[AMRadio] AM Broadcast Quality


W2XJ w2xj at w2xj.net
Wed Dec 18 15:09:42 EST 2013


Taking things back to the original subject, there is obviously a big difference between broadcast quality and communications quality. Most accept 300-3KHz as being communications quality. Many amateur rigs were designed around that concept. This includes the size of the power supply and in older real AM rigs the modulation iron. The internal processing in most rigs is also communications grade. Rigs like that can only go so far quality wise. Properly adjusted and operated they are clean and intelligible, though. 

Broadcast Quality is a subjective term and means different things to different people. To me it requires a transmitter actually designed for AM. That would be a modified broadcast transmitter or some kind of home brew such as many of the Class E rigs being built. The transmitter itself should be as wide as possible and be preceded by processing that establishes the transmitted bandwidth. Commercial broadcast processors can be configured for bandwidths as low as 4.2 kilohertz. Studies show that a bandwidth up to 7 kilohertz improves the intelligibility of speech but in the context of this discussion that would go hand in hand with a good signal to noise ratio and low interference. There is also the problem of finding receivers with a wide enough bandwidth to complete the system. An R390 will do it as will some SDR packages but not a lot of other hardware is out there. I think when band conditions are poor or too conjested that PSK31, CW or SSB are more appropriate modes for those times.

Some may find this report interesting:

http://docs.polycom.com/global/documents/whitepapers/effect_of_bandwidth_on_speech_intelligibility_2.pdf

Sent from my iPad

On Dec 18, 2013, at 1:41 PM, "Donald Chester" <k4kyv at charter.net> wrote:

>>> From: "W. Harris" <nbcblue at hotmail.com>
> The specs on my Ranger audio chain and other Johnson transmitters is flat
> response from 250-3000 cycles. For the Collins 32V-3 it is 200-3000 cycles,
> which is all very adaquite (sic)  for good AM good voice  communications. I
> get very good audio reports on the Ranger. Any more response just ads to the
> bandwidth. For AM broadcast stations, 40-7,500 cycles is typical. We are not
> broadcast stations.>>
> 
> Bill,
> 
> The selectivity of your Drake 2-B  is much narrower than the audio specs of
> your Ranger or 32V, even when it is set to receive in the AM mode. I have
> listened to AM on a 2-B in the past and recall having to tune to one side of
> the  carrier to make the AM signal fully intelligible, because the
> bandwidth was so pinched, even in the nominal "AM" position, that it was
> impossible to  receive both sidebands at once, rendering the full frequency
> range of a double-sideband AM signal is impossible to hear in any case.
> Could this have something to do with your lack of interest in anything
> beyond the frequency range of a typical SSB signal?
> 
>>> 97.307 Emission standards
> (a) No amateur station transmission shall occupy more bandwidth than
> necessary for the information rate and emission type being transmitted, in
> accordance with good amateur practice.
> 
> 10 or 20 kHz bandwidth is not necessary for good AM communications on the
> ham bands. We are communicating, not broadcasting.  >>
> 
> It depends on your definition of "communication". As I said before, simply
> getting the basic words of a message across can be accomplished perfectly
> well, if not better, with CW. "Communication" is multi-faceted; there is
> more to it than merely getting a string of words or data across to a second
> party. Recognition of who is  doing the talking by the sound of their voice
> with all its subtleties is part of what is being communicated, as well.
> Those of us who work phone instead of CW opt to use the additional bandwidth
> because we prefer the pleasure and convenience of communicating by voice.
> Those of us who work AM instead of SSB opt to use the additional bandwidth
> because we prefer the pleasure of  communicating using the characteristics
> of the AM mode. Those of us who use wider frequency  response instead of
> space-shuttle audio, opt to use the additional bandwidth because we prefer
> the pleasure of communicating  with a natural sounding human voice to that
> of a tin-can telephone. Who is to dictate by what arbitrary standard, that
> your preferred mode of operation is OK but mine uses "un-necessary"
> bandwidth? 
> 
> Besides, this is a HOBBY, not a commercial communications enterprise. By
> definition, the vast majority of amateur radio communication is trivial, and
> tolerated only because resorting to public telecommunications services would
> not, due to its unimportance, be justified. The exception is emergency
> traffic, now termed "EMCOMM". Strictly  speaking, ALL amateur radio traffic
> with the exception of emcomm, by definition, occupies "more bandwidth than
> necessary".
> 
> Good amateur practice merely tells us to use the appropriate bandwidth to
> match band  conditions and congestion in order to keep harmful interference
> to a minimum. Dropping a strong, 20 kHz wide AM signal on  to 3875 during
> prime evening hours, rendering 3860-3890 unusable to other AM and SSB
> traffic, would hardly be considered good operating practice. But if this
> same act is done in the wee hours or during the middle of the day when
> no-one else is operating in this entire swathe of frequencies, no harm is
> done, and there is no reason to say this isn't good operating practice,
> assuming the 20 kHz bandwidth is due to the frequency  range of the audio,
> not overmodulation, flat-topping on positive peaks or spurious distortion
> products. 
> 
> The bandwidth "saved" by cutting off the low frequencies at 250 or 300 Hz is
> negligible and serves no useful purpose, even when the total bandwidth of
> the signal is limited to less than 3 kHz. I doubt if anyone would notice the
> difference between the interference caused by a 2.7 kHz and that of a 3.0
> kHz SSB signal. With AM, due to carrier and sideband spacing, cutting the
> lower audio frequencies does  nothing at all to reduce the total bandwidth.
> We don't want to boost the low frequencies to the point that they overpower
> the rest of the frequency range and make the signal difficult to read. But
> that doesn't mean  we have to cut them off, either.
> 
> Furthermore, what does "broadcasting", which by definition means
> transmitting the same information to many listeners, versus point-to-point
> communication, which by definition means transmitting the information to
> only one listener, have to do with audio quality or bandwidth?
> 
> 
> Don k4kyv
> 
> 
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