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G0FTD > TECH 28.04.07 14:23l 78 Lines 3036 Bytes #999 (0) @ WW
BID : 000327117PZT
Read: GUEST OE7FMI
Subj: Old versus nu-age AM.
Path: DB0FHN<DB0FOR<DB0SIF<HB9EAS<DB0FSG<OE7XLR<OE2XUM<OE5XBL<OE6XPE<IW2OHX<
I0TVL<DK0WUE<7M3TJZ<ZL2BAU<GB7MAX<GB7PZT
Sent: 070428/1106Z @:GB7PZT.#24.GBR.EU [Kidderminster] #:32700 XSERV410h
Date: Sat, 28 Apr 2007 12:06:33 +0000
From: g0ftd@gb7pzt.#24.gbr.eu (Andy)
To: tech@ww
Subject: Old versus nu-age AM.
GM7HUD wrote:-
> I'm not actually disagreeing with anyone for a change! I'll believe that
> high-level sounds different to low-level but I want to know why. 100%AM is
> 100%AM so any difference between low and high level modulation has to be
> something do with distortion products as my namesake mentions below:
Hmm, lets have a look at typical (usually valved) high level modulated
amateur rigs of old.
- Wafty xtal and moving coil mics with uneven freq response. I've measured
them.
- Lack of care taken with impedance matching is often found affecting freq
response.
Impedance matching from mic to modulator amp introduce frequency
response variations.
And then modulator drive impedance to high level power supply impedance
where the high level mod is fed through to the power amplifier.
- Amplifier freq responses AND non linearity with drive vs output levels.
- Loadsa wide freq response was order of the day. No brickwall filtering
like today for bandwidth efficiency. More bandwidth allowed more intermod
products to form.
- Transmitter PA loading. Remember high level mod (feeding audio into PA)
is being mixed with the RF. Output loading (antenna loading) affects
the modulation quality too. With the olde worlde valved stuff the PA loading
could be varied directly with the direct to tank circuit combo.
Now consider how all these variations, tolerances and mixes add up. It's what
gives communications audio it's colour/punch. It's also what unfortunatly
creates splatter and spitch :-(
These days with signal purity being paramount our modern rigs on AM are
pure, and so *appear* to lack punch.And so we deliberatly want to introduce
controlled intermod products to create sound "reinforcement" ;-)
In the broadcast world that's where the ever popular Optimod boxes get used,
with special AM and FM versions to create the right effect.
If I take my IC706 or FT817 and feed music into the mic socket via an
isolating transformer I get broadcast quality audio. But for communications
purposes it will appear to lack punch.
For communications punch that's why we use compression to sound louder and
increase average power output. A simple pre-amp with clipping diodes does
the trick.Fidelity suffers but the signal gets through better,which is great
for dx.
Strange how we've come full circle these days - wanting to create old
world distortions. It's why bands like the The Strokes (or was it the White
Stripes ?) deliberatly recorded an album using original 1960's recording
equipment. They wanted it's original foibles to deliver a certain sound
(distortions). Or vinyl effect software processors on our PC's.
Yep - full circle..
> I have to say that I've never seen the attraction to AM on amateur
> frequencies.
Because we can ?
Or that top band is full of it.
(We're avoiding 80m which is full of Russian's using FM !)
- Andy (Q.C.) -
G0FTD @ GB7PZT
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