When you have HdphX engaged in Crescendo, it produces an effect that mimics listening to loudspeakers at +/-30 deg azimuths. This is intended for the sake of headphone listeners, to remove the artificial and extreme stereo separation that you get because the headphones are applying left and right channels directly to each ear.
Normally, when listening through loudspeakers the two channels mix together in the room air. And so each ear gets to hear its side mixed in with some of the opposite channel.
We simulate that effect with a shaped/delayed crossover between the two channels. This figure shows schematically how that shaped filtering is constructed:
The signal from the opposite channel is fed through a high-shelf filter tuned to 750 Hz, with a shelving attenuation of -20 dB, and a Q of 0.29. This simulates the shadowing caused by the head and torso. That low Q produces a gentle slope that almost mimics a 1/F filter slope, which would be ideal. But it is easier to approximate this with the high-shelf filter than to build a real 1/F filter.
Simultaneously, we feed some of that signal through a narrow bandpass filter centered at 5 kHz, a Q of 1, and with a peak level at -12 dB. This simulates the propagation of a surface wave across your face, that enhances some of the high treble from the other side.
These two filters are added together, then their sum is attenuated by -6 dB, and delayed by 600 microseconds, before combining with the main channel feeding your ear.
That summing is shown here:
I also show the combined filter shaping in the little graph in this figure.
Suffice it to say, it sounds very natural. Even when a track has some sounds that are solely in left or right, a little bit of that gets fed to the opposite ear, and it prevents the excessive head pounding you get when stereo separation is very strong.
That 600 microsecond delay induces a Haas effect, which keeps the cross-feed sounding like it really came from the opposite side. It simulates the fact that your ear is a bit further from the opposite speaker.
As a result, I can listen for indefinite periods without feeling any listening fatigue. And, as with all sound shaping, this effect is applied just before Crescendo works on restoring your hearing.
[ PS: This block diagram DSP design tool is another of my Lisp creations, called “GigaDSP”. It was originally invented to support our efforts to create extreme high speed local area networks, using radio channels at 115 GHz. A full length movie would download in about 4 seconds.
With this tool, we can wire up prototypes in seconds, and watch them run, even as we rewire the connections, or probe with additional visualization tools like oscilloscopes, spectrum analyzers, transfer function analyzers (like shown above), etc. ]
[ PPS: And obviously, when you really are listening through loudspeakers, instead of headphones, you should disengage the HdphX. The room air is already doing this for you. ]