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/