Floyd Davidson wrote:
opcom wrote:
Not studio quality.
Studio quality AM is +/- 5KHz.
"Studio quality AM" is not a valid term. AM Broadcast has never
been up to "Studio Quality" standards, which would be _at_
_least_ 20 to 20K Hz.
AM Broadcast Band quality is 5K Hz.
It's semantics at this point, the AM studio is perfect if it is limited to about 5KHz, something which is quite a different case than an FM studio or a recording studio.
+/- 3000-3500 Hz is communications quality, and fine for
AM. but we don't need more rules.
Why so wide?
I answer below..
SSB? 2.5KHz is fine. I am speaking up for the old iron. I do
limit my AM to +/-3KHz in the speech amp.
There is no difference at all between the audio response
necessary for SSB and AM. AM, because it has both sidebands,
will necessarily take up twice the RF spectrum for the same
audio response, but in fact 2.4KHz (400Hz to 2800Hz) is actually
*preferable* to higher fidelity audio response when the purpose
is voice communications. (Ma Bell did a bazillion studies on
this decades ago, so it is not exactly new information.)
Can you find this data for me? I am very interested, it might shed more light on the subject. I understand it may be hard to find today.
Since SSB has no carrier, you can shave the bandwidth a little closer by getting rid of the lower frequencies. With AM, you may as well use the lower frequencies if you want, since you have a carrier at 0 Hz so to speak. 2.4KHz
bandwidth on AM voice sounds a little muffled to me, and to me, 3.0KHz or (very) slightly more, is much more readable, especially when there is alot of noise. Everyone hears differently in some respect, maybe I have hearing
damage. In amy case I am not advocating more than 'necessary', but I do have trouble with speech signals that are cut off too sharply. I always tune my SSB receiver so that the voice is higher pitched than natural, then it is
easier for me to hear. It doesn't work with AM that way, the tuning.
just as an example,
2.4KHz ssb = 300 to 2700 Hz audio.
+/-2.4KHz AM = 0 to 2400 Hz audio.
I can't remember what volume I read it in so take it with a grain of salt, or a whole bag of salt, but I recall that for good intelligibility and having a signal that does not tire the operators, there was some mathematical
relationship between the highest and lowest audio frequencies to be reproduced. Anyone remember that? I regret I cannot quote the source. Maybe there is a some experimentation to be done. I would like to see a reprint of the
Bell studies.
But anyway, here are my sources to support the freedon to employ at least some leeway for bandwidth in the matter of communications quality AM. It's definitely your right to interpret them how you wish:
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"Understandable speech requires the reproduction of all
frequencies from about 250 to 2700 cycles, or sideband
frequencies ranging from 250 to 2700 cycles above and
below the carrier frequency."
FROM: "RADIO ENGINEERING", second edition, 1937, chapter 9,
section 72, page 396, "Waves with Amplitude Modulation",
Frederick Emmons Terman, Sc.D., Professor of Electrical
Engineering, Stanford University.
MY Opinion: Mr. Terman's text says 'requires',
therefore this is taken as the minimum requirement for
speech to be 'understandable'. This does not necessarily
imply good communications quality, but rather
'understandability'.
================================================== ==========
"Modulation frequencies Corresponding to Typical Signals
(minimum frequency range that must be met)"
"Long-distance telephone quality.......250-3500 c/s."
FROM: "RADIO ENGINEERING", third edition, 1947, chapter 9,
section 9-1, page 469, table 9-1 --Modulation frequencies
Corresponding to Typical Signals (minimum frequency range
that must be met)., Frederick Emmons Terman, Sc.D., Professor
of Electrical Engineering and Dean of theSchool of Engineering,
Stanford University. Past president, Institute of Radio Engineers.
My Opinion: Please consider the audio quality of long
distance telephone service in 1947.
================================================== ==========
"...For ordinary SSB telephony, M=3000 Hz. .."
"...For high quality SSB Telephony, M=4000Hz. ..."
"...For ordinary DSB telephony, M=6000 Hz. ..."
"...For high quality DSB Telephony, M=8000Hz. ..."
FROM: "THE RADIO MANUAL", fourth edition, 1950, appendix 5,
page 859, "Table of necessary bandwidths", George E. Sterling,
Commissioner, Federal Communications Commission, and
Robert B. Monroe, Radio Engineer, Columbia Broadcasting
System, D. Van Nostrand Company, Inc. 4th edition, 1950.
================================================== ===========
"Frequencies up to at least 2,500 cycles, and preferrably 3500
cycles, are necessary for good speech intelligibility."
FROM: "RADIO HANDBOOK", fourteenth edition, 1956, chapter 12,
section 12-1, page 225, Editors and Engineers, Ltd., edited by
William I. Orr, W6SAI.
================================================== ===========
"...Mediocre reproduction may be restricted to 100-5000 c/s.,
while many radio receivers are limited to 100-3500c/s. It
should be remembered that the frequency range is taken as
overall, including the loss of sidebands and including the
loudspeaker. Wide frequency range is only comfortable to
the listener so long as other forms of distortion are
negligible."
FROM: "THE RADIOTRON DESIGNER'S HANDBOOK, THIRD EDITION", 1941,
chapter 5, page 32, "frequency distortion", THE RADIOTRON
DESIGNER'S HANDBOOK, THIRD EDITION", F. Langford Smith,
S.SC., Member I.R.E, M. I.R.E., A.M.I.E.E., A.M.I.E
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