ITunes Distortion Measurements
From Benchmark
Contents |
Distortion and Noise (THD+N) Graphs from iTunes 6 on OSX 10.4
iTunes 6 with all setting correctly configured
| This is the ideal, bit-transparent result for 16-bit audio. Notice the noise floor is 'flat' and never gets above -129 dBFS. If any distortion artifacts are louder then -129 dBFS, it result in significant, audible losses in audio quality. |  A 16-bit 10k sine wave played bit-transparently through iTunes 6 on OSX 10.4.6. |
Distortion due to iTunes 6 volume control
| The distortion seen in this graph is caused by setting the volume control in iTunes 6 near 50%. Compare this illustration with the bit-transparent illustration at the beginning of this section to see the amount of distortion caused by the volume control.
This distortion is most likely due to truncating the digital word after the volume calculation is performed. A properly designed volume control dithers the resulting digital word, resulting in far less distortion. Read more about digital volume controls. |  A 16-bit 10k sine wave played through iTunes 6 on OSX 10.4.6 with the volume control set near 50% (-19dB). The resulting distortion is well above the -129 dBFS noise floor and, consequently, is detrimental to the quality of the audio. |
Distortion due to sample-rate conversion with iTunes 6 on Mac OS X 10.4.6
| The distortion seen in this graph is caused by sample-rate conversion from 48kHz to 44.1kHz. Compare this illustration with the bit-transparent illustration at the beginning of this section to see the amount of distortion caused by the sample-rate conversion in iTunes 6 with Mac OS X.
Read more about sample-rate conversion.
|  A 16-bit 10k sine wave played through iTunes 6 on OSX 10.4.6, with sample-rate conversion from 48kHz to 44.1kHz. The resulting distortion is well above the -129 dBFS noise floor and, consequently, is detrimental to the quality of the audio. |
Distortion due to word-length truncation with iTunes 6 playing 24-bit files
| The yellow graph corresponds to a 10-kHz pure tone with sample-rate and word-length 44.1-kHz, 24-bit. The red graph corresponds to a 10-kHz pure tone with sample-rate and word-length 48-kHz, 24-bit. When played through iTunes 6 on OS X 10.4.6, the word-length is reduced to 16-bit.
The distortion seen in the red graph is caused by word-length truncation from 24-bit to 16-bit without dither. This also happens with 96kHz signals. The yellow graph, however, indicates the 44.1 kHz sample-rate does not experience the same distortion when its word-length is reduced to 16-bit. This also happens with 88.2-kHz signals. From this, we have concluded that iTunes 6 and/or OS X will dither 44.1-kHz (& 88.2-kHz), 24-bit audio but will not dither 48-kHz (& 96-kHz), 24-bit audio. |  44.1kHz (yellow graph) and 48kHz (red graph), 24-bit 10k pure-tones played through iTunes 6 on OSX 10.4.6. These signals were captured via the Macbook's built-in optical (S/PDIF) audio output. |
