Frank Dresser wrote:

"Paul Sherwin" wrote in message
...

[snip]

Modern AM transmitters have a very sharp rolloff above a certain
frequency. Broadcasting above this would just waste transmitter power,
since (almost all) radios wouldn't be able to receive it because of
their IF selectivity characteristics. The 9kHz or 10kHz AM channel
width is just a convention, but once it has been adopted there's no
point in trying to receive a wider bandwidth - you'll just get
interference from adjacent stations.

If the received signal is very strong, the tuner's gain will have to be very
low. This will supress the adjacent channel interference quite well.


In the US and Canada, AM stations are allocated 10kHz bandwidth,
giving a theoretical 5kHz treble cutoff. In most other place that's
9kHz/4.5kHz. Stations transmit a more restricted frequency range than
this though, for a number of technical reasons. That's where my rough
and ready 3.5kHz figure came from.

RDH4 says most AM BCB radio makers tried for a final IF bandwidth response of
3.5 kHz
That was in 1955/
Since then, the BW has shrunk in many sets to even less than 2 kHz, especially
in solid
state gear, giving horrid state AM listening.
No good turning up the treble control knob, there is no treble there to boost.


Best regards, Paul
--
Paul Sherwin Consulting http://paulsherwin.co.uk

The FCC requires US AM radio stations to have an audio bandwidth between 4
and 10 kHz or a total bandwidth from 8 to 20 kHz. Typical radios with IF
transformers, rather than crystal or ceramic IF filters, don't have very
sharp skirt selectivity. Few radios will be able to block out a strong
adjecent channel 10 kHz off channel. Many can't block out a strong adjacent
20 kHz away. Some can't even block out a strong adjacent channel 30 kHz
away.

Oz local stations are rarely closer than 45 kHz, which is 5 x 9 kHz spaces.

In Canberra, we used to have 2XX community station of 300 watts on 1,008 kHz,
with 2CA of 5 Kw at 1,053 kHz, and it was a good test of any AM radio if
2CA couldn't be heard when tuned to 2XX.
My own radio allows me to pick up a weak signal at 27 kHz away from 2CA
without 2CA being heard.

Most simple transistor based tuners fail this test.
They have high Q single tuned IF coils.

7AD on 1008 kHz sometimes drifted in late at night all the way from Tasmania,
if conditions were freaky. Antenna type and location/direction minimised this
effect.



The FCC limits interference only partly by bandwidth restrictions. Mostly,
it uses geographic seperation and power restrictions.

By ear, I think most stations go to about 7 or 8 kHz audio. Many of the AM
stations are talkers, but the ads can really sparkle. There's one I hear
which sounds like it goes to the 10 kHz audio max.

Much AM is talkback from mobile telephones, and its pretty dreadful....



Patrick Turner.

"Patrick Turner" wrote in message
...




But how does one know how to apply an expander to exactly match
the inverse of the compressor characteristic?
I doubt two wrongs will make a right.

I had a book which described a very simple expander which was just a light
bulb in parallel with the speaker, if I recall. Loud passages would heat
the filament (probably not to incandesence) reduce the load of the bulb and
increase the volume even more. Quiet passages would let the bulb cool, load
the circuit and reduce the volume. It sounds goofy to me, and it's a
circuit which wasn't popular.

There were probably more sophicated expander circuits back then.

A modern sophicated decompressor circuit could match the curve of the
compressor, just as the dolby system does.




Anyhow, in Oz there isn't to much evidence of compression or emphasis of
audio HF on the stations worth listening to; I find the better the
receiver,
the more like FM reception the AM signal becomes.

Patrick Turner.


Here's some of what's been happening in radio audio processing over the
years in the US:

http://www.bext.com/histproc.htm

Frank Dresser

In article ,
Jon Noring wrote:
Well, being the "OP", I want a high-audio performance, modern design
AM tuner to integrate into my audio system -- and I believe a lot of
tube-o-philes likewise want that -- but not everyone obviously. There
are several reasons why most higher-grade audio systems use separate
components, the reasons of which are obvious to most everyone. The AM
tuner is no different than other audio components in this regard.

