Also note that in
Stereophile, April issue where they tested the Headroom Desktop and the Grace m902, USB had the highest jitter rate of all the interfaces:
http://stereophile.com/headphones/406head/index4.html (Headroom Desktop)
Quote:
| Wes Phillips reported getting good sound using the USB input from his computer. This gave the same maximum output level as the conventional digital inputs, but did raise the jitter level to 682.5ps, mainly due to the introduction of some high-level sidebands at the data-related frequencies of ±230Hz and its harmonics (grayed-out trace in fig.15, red-circled spectral lines). The central peak in this graph was also slightly wider than with the S/PDIF connection, implying the presence of some very-low-frequency random jitter. Even so, this was very much better jitter rejection than from any other product I have tested using a USB data interface. |
http://stereophile.com/headphones/406grace/index3.html (Grace m902)
Quote:
Wes Phillips reported that the m902 sounded "murky" when he connected it to his Macintosh G5 via USB. I therefore used the Miller analyzer to measure the jitter driving the Grace with USB data from my lab's PC. The result was the highest jitter level I have encountered, at 28.6 nanoseconds—almost 100 times greater than via the well-behaved S/PDIF connection. However, when fed a continuous, constant-level sinewave via USB, the m902's output level kept changing slightly. It appeared that this was due to a high level of hum; no matter how I arranged the m902's grounding or lack thereof, I couldn't eliminate what must have been a ground loop between the computer and the Grace.
The only way I could get hum-free audio from the Grace via its USB input was from my battery-powered PowerBook laptop. Fig.10 shows the jitter spectrum taken under these circumstances. The jitter level was lower than before, though still high, at 1.3 nanoseconds, and the spectrum is dominated by strong sidebands at ±23Hz (purple "1"), ±34.7Hz (purple "2"), ±230Hz (red "4"), and ±690Hz (red "9"). Note that there is also some spectral spreading of the central tone, due to the presence of low-frequency random-noise jitter. The correlation between jitter levels and spectra is unclear. However, I am not surprised that WP found the Grace's USB input to sound "murky." I must point out, however, that I have generally found much worse measured performance from USB-connected D/A converters. |
The same units tested from 420
picoseconds to
236 picoseconds for SPDIF direct.
USB, apparently, has significantly more jitter due to having no timing data and it is up to the DAC to reject it.
Stereophile's review of the BitHead does note a good amount of jitter in the USB datalink and this might be the lack of clarity that you are hearing. It is possible that you have a ground or excessive jitter issue when your BitHead is connected to your home computer's USB.
