wrote:
Tom wrote:
wrote:
Tom wrote:
wrote:
Tom wrote:
[snip]

The point of my message was that you cannot make a SW radio out of a
laptop alone ( the question he asked) but could make a VLF receiver out
of a laptop that could be used with a downconverter to tune SW. Many
have done so. The downconverter is typically a superhet radio whose
last IF is downconverted through an add-on to the audio frequency
range.

The audio frequency laptop receiver need not have especially high
sampling frequency or sample size when used with a downconverter for
SW. A 10 kHz wide passband is adequate for most transmission modes and
is readily provided by a 48 kHz sample rate - even 24 kHz could be
enough. Because the front end tuner has AGC and can regulate levels
into the sound card, the latter's A/D converter having something
approaching 16 bit resolution will have more than adequate dynamic
range - even 8-bits could be sufficient.

Of course, the state of the art is for higher resolution A/D converters
and higher sampling rates for direct conversion for DSP. As discussed
elsewhere, great care must be taken in controlling interference from
the SDR to itself, via the antenna or other unintended coupling.

Tom

All you are really doing is using the PC for the demod, which is
technically not a software defined radio. Mind you it can be done, but
it is really just an analog radio with some DSP.

Technically, it is a VLF SDR with an analog downconverter. By itself,
it can receive wireless energy, tunes over a spectrum of less than 1/2
the sample rate, has variable IF bandwidth, can demodulate many modes,
does noise reduction, all through software. How is that not a software
defined radio? Practical SDR's are going to have analog elements in
them, if the modulation is to be interpreted by one's hearing.

Tom

In the strict sense, the SDRs are ADCs and massive DSP. You wouldn't
have analog IFs because that restricts the BW of the signal. That is,
nothing can be wider than the IF.

A true SDR could demod multiple signals at one time.

From Wikipedia and what looks like an authoritatively written article:

"A software-defined radio (SDR) system is a radio communication system
which uses software for the modulation and demodulation of radio
signals."

From the ARRL Technology Task Force Report 2001
http://www.arrl.org/announce/reports-01/tt.html:

"Most software receivers have an analog front end consisting of
band-pass filtering, a low-noise RF amplifier to set a low system noise
level, a local oscillator and mixer to heterodyne the signal to an
intermediate frequency (IF) where analog-to-digital (A/D) conversion,
digital filtering and demodulation takes place. Recently, however,
there are some software receivers that perform A/D conversion
immediately after the antenna."

Your "strict sense" and "true SDR" interpretations are much too narrow
and illogical. that's like saying a radio is not a radio unless it can
receive the highest possible frequency. A crystal radio is still a
radio; a VLF software defined radio is very much a software defined
radio whether it demodulates one or many signals. Neither needs to use
state of the art technology to continue to be so defined. A notebook
computer capable of demodulating via software a VLF radio signal
coupled to its sound card input is therefore a radio receiver defined
by software.

Putting a downconverter in front of either the crystal radio or the
software defined radio simply adds the adjective "superheterodyne" to
their descriptors. These descriptors define the sub-classes of radios
to which a particular implementation belongs; the sub-class of radios
called software defined radios has many sub-classes of its own,
including both purpose built hardware/software systems and general
purpose hardware such as a personal computer running SDR software.

Tom

Thirty years ago, that would qualify as a SDR. Look at the block
diagram of what it takes today to be a SDR:
http://www.monteriallc.com/downloads/Aquila.pdf
Bandlimiting on the front end, then high resolution ADC, followed by
DSP. This is not just DSP demod.

A radio is a radio, whether it is a crystal radio, a regenerative one,
a superhet, single, double, triple conversion. A software defined radio
is a software defined radio whether it receives VLF only or operates
into the microwave region. Your example is a "HF Wideband SDR" and the
way you talk, pretty soon it, too, will not "qualify as a SDR". How can
it qualify one year and not another? Its block diagram is functionally
similar to that of the PC running SDRadio softwa bandlimited front
end (anti-alias filter), ADC (16 bits to 24 bits depending on sound
card, therefore greater dynamic range than your 14 bit example),
followed by DSP software. So what if it's a general purpose CPU running
DSP software or a DSP IC - it's still DSP, just different levels of
performance.

Now if you were to substitute "state-of-the-art SDR" wherever you said
"SDR" in your last message, I'd agree with you, except SDRadio and
Dream are not "just DSP demod" - they also filter before demod and do
noise reduction and AVC. [30]

Tom