DSP Audio Recovery

[pjxii]

Despite my "old-schoolness," I am blown away by the recovered audio of the Cubic CDR-3250. Being that its a stand alone receiver and not a PC black box was my attraction to it. I was doing some low sunspot/high RFI reading (meaning its pointless for me to try to DX for now) and came across a discussion in the Premium-RX archives on the header's subject. It's part of a thread about the Collins HF-2050 from twenty years ago but I find it relevant to today.

I tried to contact Chuck Rippel to ask permission to quote him here but could not find a valid email address. My apologies to Mr Rippel if he objects (hopefully he won't mind my sharing his insight on this aspect of our hobby). There are several key follow ups which I will add if anyone wants to read them.



Date: Mon, 18 Jan 1999 18:14:10 -0400 From: "Chuck Rippel" <crippel@erols.com> Subject: Its about audio recovery, stupid

If his supreme highness, Dr. Baily is done holding trash cans down with R390A's .... I like to actually submit some topical material for the consideration of the List.

Remember during the 92 election when the phrase du jour from the [Clinton] camp was, "Its the economy, stupid." Well, we have our own version of that concept as it relates to radio receivers and receiving techniques. That's right, what its really all about is audio recovery.
Had a long, technically intensive phone chat about the relevance of audio recovery with long time friend and hobby colleague, Dave Clark last night. In as much as the HF-2050 has outstanding audio recovery, Dave and I discussed what component(s) of contemporary receiver specs, sensitivity, noise floor, selectivity, dynamic range, etc... were responsible and how they could be defined by reviewing published specifications.
The answer is: Specifications do not necessarily define a given receivers audio recovery ability.

Lets face it, audio recovery is&#8232;>everything< ! Continuing to cite the HF2050 as an example, consider that its sensitivity specs are not all that spectacular and its a fairly noisy receiver. Yet, is arguably it is near the top with regard to having excellent audio recovery.
What comprises Audio Recovery is difficult to define. Its about like trying to grab smoke floating in the air. Its easy to illustrate good audio recovery however. Simply tune an R8/A/B and the HF2050 to the same station, listen and compare.

As Dave pointed out to me last evening, the R8's audio is superb but very mellow, making it easy to listen to. The audio from the '2050, on the other hand, is very defined and accurate. I would submit this is made possible by characteristics which include above average attention to enunciation combined with a certain [sibilance].
Whether using a speaker or headphones, (Dave suggests the 600 ohm JRC phones) the audio recovering abilities from the 2050 seem to pick up the voice and music while at the same time, ignoring or not fully processing band noise. Thus, my judgement gives the 2050 what I would call a "6db" improvement of recovered audio over the R8A.
Just thinking.... the HF-1000A exibits this characteristic, albeit to a slightly different degree, as does the Kenwood TS-870. When a group of us listened to the KWZ-30 (albeit under far less than optimum condx) this past August, its excellent audio recovery abilities were also in evidence. I discussed with Dave that my R1051H sounds a shade better using the Timewave DSP-599. He shared that he had heard that the 2050 with a Timewave is a very effective combination.
Is the thread of commonalty here that these are all DSP based receivers? Does the secret lie in the digital detection techniques or is it because the digitally derived filters have no audio "muddling" group delay?
The only receiver that casts doubt on this whole theory is the JRC-545. We listened to that receiver at the same time the KWZ was reviewed. While the conditions were equally poor, that receiver was simply not acceptable, in my observation anyway.
In closing, I would suggest that a superb receiver is not all about <.1uv sensitivity numbers, filters with 1:1 shape factors, 34dbm intercept points or 145db noise floors. Certainly these specifications can combine and help define a truly "premium" receiving system.
I submit to those still awake that our battle will be fought and won on audio recovery and how a given receiver detects, processes and presents it to the human ear. Perhaps during a receivers design phase, the engineering focus should include ear and brain as part of the receiving system with electronic allowances made to optimize use with that system? ============

 

[nanZor]

It's all about audio recovery

Wow - great find, thanks for bringing that up.

The matter is not so much a "dsp thing", as it is audio recovery. The customers for those radios usually spent long hours behind the dial, but not for fun. If they did listen for fun, it was perhaps on their lunch break.

