FAKED HIGH RESOLUTION AUDIO
We want to analyze a high resolution audio track where we have the suspicion that it is just an up-sampled version of a CD source.
The Format Display tells us that the audio file contains data in the format of 24 Bit / 88.2 kHz. This would mean that the highest frequencies could be 44.1 kHz (½ x sampling frequency / see Nyquist).
If we check the spectrum and spectrogram then we see that the music just contains frequencies up to around 21 kHz. The steep frequency decline around 21 kHz and spectrogram values mostly above -100 dB (mouse pointer measurement) indicate that this was originally a 16 Bit (max. -96dB) recording.
We simply proved that this was a CD recording with 16 Bit / 44.1 kHz, up-sampled with dedicated software, pretending being a high resolution audio track.
INTER SAMPLE PEAKS
The recording shows several Inter Sample Peaks indicated by TPL values of about +3 dBFS and a History Display with red marked overs of up to around +3 dB.
Theoretically it isn’t possible in a digital system to have higher levels than 0 dBFS, but if the signal is converted back into the analog domain then those Inter Sample Peaks cause distortions because the headroom of the converter is not large enough to reflect the resulting amplitudes.
PERIODICAL INTERFERING SIGNALS
STEREO IMAGE QUALITY
The MusicScope provides several means to asses the quality of the stereo image.
a. Vector Scope
b. Balance Indicator
c. Correlation Meter
2. Mid / Side Mode
This is a special analyzing mode activated by a single mouse click on the Levels-Meter. It allows the separate measurement of the mid (mono) and side (stereo) information. Within Mid/Side-Mode the History displays the mid (green) and side (orange) signal. A good Stereo Image allows an easy localization of instruments and voices.
If the following conditions are met then these are a good indication for an acceptable stereo reproduction:
- The Vector Scope shows a graphic which has a more vertical than horizontal distribution.
- The balance indicator is most of the time around the mid of the display and not too wide.
- The Correlation Meter stays in the green area
- In Mid/Side Mode the History indicates that the mid signal is always larger than the side signal.
MP3 VERSUS UNCOMPRESSED AUDIO
This is a comparison between an uncompressed and a MP3 (128 kBit/s) version of the same audio track. The first Spectrogram shows that the MP3 version is heavily frequency limited and that parts of the music have been removed on basis of a psychoacoustic model.
DIRECT STREAM DIGITAL (DSD) ANALYSIS
The following two examples represent a DSD64 (1 Bit / 2.8224 MHz) and a DSD128 (1 Bit / 5.6448 MHz) audio track.
The MusicScope is able to analyze and to playback DSD tracks without the need for a dedicated DSD Digital to Analog Converter (DAC).
The analysis of the DSD64 track indicates that there is still music content around 35 kHz which goes over into quantization noise given by the 1 Bit digitization method.