[HBQRP] Testing the HPM audio filter, take 3

[Sean Pepin]

Edit again: I guess it doesn't like bulleted lists.

There's a free program out there that can be used to test audio filters 
and the like:

http://www.hometheatershack.com/roomeq/

Most of the features here are far in excess of our needs, and 
configuring REW properly to get what you want isn't for the faint of 
heart. I'd have simply brought it on my laptop to the April 7 meeting 
ready to go, but alas it has perished.

My hope is that there's someone willing to plough through the setup and 
bring theirs so that others can test their stuff. I'll be there to help 
on the 7th with configuration troubles as well.

I'll outline the basic procedure here, but I can't really go into too 
much detail given that much of the setup is OS specific.

The goal is to use the 'Generator' module to output 'white' pseudorandom 
noise (PN) at a measured amplitude to give us a flat reference line, 
then use the 'RTA' (realtime analyser) module to view the change 
imparted by the filter once it's inserted into the loop.

* Set up sound card input and output under 'Preferences' in the main window.

* Connect the input and output you've configured directly together with 
a patch cord or by direct wiring (Mic to headphone, for example)

* Open the 'Levels' window so you have something to go by while you 
fiddle with settings

* Open the 'Generator' window and set it for 'White PN', -10dB FS, and 
hit the play button

* You should see the Levels change, if not it's time to fiddle with your 
soundcard settings and preferences

* Fiddle with your OS's microphone or line out levels so that the Levels 
all come in roughly the same. This usually involves adjusting 
microphone/line in and output volume settings so    that your output 
volume isn't blasting the snot out of your sensitive microphone input.

* Once all the levels look good, stop the 'Generator' module by pressing 
the play button again and open the 'RTA' module

* Mouse over the upper left corner of the graph and change it from 'db' 
to 'db FS' (decibels full scale)

* On the upper right, click on the box with the arrows going each way 
that says 'Set limits' when you mouse over it and change the numbers to 
the following and hit 'apply':
     Top: 10
     Bottom: -130 (the minus sign is important)
     Left: 20
     Right: 20,000

* Now hit the red circular record button in the upper right. You should 
see an FFT plot of the background noise in your microphone connection. A 
normal graph will have all sorts of odd harmonics and moving mess below 
about -70dB, especially at lower frequencies, and this is ok.

* Now hit play again on the generator. That mess of low level noise on 
the RTA should resolve to a flat line at some intermediate value on the 
graph, the exact value of which depends on your settings. This is our 
baseline against which we well test.

* Now for the fun part. Unplug the patch connector and wire your HPM in 
by whatever method you choose. You can take either channel coming from 
the computer to put through the HPM, but be sure to wire the HMP to the 
right channel on the cable going back into the computer. If you mix this 
up you'll get nothing but noise or a little bit of generator coupling in 
the RTA. No worries, you can change which channel is used as input in 
the REW main window under 'Preferences' on the 'Soundcard' page.

* If the line on the RTA display is sloping down at high frequencies, it 
worked! Now you can mess with the 'Set limits' section of the RTA to get 
a better view, use the cursor on the screen to get -3dB measurements, 
and generally just mess around and enjoy.

I should have time soon to build up my prototype, and when I do I'll 
post links to some screen captures of what I'm getting on my RTA display.