The following are just my opinions. Those with different experiences
will have different opinions. So read this with a grain of salt.
YMMV, etc.
Analog modems and software modems both attempt to approximate the
solution of demodulating RTTY.
It is a matter of how close each one gets to solving the ideal
equations.
In the case of modems, you also have to do it in real time.
30 years ago, you can only do rudimentary digital signal processing in
software because machines that were fast enough are also very
expensive. I was micro-programming AMD bit slices and using TRW
multiplier-accumulators back then just to process signals sampled at
10 Ms/s. Audio processing is much simpler of course, but even
something like an 8th order audio filter which can be built for a few
dollars using op-amps would have cost hundreds to build in the digital
domain.
By the 1990s, desktop computers were fast enough (especially when you
add the arithmetic processors to them; remember the 8087?) to start
competing with the best analog modems. Back then, Brian K6STI had
already posted comments to this same reflector that there are things
(such as AGC that "looks forward" in time, or the "digital flywheel")
that are simply impractical to build with analog hardware. If I
remember, Brian had also mentioned the fact that you can do non-linear
implementations that would be impractical with hardware.
Approaching 2010, we now can do even more math on desktop computers
than in the 1990s. We can get much closer to implement the
demodulation equations that we want by using software than by using
the analog approach. Even newer devices sold as hardware modems today
are really DSP based modems (but they are much more limited in what
they can do compared to the flexibility that you get from your desktop
computer).
Sound cards also became better and cheaper between 1990 and 2009.
One of the problems with some software/DSP designs is that instead of
starting at the basic first principals of demodulation, they were
emulating what analog modems were doing, so they never were better
than the analog modems. As a result, software modems that are
crippled that way fared even worse than analog modems once you go
through poorly adjusted A/D converters.
To an engineer, hardware and software are merely parts of the set of
tools. With today's technology, low speed modems designed with
software tools are incredibly more malleable and flexible than the
hardware approach, not to mention the cost and the development time.
Imagine trying to build an Olivia modem without software/firmware.
True analog modems (not the DSP modems in a box) had two things going
for them -- resolution/dynamic range and latency.
However, most sound cards today have dynamic ranges that already
exceed the dynamic range of the transceivers they are used with.
Plus, if you are using narrow I.F. filters, the dynamic range of a
sound card does not even come into play until the signal is way weaker
than the AGC threshold of the receiver -- and all you need to do in
that case is make sure that the noise floor of the sound card is below
the noise floor of the receiver.
Latency is also less of a problem as computers become even faster, and
do you really care if the character the other person types appears on
your screen 0.1 of a second later than if it were coming from an
analog modem?
Latency is often a problem when dealing with real-time tuning
indicators such as the crossed ellipse. However, by transferring
small sound card buffers and using decent sound protocols (ASIO in
Windows and Core Audio in Mac OS X), even crossed ellipse lag should
no longer be a problem -- if you see a significant tuning lag when
using a crossed banana, it is a problem with the particular software
implementation.
With the speed of a Core Duo machine, you can emulated every detail of
the HAL ST-8000 analog processing in software to achieve the same
performance. Note that the ST-8000, with its analog AGC, has a
dynamic range of 75 dB to 85 dB, something that most 16 bit sound
cards can achieve *without* any analog AGC. Emulating the KAM's
filters and slicer is child's play.
But why would you want to emulate an analog modem when you can do much
better with good software algorithms?
73
Chen, W7AY
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