[CW] DSP "Smart" Audio Detection-It's about Audio Detection, Stupid
- LONG
Ed Tanton
n4xy at earthlink.net
Sun Jan 1 12:44:22 EST 2006
I was re-examining Chuck Rippel (WA4HHG)'s website and ran into this.
It made such an impression that I had to share it. It's terrific!!!
DSP "Smart" Audio Detection
Copyright 1997, 1998, 1999, 2000 by R. Charles Rippel
Manufacturers of radio receiving equipment are increasing relying on
Digital Signal Processing (DSP) for filtering and
detection.demodulation. Unfortunately, the entire spectrum of
benefits derived from this 21st century technology have not been
fully explained by the manufacturers. Below is a reprint of my
treatise entitled: It's about Audio Detection, Stupid," which
appeared on the Premium Receiver mailing list and in the SWBC weekly,
NU on the practical benefits of DSP ``smart`` audio detection. There
follows a response from Her Hans J. Knieser of KD Elektronik GmbH
builders of the excellent KWZ-30 receiver.
It's about Audio Detection, Stupid !
Several noted SWBC DX'ers obtained Collins HF-2050 military rx's some
months ago and have been discussing their performance. Some initial
background: The HF-2050 was the first production DSP receiver offered
with about 1,100 manufactured during the late 80's until about
1991. They ware largely employed by the Canadian military and the
reported cost of the receiver was in the $30K (CDN?) range. I think
its safe to say that using this receiver has changed the way the
owners think about receiver design. Speaking for myself, I can share
that using the HF-2050 has totally caused me to re-evaluate the
attributes I view as important in receiver design.
While I have yet to actually test mine, Dave Clark was kind enough to
forward a copy of the original Collins specs. The HF-2050 is not
particularly sensitive at rated 1.25uv "soft" for 10db S/N + N. One
might also expect reasonable but only average performance from the
filtering line up of 6.0 & 3.2kc for AM, 2.8kc for SSB and 1.0 and
.3kc on CW. 3rd order is reported at -25dBm and IMD is -40dBm. To
consider specs alone would logically support a conclusion of "nice,
average radio; nothing really special. New Drakes and AOR's are
better for a bit less money than a used HF-2050."
As soon as 3 months ago, I would have enthusiastically supported such
a conclusion but using the HF-2050 has caused me to re-think my
position on receiver specs and their ability to reasonably predict
performance outcome. On a given listening situation, I can hear more
intelligibility, more audio detail, more copyable audio from the
HF-2050 than anything I use save for maybe the
HF-1000A. Additionally, Dave Clark, Tony Ward and John Bryant have
all expressed their surprise at the ability of the HF-2050 to recover audio.
The question is: How is this possible?
While I don't have a specific answer, I can offer some observations
and some initial discussion points that might lead to some
"educated," reasonable speculation.
That short answer is Collins must not be not using a diode detector
for AM nor a product detector for SSB. The detection functions must
be taking place in the DSP realm directed by very sophisticated
programming that was optimized for SSB, CW and to a lesser degree,
AM. Clearly, the receiver recovers audio better in the SSB mode
although the advantage is not alone supplied by applying ECSS techniques.
My initial feelings are that its the HF-2050's DSP detection process
rather than its filtering which is responsible for the clear
advantage in audio recovery. Commonly available receivers today
apply the output of a highly amplified and very quiet RF stage to an
IF stage where mode specific filtering and further amplification take
place. This output is directed to a diode detector for AM or in the
case of SSB, a product detector. As we are all aware, of late, AM
synchronous detectors have become popular by reducing fading
distortion in AM signals. Some sync detectors, such as that found in
the Drake R8B and Sony ICF-2010 are also sideband selectable allowing
additional isolation from QRM up or down frequency from the target
station. This detected audio is then amplified by a common audio amplifier.
There are several "flavors"of DSP receivers represented by
application and implementation of digital technology. Effectively
applying DSP in the receiver IF i requires significant processing
power and speed. At today level of technology, these requirements
translate to the consumer as significant cost items. Some receivers
simply redefine "what constitutes an IF." Then, DSP is applied at
the audio level then label this new ``stage" as an additional
IF. That would be like adding a Timewave DSP to your Drake R8x and
then calling it Triple Conversion. Where there is technically some
truth in such a label and a performance advantage, such an
explanation certainly deviates from accepted theory.
Receivers such as the Watkins-Johnson HF-1000 and 1000A, the K & D
KWZ-30 and Kenwood TS-870 have successfully applied DSP at the IF
level for not only filtering, but also detection. With the possible
exception of JRC's recent attempt at DSP, most radios which employ
this technology have received wide acceptance.
Having used the HF-1000A and now Collins HF-2050 under challenging
conditions, I would suggest that the DSP programming is actually
capable enhancing desired information while ignoring unwanted
information in the actual detection process. The selection of
"desired information" goes much farther than implying the receiver
suppresses off frequency signals, a task delegated to IF filters in
conventional designs. I am theorizing that DSP technology actually
goes a step further and is capable of discerning between wanted and
unwanted information actually present on the desired frequency of reception.
To get a glimpse of why the Collins 2050, KWZ-30 or WJ HF-1000 might
accomplish this, a visit to KWZ's WWW page describing their
detection technique might be in order. Their detection technology is
described at:
http://www.kd-elektronik.com/index_e.html
Don't consider this information to grasp the finer design details of
its specific technical application. Rather, consider it as a glimpse
of how DSP technology might make what would arguably be presented an
a quantum leap forward by an order of magnitude in delivering a new
level of performance to be used by radio receivers for decoding an
analogue signal or broadcast.
