[Icom] Audio response, articulation etc.

Mon, 06 May 2002 00:16:35 -0700

Andy,

Precisely. This one of the reasons why ITU-R Recommendation M.1173 (3)
(technical requirements for transmitters in the maritime HF radio services)
explicitly states:
"The transmitter audio-frequency band shall be 350 Hz to 2 700 Hz with a
permitted amplitude variation of 6 dB." The 350 Hz lower limit is set for
two reasons; the degradation of articulation in the presence of strong
low-frequency spectral components, and the need to conserve transmitter
output by confining it to the area of the voice spectrum where it will
contribute most to articulation at the receiver.

Similar criteria apply to the codec/filter chips used in line telephony
applications, for the same reason. The nominal frequency response of the
filters is typically 300 ~ 3000 Hz at the -3 dB points (nominal). The
handset transducers (mic and receiver inserts) are matched to the filters.
In all cases, the highest audio frequency will always be lower than one-half
the sampling frequency of the codec. The same rule applies to the
analogue/digital and digital/analogue converters used in a DSP radio.

A correctly-designed receiver audio amplifier should already be 6 dB down at
about 3 kHz, to ensure that any white noise generated in the audio amplifier
or the preceding circuits will not degrade the output S/N ratio. (Remember
the infamous "Icom audio amp hiss"?)

The "1950's AM Radio Mellow Sound" has no place in radiotelephone
communications, if one recognises that the purpose of radiotelephony is to
transmit human speech by radio, with maximum intelligibility,  under the
widest possible range of propagation conditions.  Its days in broadcasting
may also be numbered, once DRM (digital re-farming of AM broadcast bands)
becomes established. http://www.drm.org/indexdeuz.htm

Best 73,
Adam, VA7OJ/AB4OJ
North Vancouver, BC, Canada
http://www.qsl.net/ab4oj/
Note new e-mail address:
mailto:[email protected]

-----Original Message-----
From: [email protected] [mailto:[email protected]]On
Behalf Of Andy Wallace
Sent: Sunday, May 05, 2002 18:11
To: [email protected]
Subject: Re: [Icom] RE TS940 and IC751A

Adam-

Doesn't an attempt to transmit low frequencies--which don't add to voice
intelligibility--also consume a disproportionate amount of your transmit
power? Bob Heil has very strong feelings on this point I think. The "full,
mellow" AM type sound may be about the least efficient way to communicate
long distances with a minimum amount of power? It would also seem that a
"hi-fi" speaker sensitive at the high end would simply add to hiss and
static, not voice intelligibility. I have a high end sound system for music
but would never consider hooking it to an HF radio.

Andy K5VM