Hi all,
re measuring passbands:
Jay is quite right: Using a different setup (microphone, speaker) doesn't
allow a valid comparison of results obtained by different people. But the
differences are not as great as you might think.
Bill is also right: recording directly from the receiver (bypassing speaker.
mike) DOES allow a valid comparison. I have done this too, using the
phone-jack.
I started recording from the speaker, using a mike, because
1. It was very easy to do,
2. I usually listen to CW through the speaker anyway - unless conditions are
very bad, when I put the phones on. I found that most people in ZL ALSO
listen via the speaker. They, and I, wanted to see the passband that "my
ears were hearing", and see the effect of the ambient noise in the shack (i
usually have 2 computers going).
I've also recorded through earphones by clamping the phones around the mike.
I've found that
- the speaker/enclosure usually has a pronounced effect on the audio
response,
- 2 different radios of the same model can have markedly different passband
responses,
- the responses I measure can be markedly different from what the
manfacturer's specs say they should be.
However, the only "high end" radios I've measured are an FT1000MP and an
FT767. I own, and have measured, an IC701 and a TS520s, but these are hardly
state of the art. I'd like to see a passband using one of the filter
combinations Ten-Tec owners rave about.
My (relatively cheap) UD-129 dynamic microphone has a quoted variation of
plus/minus 2 dB over the audio range, but I've compared it with a high quality
one and it's much better than that. I am more interested in looking at
passband skirts and filter shapes, over ranges of 30 - 40 dB.
If it is EASY for people to digitize 10 second, 16 bit, 8000 Hz sample rate
responses from the earphone socket say, I would be very interested in
processing responses from a variety of rigs and posting the results. I
produce, and can send PostScript files, but these are big, probably not a good
idea to post).
I'm about to prepare a set of instructions for doing this (later, lunchtime in
ZL has just ended). Let me know if you want them.
Later today I'll try to post a sample response (ascii numbers) from whatever
files I have in my office.
73, Gary ZL1AN.
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