Audio accuracy with good amplifier circuitry (be it dsp or analog) was vitally necessary to listen to languages where syllables, intonation, emphasis and so forth could mean the difference between hearing "can I launch?" or "can I go to lunch?"

It is interesting to see him performing this test, and mentioning the venerable R-390 from a consumer level standpoint. For the customers these receivers were designed for, it would not be uncommon to see a Cubic, Rockwell Collins, Racal, or other high end receiver bolted right next to or under a Korean war / Vietnam war era R390 getting a shakedown in a similar test.

Unlike pristine restorations of R390's, the higher level receivers had *better* have as good an audio section or more than those R390 workhorses that may have been in and out of the maintenance shop numerous times in it's life - and still going. Otherwise, the contract would have been quickly killed.

For us SWL's, and amateurs, the cost-cutting measures are put into place, usually slapping a poor audio amplifier design - or manufacturing implementation - figuring that we are just listening for fun, have noisy background noise to begin with, and are not likely to put a few hours behind the dial daily - if that.

Audio specifications are also not something one can easily "bench race" when deciding upon a purchase, like overall sensitivity, IMD, and things like that where the customer won't ever truly realize the specifications. Like placing a coat-hanger antenna on the back of an R-8600.

But certain consumer types DO want good audio, and recognize the importance of that last-mile of audio recovery being important. Hence the popularity in the past of the Kiwa style of audio upgrade kits.

It really is beneficial, but about the most one can bench-race an audio spec on is the THD distortion. Gosh, I remember doing that kind of bench-racing as a kid reading Stereo-Review magazines and comparing thd against differing amps, not knowing anything at all about how you can tweak that value with lots of negative-feedback and smearing the sound. Spec was great, the the overall sound was mush.

In the end, it is SUPER important to me, but maybe the vast majority of manufacturers can't crank up the price of their gear for substituting a .05 cent part for one that is vastly superior at .10cents apiece in a consumer level marketplace.

 

[Boombox]

Interesting thread, although I'm not certain what is exactly meant by 'audio recovery'.

I think audio processing is important in DXing / SWLing / MW DXing and listening. Some IF chips seem to produce better audio than others, and some radios are gems when listening through headphones.

I don't have any monster receivers, but some radios have better audio than others, and -- after all -- radio is an audio pastime. The GE SR series have smooth audio overall, as do some of the smaller Sangean analog portables (through the headphones). My FRG-7 has terrific audio, it's probably my best SW receiver for audio.

Some radios are a bit thin on the bass response where they don't need to be.

Ironically, several of my old boomboxes, or 80's era Walkmen, have pretty smooth audio -- both on MW and FM.

 

[pjxii]

Interesting thread, although I'm not certain what is exactly meant by 'audio recovery'.

A later post by Mr. Rippel included this definition: "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"

The late Hans-J. Kneiser, designer of the DSP-based KWZ-30, replied with this information:

"Dear mr. Rippel, thank you for sending the copy of the text to numero uno. Here I will only comment on the text and write about the other items later in a separate e-mail.
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:&#8232;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 [receiving] fax pictures. Due to the phase distortion the vertical lines get fuzzy [or] 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.
The 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 resolution error is not a big deal. It can be said that the digital downconversion 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 Timewave filter. Do, not mention that thing again. 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.
Best regards Hans-J. Kneisner K-D Elektronik GmbH - "

Now you must remember that this was very early in the DSP-based receiver process. How things have come along I do not know, as the only DSP unit I have is the Cubic from the same time frame. I just found it a fascinating discussion and helps explain the ability of the CDR-3250 to make speech very understandable (and music cleaner) on AM much more so than my Lowe HF-225 Europa, which has a reputation for excellent audio. I wish the RFI here wasn't so brutal so I could spend an evening with the Cubic and Drake R7A in a shootout.

 

[pjxii]

The matter is not so much a "dsp thing", as it is audio recovery. The customers for those radios usually spent long hours behind the dial, but not for fun. If they did listen for fun, it was perhaps on their lunch break.
...
But certain consumer types DO want good audio, and recognize the importance of that last-mile of audio recovery being important. Hence the popularity in the past of the Kiwa style of audio upgrade kits.
...
In the end, it is SUPER important to me, but maybe the vast majority of manufacturers can't crank up the price of their gear for substituting a .05 cent part for one that is vastly superior at .10cents apiece in a consumer level marketplace.

So true, especially that last line!

