Right, and the roll-off becomes less severe the higher the sampling rate you use. I'm assuming you're still limited to the setup(s) you mentioned earlier. Or, if you're like me, the roll-off can be desirable at times.

That's an interesting test you did there! I might see if I can do something similar in the future.

Here is the spectrum of a band-limited white noise signal (brickwalled maybe around 16 kHz) when using the following signal path:

WDTV-Plus (optical pass through) > Metrum (44.1 kHz) > 2i2 (96 kHz)
Finally got sick and tired of making CDs, dealing with my PS2 CD-R impotence, and other issues w my senile Xbox 360...



I think a NOS DAC (zeroth order S/H) should generate a mirror image (brickwall and all) starting at 22.05 kHz and centered at around 44.1 kHz. This might explain the sharp gap between 16 to 28 kHz (the gap gets mirrored from 16 to 22.05 kHz into 22.05 to 28 kHz).

Moreover, the S/H operation should also modulate a sinc function into the FR, with nulls occurring every integer multiple of the 44.1 kHz sampling rate. This explains the null at around 44.1 kHz.

The images and nulls will continue to repeat at every integer multiple of 44.1 kHz, but more attenuated by the sinc function shape (a result of the S/H operation).

Here is another white noise signal spectrum using the same set up, but this time the noise does not seem that brickwalled but proly alias a little bit (notice the discontinuity around 20 kHz). This I think shows the sinc deal better.



While sinc function related losses around 20 kHz and 15 kHz are proly about 3 and 1.6 dB respectively, I liked the laid back nature of the DAC. This is a worst case 44.1 kHz mode situation, at 96 kHz (thu SW upsampling or so) the losses might be closer to  0.6 and 0.3 dB (20 and 15 kHz). These theory numbers seem to correlate well w another similar NOS DAC discussed here. The effects of the ultrasonic stuff probably depends on the amp, but I also did not seem to have problems with that.

Here's what a pure 10k sine wave looks like from the Quad, vs the gamma2 (a delta-sigma WM8741 D/A chip)





Interesting to note the stair-steps. Even with a 1k sine wave they were visible on my 'scope, although I can't seem to find those pics that I took.

For reference, signal path was:

Asus Laptop -> gamma1 USB-SPDIF -> Quad -> HP Scope
Asus Laptop -> gamma1 USB-I2S -> gamma2 -> HP Scope

It was also interesting to see how the response in the transition band changed with the gamma2's different filters vs. the Quad.

Cool, thanks, OJ!

Solderdude, it would indeed be interesting to see what the signal looks like coming through headphones or speakers. As for the lack of filter ringing on NOS, or when considering filters with just a bit of post-ringing, my upsampling tests with the Hex and comparisons against other DACs make me think this might actually have audible differences. Much more subtle than the many other benefits you gain from upsampling/oversampling, but there were still certain selections where I thought transients were slightly smeared on OS DACs or when using upsampling filters with pre and post-ringing on the Hex. It is very hard to describe...

Of course, YMMV, and it's definitely a possibility I was not actually hearing what I thought I was hearing. If I was, though, it's a trade off. You can definitely hear the rolled-off treble and other "unique" characteristics of NOS DACs, but I don't mind due to easily getting fatigued while listening.

I would be very curious to see what your subjective impressions would be in these situations as well.