The rated output/distortion topic is, by most manufactures, a means or a
method of expressing the performance of the receiver audio system. There's
always three required ingredients being (a) rated power output expressed in
watts, (b) THD or THD+n usually measured at rated output power and expressed
as a percentage, and (c) load impedance expressed in ohms. And of course
one could add frequency response, but for ham receivers it's really not that
important. I don't ever recall anyone publishing the audio spec using the
value specified at the level where any degree of signal clipping takes
place. Oooh, that would be nasty.
Home audio systems typically use much higher voltage output devices as the
same for automotive systems that incorporate switching inverters to get the
12 VDC to values ranging from 39 to 75 volts. Thus with the higher voltage
systems, more voltage swing with higher current values can be developed
across a given load and hence more power output watts. This is, as I see
it, one reason we find higher distortion and typically 2 to 3 watts of audio
output on receivers as they must operate at a nominal 12VDC and in certain
cases with limited values of current. This also invokes the reason, to some
extent, we find the transmitter IMD numbers at less than desired values,
although they meet the current FCC required specifications for spectral
purity.
Regardless if we like it or not, we ARE dealing largely with voice
communication systems or systems that are intended to produce audio in the
most sensitive area of human hearing. The old Bell Telephone system numbers
of 300 Hz to 3000 Hz still prevails. That's a bandwidth of 2700 Hz.
Agreed, any added distortion to the signal does or is likely to produce
hearing fatigue. At this point one must then add the SPL value applied and
the time duration figures to determine the figure of merit with that regard.
If one desires a higher listening level than the ham receiver audio system
produces, being some 2 to 2.5 watts into a 3" to 5" communication speaker,
then as said in an earlier post, use the Line Output fed to your favorite
high power audio amp and speaker system. Still the distortion figures may
not greatly improve as different designs of product detectors along with AGC
characteristics will add certain distortion artifacts to the recovered
audio.
The point to all of this, we rarely operate our radios at audio levels that
approach their distortion limits. Of course the basic objective is not to
have any peak distortion hence the need for the headroom, but in practice,
the average audio level is much less than 1 watt into 8 ohms with an degree
of an efficient speaker system.
73
Bob, K4TAX
----- Original Message -----
From: "Richards" <jruing@ameritech.net>
To: <k9yc@arrl.net>; "Discussion of Ten-Tec Equipment"
<tentec@contesting.com>
Sent: Thursday, May 17, 2012 12:09 AM
Subject: Re: [TenTec] Top receivers
On 5/16/2012 10:42 PM, Jim Brown wrote:
With some receivers it certainly does, especially those that are power
or supply voltage limited. BUT -- I like to think that I know the
difference between my ham rig and my home entertainment system.
I KNOW I know the difference, and kinda resent
the sarcastic implication... ;-)
FWIW, for ham radio at home, headphones have ALWAYS been my primary mode
of operation. I use a loudspeaker only for casual listening.
ME TOO... in fact I am a huge headphone / headset
junkie. I am blessed in that my XYL does not whine
about me spending money on stuff... including another
headset. We both use the best wireless headphones
for listening to old time radios shows - the sound never
alters no matter where we roam throughout the place.
Makes raiding the fridge safe - we never miss any of
the show even though we leave the room.
My preference on headsets is also related to the fact
the microphone is always a close talking type, which
stays put in the same place, no matter how far back
from the desk I lean, or whether I turn my head
from side to side, etc. A close-talking microphone
tends to be more noise rejecting (I don't like the
term noise cancelling... but that is another topic
for another day....)
Every
radio I've ever owned has had quite adequate drive for headphones. On
the other hand, a mobile radio, including a talkie, needs enough audio
to get over ambient noise, and many of these rigs don't.
10% distortion is a standard spec for industrial paging systems.
I suspect this tends to reinforce my point...
I think... ;-)
10% distortion is a standard spec for industrial paging systems.
That's the distortion at the peak of the signal just below clip,
and is typical of many computer sound cards.
Geeze...are you sure about that ?
I have been testing sound cards and microphones for
speech recognition companies since 1992... and no sound
card I ever used was rated at 10% THD.
Typical specifications for cards in my machine might
look like this:
Output THD+N at 1kHz (Front-out) : <0.0025 %(-92 dB)
Input THD+N at 1kHz : <0.0022 %(-93 dB)
Even the venerable old Creative Soundblaster Audio PCI 16
had numbers like this back in the 1990s :
Signal to Noise Ratio: 90 db
Total Harmonic Distortion @ 1 Vrms (10 KOhm load): 0.01%
I cannot address the rest of the post, as I am embarrassed
to admit I don't follow how it relates to the topic of ham
radio audio ... sorry.
------------------------ K8JHR -----------------------
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