In article writes:

"Phase response" is a property of a system, not a signal.

Exactly - so you can't talk about it in isolation. But a "system" can
be two signals generated by one or more black boxes.

In other words, the phase response of a
system provides, indirectly, the frequency-dependent delay through the
system.

Right. This is what I've always described as "group delay", the group
being the entire collection of frequencies within the waveform. There
is no single number because, as you say, each frequency gets its own
little delay unit in the big black box model. I could have been using
the term incorrectly all my life (which probably is a total of about
five times in more than 40 years of engineering). If "group delay" is
the length of time between when the leader of the pack goes in to when
it comes out, that's just plain "delay" in my book.

1. What physical device can be used to apply a 90 degree phase shift
to a complex audio signal? You can ignore anything below 20 Hz and
above 20 kHz.

Let me emphasize that the phase response is a property of a *system*,
NOT a signal. So if the system has a 90 degree response at all
frequencies, then that characteristic is going to be imparted to ALL
signals that are passed through it, complex or otherwise.

In other words, you needn't consider a "complex" audio signal to
validate the system - a simple sine wave sweep will do.

A "simple sine wave sweep" isn't a simple waveform. A series of sine
waves that remain stable long enough to ignore the start/stop effects
would suffice. If you could put in 1 kHz and get it out .25 msec later
(ignoring the fact that you can't tell one cycle from another and
might have 90 + multiples of 360 degree phase sheift), then put in
a 2 kHz sine wave and get it out .125 msec later, that would be a
start.

2. How you know you've accomplished it? (how would you measure it?)

One easy way would be to connect a sine wave signal generator as an
input into the system. Connect the output of the system into a scope
and Y the output of the signal generator into the X input of the
scope. Place the scope into X-Y mode (ala Lissajous patterns). Then
at any one frequency, you should see a circle displayed on the scope
because the output and input are 90 degrees apart. The geometry should
remain a circle as you sweep the sine wave generator across the
frequency band.

That'll do it.

3. What change would you expect to hear as a result of this phase
shift if the content was, say, human
speech?

Listen for yourself:

http://www.uspsdata.org/OurHouse90.wav

Not knowing what the original sounded like, it's hard to guess the
change, but thanks for the example. It sounds a bit like incoherent
stereo, which the original may well have been considering the era of
the recording. Did you do this with a math program?


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