Pure Vinyl Sample Rate Converter

Pure Vinyl's built-in Sample Rate Converter (SRC) can be used to convert high-resolution audio (up to 192 kHz 24 bit) to CD format, 44.1 kHz (with a choice of either 16 or 24 bit output). (The same sample rate converter design also is used for optional, real-time upsampling playback, to up to 24 bit / 192 kHz, of CD or other format tracks while in music server mode.)

If starting with a "flat" vinyl recording (no RIAA correction curve applied), the RIAA correction optionally can be applied during the conversion step, while the audio is in the 64 bit high precision internal format used by the Pure Vinyl SRC.

Pure Vinyl's SRC works equally well for power of two conversions (from 88.2 or 176.4 kHz) and for factored conversions (from 96 kHz or 192 kHz). Factored conversions take longer to perform, however.

Pure Vinyl Sample Rate Converter: Signal Integrity / Purity Test

A perfect result for a sample rate converter is a single trace with no spurious generated tones (completely black background), and little or no "V" reflection (aliasing) after the sweep passes 22.05 kHz, which is the Nyquist limit (this happens at about 19.5 seconds on the spectrogram display).

Anything besides a single, parabolic trace represents unwanted distortion and audio "dirt" added to the signal.

Test Conditions:

  • Input Sample Rate 96 kHz, 24 bit resolution
  • Swept Sine Wave File, 20 Hz to 48 kHz, -1 dB vs. Digital Full Scale
  • Sample Rate Conversion to 44.1 kHz, 24 bit format
  • Spectrogram amplitude scale -160 dB to 0 dB vs. Digital Full Scale
  • Test file duration 46 seconds (only the treble part of the analysis is shown)
  • Conversion time measured on 2.1 GHz G5 single-processor iMac

Sample Rate Conversion Processing Time: 15 seconds

Compare above with the other traces shown here, and with sample rate converters measured here: http://src.infinitewave.ca/ (The frequency axis is reversed in measurements at that website, where high frequencies are displayed at the top of the spectrogram, the opposite of what's conventionally used for spectrogram or "voice print" displays.)

Below are additional results, from testing the sample rate converters of some other Macintosh audio applications, using the same 96 kHz input file, resampling to 44.1 kHz, 24 bit, as above. Also note the conversion time when comparing the sample rate converters. If a choice of "highest" / "best" quality SRC was provided, the highest quality was used. Note that two of the applications apparently use the same SRC algorithm:

Sample Rate Conversion Processing Time: 268 seconds

 

Sample Rate Conversion Processing Time: 24 seconds

 

Sample Rate Conversion Processing Time: 21 seconds

 

Sample Rate Conversion Processing Time: 5 seconds

 

Sample Rate Conversion Processing Time: 5 seconds

Note: there is no inadvertent repetition of two graphs above. Apparently, both software packages use the same sample rate conversion software / algorithm!

 

Sample Rate Conversion Processing Time: 14 seconds

Note: 16 bit output file. The internal format used for conversion was 32 bit, but Audacity only supports 16 bit output when saving audio to uncompressed file formats such as AIFF or WAV. (See next test for 24 bit output, via FLAC format.)

 

Sample Rate Conversion Processing Time: 14 seconds

Note: 24 bit output file, converted from FLAC 24 bit lossless compressed format to uncompressed PCM format (Sound Designer II) for analysis. Audacity only supports 16 bit output when saving audio to uncompressed file formats such as AIFF or WAV. However, 24 bit resolution is available if using FLAC format.

 

Sample Rate Conversion Processing Time: 250 seconds

 

Sample Rate Conversion Processing Time: 16 seconds

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