 

[nanZor]

In my case, two receivers that I've own/ed have the desirable "it factor" when it comes to audio clarity that goes beyond mere frequency response:

Commradio CR1a
CCrane Pocket / Skywave / Skywave SSB

My Kenwood 590s is nice, and has plenty of frequency response options, but it still doesn't have the absolute clarity of the above. Nor my Alinco, Target or anything else I've used. They all sound analog, slow.

The additional information from the 2 dsp based receivers above have widely different performance from an rf perspective, but the audio in either has that additional clarity I've come to enjoy - and mostly for my cw / ssb listening (aside from the CCrane's which I use mostly for music but ssb or cw is no slouch either - for a pocket radio that is.)

In the end, I think it *can* be duplicated by analog circuitry by paying attention to things like signal-to-noise, phase, high quality componentry that doesn't smear or smooth the sound etc. But are consumers willing to pay for that? I am.

That additional detail is the audio-recovery I think we're speaking of here that makes all the difference. I guess If I were to go for a modern Icom or similar, I'd have to demo the audio personally with my own headphones to see if it lives up to at least the CCrane in the audio response - for that super fast dsp clarity first, and things like overall frequency response etc second.

But just having a dsp doesn't automatically give a radio's audio that "it factor" for me. The Tecsun Pl880 doesn't bring out the best of the SiLab chips for me for example.

 

[Boombox]

How does the Skywave sound on AM?

 

[nanZor]

Sounds great on AM, but I have to bring this up about source quality ...

Stations, both domestic and shortwave, that seem to take pride in their signal audio quality sound just fine.

However, there are times that the Crane's added clarity can make a careless station or network hard to listen to.

Example - some networked stations don't really run any sort of quality studio, and just pump streaming audio out to their analog transmit chain. More often than not, that produces a metallic, ringing edge - basically making it sound like the studio is inside a steel drum.

Of course I can hear that stupid metallic ring on my analog sets, but with the clarity of dsp here, it really bothers me.

One station in particular here in LA has that studio in a steel-drum effect -- and they run ads for the CCrane's themselves! Shame on them for not taking pride in their signal and possibly driving away the listeners that may have been persuaded to get one. Hah, their station sounds actually WORSE than before, and an uneducated listener may not know that the CCrane is dutifully revealing that studio in a steel-drum as it should, and not hiding it. If one of their engineers actually listened to their own signal on the CCrane, they'd probably do something about it.

The biggest irony about using poor quality transmitter audio is that their own ads promoting streaming the iHeart radio app sound like crap! Hmm - we could go into a lot of conjecture about why that is so ....

I think I probably covered this in more detail in the Pocket review, or the home theater subforums..

 

[Boombox]

Does the Skywave have bandwidth selections, and which bandwidth do you use when listening on MW, as opposed to serious DXing?

My three SiLabs equipped radios (PR-D5, G2, and RS Pocket Radio) have fixed bandwidth on MW (the PR-D5 seems to be around 4-5 khz, the other two are more like 6 or so), so it's a passable but not awesome MW listening experience as it can be on, say, a Superadio (even via headphones), Sony ICF-38, my Sanyo Walkman or some Sangean models.

 

[jim202]

In this whole thread, no one has mentioned the fact that if you roll off the high end of the audio spectrum, you in effect get rid of a bunch of the white noise that many receivers have today. You can even see this on a O scope that is looking at the audio out of a receiver.

Sure this may kill some of the highs in a person that is use to music fidelity. But it will make listening to a receiver looking for signals in the HF spectrum much more pleasent.

I too have used an R390 over the years and would like one today. But I don't think I could lift one these days. It might take 2 of us to move it around.

The R390 was and still is a fine receiver. Back around 1969 or 1970, I just happen to be at the dump on Fort Devens, MA. There was a 5 ton truck there that was rolling boxes of R390 receivers onto the ground. Then a huge dozzer was using the boxes to see how well the tracks of same could flatten the boxes. many of them were still in the original shipping packages.

I tried my hardest to talk the person into letting me pick one of those boxes up before the tracks got to it. But no dice. Almost had tears in my eyes watching those receivers be destroyed. Must have been close to 50 boxes that were run over and then pushed into a trench that was dug for them.

Those R390 receivers might have made stable land fill, but I sure would have like at least one of them in my shack.

