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ZL1ANM > FT817 29.04.07 01:26l 89 Lines 4289 Bytes #999 (0) @ WW
BID : ZL1ANM83
Read: GUEST DL1RX VE7HFY DG4IAK
Subj: Re: Max current from mic 5v pi
Path: DB0FHN<DB0FOR<DB0MRW<DK0WUE<DB0RES<ON0AR<ZL2BAU<ZL1AB
Sent: 070429/0013Z @:ZL1AB.#06.NZL.OC #:59540 [AUCKLAND] FBB7.00i $:ZL1ANM83
From: ZL1ANM@ZL1AB.#06.NZL.OC
To : FT817@WW
>> Also, you're aware that modern rigs generate AM in a low level
>> stage and linearly amplify it, the result being audio that lacks
>> the punch produced by high-level modulation?
>
>Neil, I think I might be being stupid here (so no suprises there) but can
>you explain why low-level and high-level modulation would sound different?
>
>73 de Andy GM7HUD
When the first true multi-mode rigs (with AM) started coming out in the
early 90's, there was discussion in QST about the ineffectiveness of the
AM mode's audio in the new rigs. Debate focused on the shortcomings of
low-level modulation, which was a cost-effective way to provide the mode.
It's possible that in the intervening 15 years, improving techniques
have made the AM generated by today's rigs more effective.
The traditional method of generating AM, dating from the 1920's, was to
couple the secondary of an audio transformer into the anode supply of
the final RF stage. This method is called high-level modulation, and
it served hams for 50 years, until SSB superseded AM in the 1970's.
Consider a final amplifier stage of that type, operating at 7,000 kHz,
whose DC input to the anode is 80 watts. If it is being modulated 100
percent by a 1kHz tone, the modulator is supplying 40 watts of audio at
the transformer secondary. Assuming 70 percent efficiency in the final
stage, the output consists of:
1. a 56 watt carrier at 7,000 kHz
2. an apparent carrier of 14.0 watts at 7,001 kHz
3. an apparent carrier of 14.0 watts at 6,999 kHz
The bandwidth is twice the modulating tone, 2.0 kHz. You can see that
fully two-thirds of the output power consists of a carrier that conveys
no information. This traditional high-level modulation technique
requires the generation of a very large amount of audio.
Now let's consider generation of the same AM signal in a low-level
modulation stage, followed by linear amplification. We can generate
the carrier and modulate it with the tone, then amplify it in linear
stages to the 56 watt level so long as the amplitude of the carrier and
the amplitudes of the two apparent carriers do not change with respect
to each other. This is relatively easy to do when the modulating
frequency is a single tone of unchanging level.
Problems arise when we attempt to transmit the human voice. The latter
is much more complex than a single tone. For a start, it has a wide
bandwidth, even when restricted to 300-3,000 kHz. Voice peaks may be
one hundred times the power (+20 dB) of the voice's minimum level.
The total bandwidth transmitted is twice the highest modulating
frequency, 6.0 kHz in the case of audio that is capped at 3.0 kHz.
To linearly amplify such a complex signal, in multiple stages, in the
presence of a carrier that is much stronger, is quite an achievement.
This situation creates the potential for intermodulation problems in
any of the stages.
Speech compression, to restrict the voice's dynamic range, can help,
but it distorts the voice and makes it less copyable under marginal
conditions. AM suffers badly from marginal conditions (noise) on HF,
so heavy compression is not a real solution.
Now, AM is a secondary mode in modern rigs. The Yaesu FT-817 manual
actually states that it is provided primarily as an emergency mode.
When you consider that the linear amplifying stages of a transceiver
are selected and set up to favour SSB and other modes, that AM is a
secondary mode, that intermodulation and linearity are big concerns,
it's not hard to see that the engineers probably restricted the AM
modulation depth in those earlier multimode radios to much less than
100 per cent, leading to complaints that the audio lacked punch.
Another question involves the receiving transceiver. Today's rigs
appear to have 6 kHz wide IF bandwidth for AM reception, but I'm
not sure whether those multimode radios of the 90's did. If they
simply used the normal SSB 3.0 kHz filter, then AM was at an even
worse disadvantage, since reception of one sideband was entirely
eliminated.
73 de Neil ZL1ANM
who in the 1960's used to 100% modulate an ARC-5 on 80m at the
100w DC input level with a homebrew modulator (pp 1625's).
T4 1.5à24
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