription via the Audirvana player.

Audirvana

What does a good Hi-Res recording technically looks like?

Wide bandwidth, high dynamics, low noise and no distortion, these are the ingredients of a good Hi-Res recording. I do not talk about musical quality or sound recording quality but only about the care that has been taken from a technical point of view during the recording or signal processing (digitizing, mastering or cutting in the case of vinyls.

I selected as a reference for a good Hi-Res recording a StudioMasters Edition of Soul Brothers record by Milt Jackson & Ray Charles. The recording was made in 1958 (Recorded in NYC, September 12, 1957) and was certainly realized on magnetic tape. The album was released in this edition on July 24, 2012 by Rhino Atlantic and received the Qobuz Reference award. On the Qobuz site the tracks are available in 24 bits and 96 kHz (24-96) and it can be found on the HDTracks.com site in 24 bits and 192 kHz (24-192).

The first thing we're going to investigate is the quality of the recorded signal to see if it has been processed without saturation and without compression while exploiting all the available amplitude. This can be seen with the curve that represents the signal over time (Waveform of the audio file).

Waveform of Soul Brothers record by Milt Jackson & Ray Charles 24-96.

With this screenshot we can clearly see that the signal is very well recorded with sharp peaks and softer moments. There is neither saturation nor excessive compression of the signal here. By zooming in on the highest area of the signal, you can verify that there is no saturation at all.

Zoom in the waveform of Soul Brothers record by Milt Jackson & Ray Charles.

The second point we can investigate is the frequency distribution of the recording to check that the whole spectrum is correctly processed, without reduction and without disturbance.

Spectrum of Soul Brothers record by Milt Jackson & Ray Charles 24-96.

In this edition Soul Brothers has been digitized in 24-96 so the theoretical bandwidth ranges from 0 Hz to 48 kHz. Here we have a very nice signal without big anomaly. The bandwidth extends up to 25 kHz and there are no big parasites or interferences. A bump at 30 Hz can be observed but has no obvious reason (magnetic tape - hum noise?). In any case it's a very nice job.

To conclude we will check the signal statistics in order to have a complete evaluation of this recording.

Waveform Statistics of Soul Brothers record 24-96.

The information that can be deduced from the statistics is that there is indeed no signal saturation "Sample peak level" (The maximum level of digital samples in the recording) is less than 0 dB and "Possibly clipped samples" = 0 (The number of samples where the true peak signal level exceeds 0 dB).

To measure the dynamic range we will use the very efficient TT Dynamic Range Meter plugin which has the goal to discover and categorize over compression (The DR Value is the difference between the peak and the top 20 average RMS measurements).

Dynamic Range of Soul Brother record 24-96.

Soul Brothers has a DR of 18 which is a very good result (there are a lot of poor quality recordings with DRs of 5 or less). A value above 20 is outstanding and can also be dependent on the style of music.

What does a bad Hi-Res recording technically look like?

This is the whole point of this article and we will discover in the following pages that many audio recordings do not meet the minimum criteria to be considered as good Hi-Res Audio recordings.

But to have an idea of what we can find, I checked the same edition of our Soul Brothers in 24-192.

Spectrum of Soul Brothers record by Milt Jackson & Ray Charles 24-192.

It is obvious that what we buy here compared to the 24-96 version is essentially a big spike at 76.8 kHz of unknown reason (this track comes from a magnetic tape recording. Some generate interference frequencies such as Revox C270 which use 76.8 kHz for the +5 V switching power supply and the pulse width modulation for the spooling motors. Source https://archive.org/stream/studer_Revox_C270_Serv/Revox_C270_Serv_djvu.txt). We can clearly see that after 30 kHz there is no useful signal apart from this interference.

Before buying a Hi-Res recording it is therefore recommended to check its real quality.

For the following of my reviews in the next post I will only look at the spectrum because the objective here is to verify that an edition of an album in Hi-Res has an added value compared to the same edition in standard version (CD for example). So I will not look at over-compression, distortion, saturation or any otherother waveform issue. But you should know that the overall quality of your album depends on many technical factors and not only Hi-Res.

Waveform saturation: Eels - Beautiful Freak (CD)

Over-Compression: Iggy Pop - A Million In Prizes (CD)

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Location: Paris, France

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