Maybe what you want is the old JW Miller passive AM tuner. No
active devices at all, just a bunch of tuned circuits and a detector
diode.

Mark Zenier Washington State resident

In article ,
"Frank Dresser" wrote:

"Patrick Turner" wrote in message
...




But how does one know how to apply an expander to exactly match
the inverse of the compressor characteristic?
I doubt two wrongs will make a right.

I had a book which described a very simple expander which was just a light
bulb in parallel with the speaker, if I recall. Loud passages would heat
the filament (probably not to incandesence) reduce the load of the bulb and
increase the volume even more. Quiet passages would let the bulb cool, load
the circuit and reduce the volume. It sounds goofy to me, and it's a
circuit which wasn't popular.

There were probably more sophicated expander circuits back then.

A modern sophicated decompressor circuit could match the curve of the
compressor, just as the dolby system does.


I am surprised at the size of this thread and how it has taken off, but
many of the comments seem to be either misleading as a result of wrong
facts or limited understanding of the technology involved. I have several
comments that I will lump together.

1. It has been variously stated that the audio bandwidth of AM
broadcasting is either 3.5 kHz, 5 kHz, or 10 kHz. In the US AM broadcast
channels are 20 kHz wide, so audio is effectively limited to a maximum of
10 kHz by law/regulation. It is my impression that most AM stations
transmit audio out to this legal maximum. Of course as HD-radio takes
hold this will change with the analog signal cutting off somewhere around
5 kHz. I know there are at least 2 active broadcast engineers that read
this group, perhaps they could fill us in on what the stations they are
involved with are actually doing as far as audio bandwidth goes?

2. The idea expressed above that a "modern sophicated decompressor
circuit could match the curve of the compressor" seems far fetched to me.
In the days of yore when audio processing consisted of a single broad band
compressor, and a broad band "peak limiter" one might have contemplated
this, at least as far as the compression part went, but today's audio
processing is much more complex. Processing today involves broad band
AGC, multiband compressors, plus multiband and broadband clippers in place
of the old "peak limiter". It isn't clear to me that this would be easy
to undo, or even possible. I don't know if the multiband aspect creates
problems for reversing the process or not, but how do you undo clipping,
and if there are any feed forward compressors involved it is possible that
the output isn't even a single valued function of the input, making
recovery mathematically impossible.

3. TRF receivers have been mentioned, and everyone seems to assume that a
TRF receiver would consist of cascaded single tuned resonators with RF
amplifier stages between. There is no reason why double tuned circuits,
similar to those used in the IF transformers of a superhetrodyne can't be
used in a TRF receiver, with all the selectivity/bandwidth benefits that
brings to the party. For examples see the Western Electric No. 10A
receiver, the J.W. Miller TRF receiver, the early Altec AM receiver, as
well as others.

4. It has been stated that constructing the various RF and IF coils,
especially IF transformers with variable bandwidth, that are required, is
one reason why people aren't doing this type of project. I would suggest
that a variable bandwidth double tuned IF filter can be built using
standard two terminal inductors, by using low side capacitive coupling. I
have a British Acoustical AM tuner that uses this approach in place of the
first IF transformer to provide variable bandwidth. Rather than using an
IF transformer with a tertiary winding to provide variable bandwidth, two
separate coils are used which are coupled by low side capacitive coupling,
where the amount of coupling can be switched to change the bandwidth just
the same as with the tertiary approach.

5. The thinking here seems to be limited to single tuned circuits for TRF
receivers, and double tuned IF transformers for superhetrodyne receivers.
There is no reason why one can't build more complex filters that will
provide better performance than an equivalent number of poles in ordinary
double tuned IFTs. Quad tuned filters are relatively easy to do, and it
is possible to go to even more poles in a single filter module, providing
an improved selectivity vs. bandwidth trade off.

6. It has been suggested that adding resistors across an ordinary IF
transformer will widen the audio bandwidth. This is not always true as
the Heath company illustrated in the manual for their BC-1A High Fidelity
AM tuner. They suggested adding a resistor to the first IFT to narrow the
bandwidth if interference from adjacent stations was encountered, and IIRC
they provide audio response graphs with and without the added resistor
showing how the resistor narrows the bandwidth. Actually I think that in
this case it is only the nose bandwidth that gets narrower, the bandwidth
further out beyond the audio range does increase as you would expect. I
think this effect is probably due to the fact that the first IFT in the
Heath, and other quality tuners, is overcoupled, and adding the resistor
eliminates the overcoupling effect narrowing the nose bandwidth. It pays
to be careful and make sure you know the theory and what you are doing, as
things don't always work as you might expect.

