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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 manufactuers. 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. |
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Its about Audio Detection, Stupid ! |
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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 maufacturered 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 copiable 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
snychronous 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 DSPto 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 benifit 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. |
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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 downconverter. 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 reveiving 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
phaselinear, which means that the delay time for all frequencies in the passband is the same. Often the expression phaselinear is used, although many people do not know what it means. It means that the phase increases
in a linaer function with the frequency. If the factor is correct, the delay time is constant. That the phaselinearity 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 absense 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 downconverted 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 resolutiuon 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
downconverters/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. |
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