In closing, consider the possible benefits from the application
of this technology when it is employed beyond "simple" IF
filtering. "Smart" digital detection schemes would add what could be
considered as an approximate equivalent of additional, filtering IF
stages but applied instead to benefit detection and audio
recovery. If enhanced audio recovery from "smart" detection schemes
is a design intent of the builders of this equipment, my only
criticism is that they have not communicated the application of this
technology in ways that we, the consumers can interpret and identify
its benefits.
Follow up from Hans-J. Kneisner - KD Elektronik GmbH
From your text I understand that you confirm that DSP-receivers
sound different from analog receivers and that the readability of
weak signals is better. But you cannot quite pinpoint the reason for
the better quality. Maybe I can. This is going to be a somewhat
longer explanation and if I tell you something that you already know,
excuse me for that. I am sending you this for the preparation of the
demonstration and I want you to tell the people the right things.
Comparison of DSP-Receivers and analog (conventional) receivers:
There are two reasons for the better audio- or signal-quality of the
DSP-receivers: one is the properties of the bandpass-filters and the
second is the properties of the demodulator or down-converter.
1. Bandpass filters
The bandpass-filters used in analog receivers are either crystal or
mechanical filters. Both filters suffer from phase distortion, the
more the steeper the skirts are. This means that the delay time of
different frequencies in the passband is not the same. The time or
phase relationship of the frequency components of a signal is lost or
at least distorted. This can easily be observed with digital signals
like fast CW or RTTY. The pulses are severely rounded or even can get
pointy. Or this can be seen by receiving fax pictures. Due to the
phase distortion the vertical lines get fuzzy of are doubled. This
does happen with audio signals too, but the human ear cannot detect
the phase error, but the sound and readability are affected. There
are very expensive receivers, e.g. from Rohde u. Schwarz, which have
quite elaborate phase compensation networks to compensate the phase
distortion, but these receivers are very rare.
The bandpass filters in the DSP-receivers are of the type FIR. These
filters are strictly phase-linear, which means that the delay time
for all frequencies in the passband is the same. Often the expression
phase-linear is used, although many people do not know what it means.
It means that the phase increases in a linear function with the
frequency. If the factor is correct, the delay time is constant. That
the phase-linearity of the filters is mathematically exact linear is
very important for the signal quality. I have always stressed this in
my brochures and publications, but the reviewers do not pay attention
or they do not know why this is so important. You can reread the
review from Radio Netherland (there is a link in our homepage). They
too write a lot about the special sound and do not know the reason.
Some reviewers even write that the sound is somewhat artificial. The
contrary is correct. The sound is more natural with a DSP-receiver
than with an analog receiver, but they have never heard it before.
The absence of phase distortion can again best be seen by receiving
digital signals and looking at the signals on a scope or by looking
at fax pictures. And the digital filters do not ring. You can receive
fast CW or RTTY with a very narrow filter, which is not possible with
analog filters. There is no analog counterpart for the
FIR-filters.They can not be built in the analog technology. Thus
these filters and their performance is really something new in the
art of communication.
It is important too, that the filters in the front-end of the
receiver or the first i.f. do not cause phase distortions. Therefore
are we using a pretty wide crystal filter in the 1. i.f. of 15 kHz bandwidth.
2. Demodulators
All demodulators are mixers or multipliers. The frequency conversion
is mathematically a multiplication. The simple diode demodulator for
AM uses the nonlinearity for mixing the carrier with the sidebands.
This is the wanted signal. But the sideband frequencies multiply with
each other too. Every frequency in one sideband generates a signal
with all other frequencies which are present in the passband. This
leads to an almost unlimited number of unwanted signals. These are
smaller because the sideband frequencies are smaller than the
carrier, but they are there. Therefore the diode demodulator has a
distortion factor of 3 to 5 %. or more. The situation is a bit better
with sync detectors and product detectors (product = multiplication),
because the added carrier is much stronger than the signal and so the
spurious signals are relatively smaller. Basically there is no
difference. It can not be prevented, that the signal components
multiply with each other.
This is completely different with the digital multiplication. As said
before, any frequency conversion is a multiplication of two
frequencies. If two frequencies are multiplied in the digital
representation, only this is performed and nothing else. A
multiplication of the signal components does not happen. So when the
signal is down-converted in the DSP, the resulting signal is as clean
as it was. There are of course different algorithms for the
demodulation of AM and SSB or other signals. But common for all is
that they do not cause a distortion like the diode demodulator or
product-detector. Basically the demodulator algorithms are free of
distortion, except maybe the resolution. In a 16-bit system the
resolution is 65,000 and in a 32-bit system it is 4.3 billion bits or
steps. In the KWZ 30 we use double precision math, which is 32-bit.
So the resolution error is not a big deal. It can be said that the
digital down-conversion and the demodulation does not cause a
detectable distortion.
The properties of both the filters and the
down-converters/demodulators were unknown before and contribute to
the special and exceptional signal quality of the DSP-receivers. A
real DSP-receiver is something completely different than a
conventional receiver with an added DSP filter. I think that this is
enough about this matter and I hope that it gives you the information
that you have missed to understand the differences between a
DSP-receiver and an analog receiver. If you need more information
about this or have any questions, please let me know.
72/73 Ed Tanton N4XY <n4xy at earthlink.net>
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