7. It has been suggested that using a 2 MHz IF frequency would allow
wider bandwidth than the standard 455 kHz IF frequency. I fail to see why
this should be true. Within reason, for bandwidths typical of audio
receivers, you should be able to build a filter at 455 kHz that has
effectively the same response as a 2 MHz filter. There is no need to
throw out the 455 kHz IF just to get wide bandwidth.


Regards,

John Byrns


Surf my web pages at, http://users.rcn.com/jbyrns/

Jon Noring schrieb:


In the last couple of years I've posted various inquiries to this and
related newsgroups regarding high-performance, tube-based AM (MW/BCB)
tuners, both "classic" and modern.

Have a look into the "Collins" S-series. These are state-of-the-art
tube sets 'til now. At least it's not the tubes alone but the fabulous
mechanical IF-filters giving outstanding results for a tube set.
Manuals with layout diagrams should be available on the web....

John Byrns wrote:

3. TRF receivers have been mentioned, and everyone seems to assume
that a TRF receiver would consist of cascaded single tuned
resonators with RF amplifier stages between. There is no reason why
double tuned circuits, similar to those used in the IF transformers
of a superhetrodyne can't be used in a TRF receiver, with all the
selectivity/bandwidth benefits that brings to the party. For
examples see the Western Electric No. 10A receiver, the J.W. Miller
TRF receiver, the early Altec AM receiver, as well as others.

I did a cursory check on the Internet, but did not yet find any
schematics for the mentioned receivers. Are they online somewhere?
Anyone?

I also found the following article from John posted back in 2000,
where he talks about the double tuned TRFs, such as WE-10A, J.W.
Miller, Collins (which I assume is the same one Volker Tonn
mentioned today), Meissner, and the Weeden (the last of which John
noted to be the best designed of all of them):

http://groups.google.com/groups?selm...&output=gplain

Unfortunately the URLs to the TRF schematics at John's site are not
working.

John, how exactly do these double tuned circuits work in TRF circuits
compared to using single tuned resonators, as depicted on slide 7 of:

http://www.technology.niagarac.on.ca...531unit6rx.ppt

John, I also recall you mentioning a while back about "modernizing"
one of these TRF receivers. What is the current state of your research
on these circuit designs? Have you advanced to the point that a
detailed schematic is right around the corner?

Thanks for posting your thoughts.

Jon Noring

Patrick Turner wrote:

I won't budge from the idea that its possible to digitise the signal
from the antenna and simply apply suitable algorithms, and get
digital decoding, without all the phase shift caused by consecutive
tuned circuits.

And I agree with Patrick. Despite my desire to have a nice, kit-made,
high-performance AM tube tuner, ultimately I think the best radio
tuner for sound quality and overall performance (whether AM, ASM, FM,
digital broadcast, etc.) is the pure digital system as described by
Patrick.

But do the necessary low-level A-D converters already exist? Is anyone
actually building radios on this principle, or are we still a few
years off?

Jon Noring

[p.s., pure Class D digital amps are continuing to improve, with
better switching and so on, so ultimately the only analog streams
we'll be dealing with will be radio signals captured by the antenna
(which will promptly be digitized), and the output to the speakers
from the last-stage PWM of the digital amplifier. Everything inbetween
will totally be digital, using advanced and inexpensive DSP to do
things not possible in the analog processing realm. The only realm
left for the audiophiles to play in will be speakers.)

Jon Noring wrote:

Patrick Turner wrote:

I won't budge from the idea that its possible to digitise the signal
from the antenna and simply apply suitable algorithms, and get
digital decoding, without all the phase shift caused by consecutive
tuned circuits.

