[CW] DSP "Smart" Audio Detection-It's about Audio Detection, Stupid - LONG

[Earle Johnson]

Believe Ripple also said someplace that if the 2050
broke it became nothing more than an "expensive
doorstop"

I have owned and used the WJ 8712A, 8712P
and Rohde&Schwarz XK2100L HF transceiver
with the receiver section (I was told) the same as
the EK895....They are without a doubt the finest
rcvrs as far as RF performance goes...recovered
audio is excellent and everything sounds clean and
natural...but here we need to define natural...I believe
as we get older our hearing changes considerably...
mine has and I just like the analog sound. Especially
the WJ 8716/18 series of rcvr or even more the
ITT Mackay 5050A. DSP rcvrs are superior in a number of ways None of my
analog rcvrs hear
better....BUT God help you if something fails in the
WJ 12A or 12P because its SMT and requires unique
repair skills and equipment that the average tech
simply does not have....DRS (WJ) charges $1k. to
remover the cover of their new products and then
after they remove the cover they start the clock. At least
I can repair the older stuff. or if I cant then I know
who can. The difference in performance does not make
up for the difference in repairability....in my opinion
Also like the sound of the 6790GM. Cubic 3030 series
rather raspy....to me, others might disagree.

W4EWJ
----- Original Message -----
From: "Ed Tanton" <n4xy at earthlink.net>
To: "noga reflector" <nogaqrp at mailman.qth.net>; "CW Reflector"
<cw at mailman.qth.net>; <racal at mailman.qth.net>
Sent: Sunday, January 01, 2006 12:44 PM
Subject: [CW] DSP "Smart" Audio Detection-It's about Audio Detection,
Stupid - LONG


> 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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