Steve-
Good point about freq response dropping off at 15kHz. My soundcard can record up to 192kHz/32bit and the Virtins software accepts sampling rates up to 192/24. I usually input the azimuth signal at 96/24 and it seems to work well. Actually I do have a generator which I purchased last spring that can generate sine, square and saw waves. But I haven't even tried it out because I don't have the test leads with a banana jack on one end! Is that ridiculous, or what? So I use my test CD.
Although only an amateur, I do want to comment that the very FIRST thing anyone needs to make sure of in a calibration procedure, and this is almost NEVER explained clearly by anyone writing their papers on the internet, is that when the VU meters read zero the machine is producing
operating level at the outputs. Your machine will have a specification in volts as to what (in the case of the A810 it's 0.775v) this is. You change this setting by playing back the 1kHz tone on your test tape and then adjust the playback output amps to show
operating level in volts at the tape outputs. After this is done you will then adjust the VU meter amplifiers (NOT the screw on the meter front!) so the meters read zero. Your meters are now displaying the
reference level of your test tape (in nWb/m) as 0db and you are producing
operating level, in volts, at the tape outputs. The syncing of the VU meters to the deck's specified
operating level is normally a one-time-only adjustment. Maybe that's why noone ever mentions it, but when buying a used machine it's certainly the first thing to check! There is a Swedish person who has a website-
http://home.swipnet.se/herbalifeinfo/Audio/reeltoreel.htm. It's the only one I know of that explains how and why to do this. Highly recommended reading for the novice. Reading this page a few years ago was the first time I ever said "Hey! I can do this!" Lots of explanatory pics there, as well. Hint: Just start reading from the 1st page. There is a link at the bottom of every page that takes you to the next one.
Steve, as you noted, calibration of most systems using a test CD can be a pain, since there isn't a way to easily change the line level without changing rec level as well. The A810 has a way to do this. The entire control area under the headblock is actually a hinged panel that folds up and can lock in a 45 or 90 degree position. This is where the cards and
periphery controller reside. So the procedure is simply to set input level from the 1kHz tone on the CD to read 0db on the meters when the machine is stopped (which produces an
operating level of 0.775v at the outputs). Then I put the machine into rec mode and monitor repro output. The VU meters now indicate that the output level changed when the machine started recording, since the rec level hasn't been set yet. It's a simple matter to now adjust rec level for each channel using the periphery controller. After setting the VU meters to again read 0db by using the rec level control of the periphery controller and not moving
the input level control I am now recording
reference level on the tape (in my case 355nWb/m) and producing
operating level (0.775v) at the outputs. I know that's a lot of words, but you can see this calibration can be done in less time than it took to read about it! Steve, I thought it might help some of the really new guys out there to hear about this. Operating level, reference level- it can all get very confusing! Also, for those who are now asking "What the heck is a periphery controller", it's a digital interface to calibration of the A810, which uses pushbuttons and precise values instead of tuning potentiometers, and each parameter is divisible into 256 steps, which are displayed on the tape counter when calibrating. Recording & playback of the machine itself is 100% analog. This system allows for 2 complete sets of values (including eq, where the time constant is programmable for recording and/or playback) to be stored for each speed! In addition, it's possible to change the settings in real-time during recording and/or playback if that is desired, and all settings, including the counter reading are stored in non-volatile memory when the machine is powered down.
George-
I tried low-frequency biasing at 20Hz with a Q457 tape that I have. No results! At least I now know what to expect from a "newer" tape that this doesn't work on! Changing the bias current I could hear a difference in the sound of the noise, but it didn't vary enough in overall character so that I could say "Ah, I hear the point of lowest distortion". The sound didn't seem to change noticeably until I knew I was way away from the optimun bias point. Going to try it again as soon as I get some more of the 226 that I mentioned earlier. I have some on the way- should be here this week! I don't know alot about 226, having only one reel of it so far. I suspect it may also be of the "higher output" variety that Bill V. says this technique won't work on...