And I agree with Patrick. Despite my desire to have a nice, kit-made,
high-performance AM tube tuner, ultimately I think the best radio
tuner for sound quality and overall performance (whether AM, ASM, FM,
digital broadcast, etc.) is the pure digital system as described by
Patrick.

But do the necessary low-level A-D converters already exist? Is anyone
actually building radios on this principle, or are we still a few
years off?

Jon Noring

There are virtual radios which can be installed in a PC.
Been around for years.
They involve a suitable antenna interface and sound card, and program on a
disc,
and were advertised for sale on the back of Electronics, the british
magazine.
The front plate of a radio communications receiver appears on the screen
and I guess you tune and select receiver functions by dabbing items on the
screen with a mouse.

[p.s., pure Class D digital amps are continuing to improve, with
better switching and so on, so ultimately the only analog streams
we'll be dealing with will be radio signals captured by the antenna
(which will promptly be digitized), and the output to the speakers
from the last-stage PWM of the digital amplifier. Everything inbetween
will totally be digital, using advanced and inexpensive DSP to do
things not possible in the analog processing realm. The only realm
left for the audiophiles to play in will be speakers.)

I think the world of totally digital is still some way off.

And while things like good vinyl replay still beats all digital disc
formats,
there will always be a following for analog.

I will be dead in 25 years, or deaf by then, so I won't give a hoot what
the human race does after that.

Patrick Turner.

Volker Tonn wrote:

Jon Noring schrieb:

In the last couple of years I've posted various inquiries to this and
related newsgroups regarding high-performance, tube-based AM (MW/BCB)
tuners, both "classic" and modern.

Have a look into the "Collins" S-series. These are state-of-the-art
tube sets 'til now. At least it's not the tubes alone but the fabulous
mechanical IF-filters giving outstanding results for a tube set.
Manuals with layout diagrams should be available on the web....

The mechanical filters are only good for reducing the BW of an existing IF
strip to make the receiver extremenly selective,
so a much reduced bandwidth is possible which isn't capable of wide AF BW.
Crystal filters are also used for the same purpose.


Since Mr Noring says he has regularly trawled the Net for everyone else's
expertise
on AM reception, but got nowhere, because he's still doin it,
why doesn't he gird his loins and put his shoulder to the task of learning
all about AM and radio engineering as spelled out so clearly in all the old
text books, and then
damn well build his own perfect AM radio???

Patrick Turner.

John Byrns wrote:

In article ,
"Frank Dresser" wrote:

"Patrick Turner" wrote in message
...




But how does one know how to apply an expander to exactly match
the inverse of the compressor characteristic?
I doubt two wrongs will make a right.

I had a book which described a very simple expander which was just a light
bulb in parallel with the speaker, if I recall. Loud passages would heat
the filament (probably not to incandesence) reduce the load of the bulb and
increase the volume even more. Quiet passages would let the bulb cool, load
the circuit and reduce the volume. It sounds goofy to me, and it's a
circuit which wasn't popular.

There were probably more sophicated expander circuits back then.

A modern sophicated decompressor circuit could match the curve of the
compressor, just as the dolby system does.

I am surprised at the size of this thread and how it has taken off, but
many of the comments seem to be either misleading as a result of wrong
facts or limited understanding of the technology involved. I have several
comments that I will lump together.

1. It has been variously stated that the audio bandwidth of AM
broadcasting is either 3.5 kHz, 5 kHz, or 10 kHz. In the US AM broadcast
channels are 20 kHz wide, so audio is effectively limited to a maximum of
10 kHz by law/regulation. It is my impression that most AM stations
transmit audio out to this legal maximum. Of course as HD-radio takes
hold this will change with the analog signal cutting off somewhere around
5 kHz. I know there are at least 2 active broadcast engineers that read
this group, perhaps they could fill us in on what the stations they are
involved with are actually doing as far as audio bandwidth goes?

The Oz situation is different to the US, as I and PA have indicated.

If two stations are 10 kHz apart on carrier F, and the both use
10 kHz modulation, then the sidebands of one station
will interfere and be heard when tuned to the other, if the signal strengths are
the same.
10 kHz notch filters won't stop the monkey chatter.