 

[pjxii]

In this whole thread, no one has mentioned the fact that if you roll off the high end of the audio spectrum, you in effect get rid of a bunch of the white noise that many receivers have today. You can even see this on a O scope that is looking at the audio out of a receiver.

Sure this may kill some of the highs in a person that is use to music fidelity. But it will make listening to a receiver looking for signals in the HF spectrum much more pleasent.

I too have used an R390 over the years and would like one today. But I don't think I could lift one these days. It might take 2 of us to move it around.

The R390 was and still is a fine receiver. Back around 1969 or 1970, I just happen to be at the dump on Fort Devens, MA. There was a 5 ton truck there that was rolling boxes of R390 receivers onto the ground. Then a huge dozzer was using the boxes to see how well the tracks of same could flatten the boxes. many of them were still in the original shipping packages.

I tried my hardest to talk the person into letting me pick one of those boxes up before the tracks got to it. But no dice. Almost had tears in my eyes watching those receivers be destroyed. Must have been close to 50 boxes that were run over and then pushed into a trench that was dug for them.

Those R390 receivers might have made stable land fill, but I sure would have like at least one of them in my shack.

Using the DSP technology in the Cubic receiver lets me bring the bandwidth in AM mode as narrow as 1.8 kHz (to reduce the static and lightning crashes) on tropical and longwave broadcast stations yet maintain excellent intelligibility. None of my analog based radio's filters can do that, it becomes "basement" audio with narrow SSB filter in AM mode.

It truly is heartbreaking (for us radio lovers) hearing those stories about the mass destruction of all those fine receivers. I know that the old 1051 receiver used by the navy suffered the same fate, and I'm sure there were many others.

 

[nanZor]

It is a shame that many have not heard the difference between superb audio intelligibility, and not just mere frequency response shaping that some of the higher end gear had back in the day.

If one hasn't heard it, then I guess it would be hard to design for it. It's something specs alone can't touch - it takes a seasoned ear. Or maybe just an engineer from the 70's era stereo department (ahem, Kenwood - pull some of those guys back as consultants - despite the generally good sound today, it can be made AWESOME)

But yeah, back to the DSP side - it takes more than just putting the chips down - they have to be integrated into the whole package with intelligibility in mind, not just frequency response. It is basically what makes a radio memorable 50 years after production, not just because it was cool, but because of how it treated your hearing.

 

[nanZor]

Does the Skywave have bandwidth selections, and which bandwidth do you use when listening on MW, as opposed to serious DXing?

I put more info in this thread:
https://forums.radioreference.com/hf-mw-lw-equipment/360588-ccrane-skywave-w-ssb.html

Remember, this radio is just slightly larger than your palm. Expecting an Elecraft experience from it might be taking it too far.

 

[pjxii]

Does the Skywave have bandwidth selections, and which bandwidth do you use when listening on MW, as opposed to serious DXing?

My three SiLabs equipped radios (PR-D5, G2, and RS Pocket Radio) have fixed bandwidth on MW (the PR-D5 seems to be around 4-5 khz, the other two are more like 6 or so), so it's a passable but not awesome MW listening experience as it can be on, say, a Superadio (even via headphones), Sony ICF-38, my Sanyo Walkman or some Sangean models.

A little off topic, but since you mentioned the Sony ICF-38, I am amazed that Sony made so many high quality radios over the years. Whenever I just needed something to listen to and I found a Sony at K-Mart it turned out to be a real winner. The SRF-59 was a wonderful surprise as well.

 

[GB46]

I have virtually no technical understanding concerning DSP or digital electronics in general. Nevertheless, I've been following this discussion with interest, because I'm curious about the DSP feature in my Sangean ATS-909X. Does anyone know where in the radio the DSP is used? Is it the audio section, IF stages, or somewhere else? This radio's audio is strikingly clear and pleasant, so maybe that's due to DSP, but some reviewers claim that the DSP is used for selectivity.

By comparison, my R75's DSP unit is for the audio, but it's only active if the noise reduction or autonotch feature is switched on. The NR feature does reduce noise, but adversely affects the audio quality, so if the NR level is set higher than 3 or so out of 10 it distorts the audio, making it unpleasant to listen to. Autonotch works well, but I've only needed it on rare occasions; I don't hear a lot of adjacent signals these days, so heterodynes are seldom a problem.