2. The idea expressed above that a "modern sophicated decompressor
circuit could match the curve of the compressor" seems far fetched to me.
In the days of yore when audio processing consisted of a single broad band
compressor, and a broad band "peak limiter" one might have contemplated
this, at least as far as the compression part went, but today's audio
processing is much more complex. Processing today involves broad band
AGC, multiband compressors, plus multiband and broadband clippers in place
of the old "peak limiter". It isn't clear to me that this would be easy
to undo, or even possible.

And two wrongs don't make a right.

I don't know if the multiband aspect creates
problems for reversing the process or not, but how do you undo clipping,
and if there are any feed forward compressors involved it is possible that
the output isn't even a single valued function of the input, making
recovery mathematically impossible.

Limiting stuffs audio, and it cannot be undone.



3. TRF receivers have been mentioned, and everyone seems to assume that a
TRF receiver would consist of cascaded single tuned resonators with RF
amplifier stages between. There is no reason why double tuned circuits,
similar to those used in the IF transformers of a superhetrodyne can't be
used in a TRF receiver, with all the selectivity/bandwidth benefits that
brings to the party. For examples see the Western Electric No. 10A
receiver, the J.W. Miller TRF receiver, the early Altec AM receiver, as
well as others.

With variable tuning? its hard to get right.

fixed IF tuning is far easier.



4. It has been stated that constructing the various RF and IF coils,
especially IF transformers with variable bandwidth, that are required, is
one reason why people aren't doing this type of project. I would suggest
that a variable bandwidth double tuned IF filter can be built using
standard two terminal inductors, by using low side capacitive coupling. I
have a British Acoustical AM tuner that uses this approach in place of the
first IF transformer to provide variable bandwidth. Rather than using an
IF transformer with a tertiary winding to provide variable bandwidth, two
separate coils are used which are coupled by low side capacitive coupling,
where the amount of coupling can be switched to change the bandwidth just
the same as with the tertiary approach.

Very hard to get right. I tried all that.
I tried tertiaries, but mechanical variation of the distance between IF coils
seemed to work best.



5. The thinking here seems to be limited to single tuned circuits for TRF
receivers, and double tuned IF transformers for superhetrodyne receivers.
There is no reason why one can't build more complex filters that will
provide better performance than an equivalent number of poles in ordinary
double tuned IFTs. Quad tuned filters are relatively easy to do, and it
is possible to go to even more poles in a single filter module, providing
an improved selectivity vs. bandwidth trade off.

6. It has been suggested that adding resistors across an ordinary IF
transformer will widen the audio bandwidth. This is not always true as
the Heath company illustrated in the manual for their BC-1A High Fidelity
AM tuner.

If it lowers the Q, the BW is widened, but at the expense of
attenuation just outside the band.
Its a bandaid measure.

They suggested adding a resistor to the first IFT to narrow the
bandwidth if interference from adjacent stations was encountered, and IIRC
they provide audio response graphs with and without the added resistor
showing how the resistor narrows the bandwidth.

??

Actually I think that in
this case it is only the nose bandwidth that gets narrower, the bandwidth
further out beyond the audio range does increase as you would expect. I
think this effect is probably due to the fact that the first IFT in the
Heath, and other quality tuners, is overcoupled, and adding the resistor
eliminates the overcoupling effect narrowing the nose bandwidth.

So its the rabbit eared response curve which is damped by the R,
thus narrowing the BW.

It pays
to be careful and make sure you know the theory and what you are doing, as
things don't always work as you might expect.

You got it.



7. It has been suggested that using a 2 MHz IF frequency would allow
wider bandwidth than the standard 455 kHz IF frequency. I fail to see why
this should be true.

Because for the same Q value, the pass band would be 4 times wider

Within reason, for bandwidths typical of audio
receivers, you should be able to build a filter at 455 kHz that has
effectively the same response as a 2 MHz filter. There is no need to
throw out the 455 kHz IF just to get wide bandwidth.

Its difficult to make a 455kHz typical old IFT produce a nice flat topped
20 kHz wide BW. Its either pointy nosed, undecoupled, or flat topped, critical
coupled,
or over critical or rabbit eared.
I have tried all that.

Patrick Turner.



Regards,

John Byrns

Surf my web pages at, http://users.rcn.com/jbyrns/