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Should PCM receivers really cost more?

Old 06-03-2003, 11:01 AM
  #26  
HarryC
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Default Should PCM receivers really cost more?

Originally posted by Kris^
IPD is nothing more than a normal FM reciever

being FM is more prone to interference than PCM,
Those statements demonstrate that you don't understand our radios. Your PCM radio IS an FM radio. Your PCM receiver is nothing more than a normal FM receiver. You need to learn quite a bit more about the workings of model radios before sending posts about what system is better than another.

Harry
Old 06-03-2003, 11:14 AM
  #27  
Mr T.
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Default Should PCM receivers really cost more?

Originally posted by Kris^
sooooo.. . IPD is nothing more than a normal FM reciever, with some signal reception and amplification circuits thrown in, a PCM-type failsafe function thrown into the mix, and better resolution (within the limitations of the PPM timing window).


IPD receiver is in fact equal to other brands PCM receivers. It consists of RF deck followed by a microprocessor rather than simple PPM decoder (4017 + diode and capacitor).

I bet if one was crazy enough it would be possible to wirte a program that would allow IPD receiver to receive other mfgs PCM signals. I even think it would require LESS processing than it goes on in IPD.
Old 06-03-2003, 01:55 PM
  #28  
HarryC
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Default Should PCM receivers really cost more?

Originally posted by Kris^
It seems that the way that sensitivity is increased, by actually widening the frequency "window" in the RF and IF amplifiers, would make it more prone to interference at the lmiit of sensitivity, which is pretty darned undesirable if the receiver is losing signal due to interference from something on the very edge of the frequency. You are still limited to the "narrow band" for your frequency, for total frequency bandwidth, and being that these "bands" are really bell-curve areas of sensitivity, the outer edges of that "sensitivity range" would actually be outside the desired 10khz (if I recall properly) of maximum frequency deviation allowed by the FCC for transmitters.. . . .
Plenty of errors there to deal with!!

When signal strength is weak the IPD widens its selectivity. Note that word selectivity. No-one said it widens the range of frequencies it looks at. It widens its selectivity. On that frequency.

Since it does not alter the frequencies that it sees, it does not look outside of its 10khz band.

Germany is one of the few, perhaps the only country in this world that has strict laws regarding receivers. Most of us just have laws regarding transmitters. Because this is Multiplex's big market (as well as being its home) Mpx receivers are extremely high quality. The notion that its receivers alone take such a scattergun approach to frequency selection is not just technically wrong since you have confused selectivity with something else, it is not legal for Mpx!

A lot of impressionable people read these threads Kris, and will assume that you know what you are talking about. Please do not send posts that are are derogatory to a specific brand or system until you actually understand model radios. When you can explain to us why it is physically impossible to compare PCM against FM, we will know that you are beginning to understand!

Harry
Old 06-03-2003, 05:33 PM
  #29  
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Default Should PCM receivers really cost more?

Harry,
For sometime now I have been trying to obtain the schematic for an IPD receiver. It seems you may be the man to answer a few questions I have relating to IPD operation.

Quote.
"When signal strength is weak the IPD widens its selectivity"

Q1. Is this the RF signal strength in your statement ?

Q2. IPD - If it means Intelligent Pulse Decoding how can it digitally decode the RF signal which is A.C. ?

Q3. How does it change the selectivity and when does it know to do this?

Q4. What is the composition of the circuitry that allows this function to operate correctly in the close vicinity of other transmitters ?

Quote
"It widens its selectivity. On that frequency."

Q5. Is it feedback that makes it frequency selective on that frequency and if so where is it being sensed from ?

Q6. What happens to the sensitivity if the selectivity is increased on that freqency ?

Q7. Is this a technique using only passive components or are active components also used ?

I'm sure a lot of technical readers out there would also like to understand the vagueries of IPD. I for one look forward with interest to you de-mystifying IPD.

DavidO
Old 06-03-2003, 07:33 PM
  #30  
HarryC
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Default Should PCM receivers really cost more?

David, I hope that I can answer most of your questions, though not the ones concerning the exact mechanisms that Mpx uses to create some of the features.

I think we need to start by separating out two different things that have arisen and are getting jumbled together, when in fact they are different. One is the selectivity issue which is purely a radio receiver thing, and the other is the IPD error detection which is a data thing - the failsafe part of the Rx.

In FM Capture Theory an FM Rx whether using PPM data or PCM data (note Kris, both are FM!) will be very good at selecting the strongest signal and ignoring weak signals. Assuming that you are the only Tx on your frequency then any other Tx bleeding over will be relatively very weak, as will background static, intermodulations etc. My Tx has a frequency scanner built in and if I switch it to high sensitivity then when 2 Tx are on, several other channels light up with low strength signals, if 3 Tx are on lots and lots of channels light up with low strength signals. So as well as the channels that are genuinely in use, our Txs mix the frequencies and the whole band can soon light up with these low power intermodulations. Your Rx can afford to be extremely selective and behave as if it is almost deaf - so it only hears the Tx that is shouting at it and can't hear the quieter IMs, static etc. But signal strength decays very quickly with distance and if your model is some distance away then the signal it hears from you may be down near the power of other Txs nearer to it, background static and so on. Now it can't afford to be so picky, other signals on that frequency may equal or occasionally overcome your signal strength. So IPD widens its selectivity - that is it becomes less deaf to your frequency so it can still hear you in amongst all the other noise, and not capture one other false but slightly stronger intermittent signal. Why not do that with normal PPM Rx? Well, they could but of course it would be getting some intermittent false data and would be glitching so it is of no benefit. To get any benefit from the lower selectivity you need error detection and the IPD part gives it that. Also, if the filed strength of the signal is strong the data is passed unmodified to the servos. Where field strength is weak the IPD will start to process the data. I don't know what the process is except that it uses recent past data so I suspect that it is a moving average or similar. This has the effect of slowing the servo response and giving you a warning that if you push any further you are likely to lose contact.

Error detection and failsafe works after the radio part has been stripped out, IPD only works on the servo data. It is not rocket science, it is the same as goes on in after-market failsafe devices, electric motor ESCs, gas turbine ECUs etc. It is just that Mpx has put it all into one Rx covering all channels and with a user programmable failsafe setting to drive the servos to, thus meaning that anyone with a PPM Tx, whether a computer or humble 2 channel non-computer Tx can have the benefit of error detection and failsafe that was previously only available in higher cost computer Tx with PCM and PCM Rx. PPM data is a time length pulse for each servo. There are max and min times that each pulse can be. If the pulse is longer or shorter than those time limits it must have been corrupted. So we have detected an error. It can not detect a corruption that has altered the time length but kept it within the normal time limits, however interference that can do that is phenomenally rare and not worth worrying about. If you turn your Tx up to 150% travel max for JR or 140% max for Futaba you generate a pulse that is either 0.9ms or 2.1 ms long. IPD's limits are set at a pulse of min 0.89ms or max 2.35ms (it needs to take account of Mpx Txs able to drive further than Far Eastern Tx). So if IPD sees a servo pulse shorter than 0.89ms or longer than 2.35ms, it says "error, impossible!!" It then holds at the last good position for 1/2 second, then if still in error it sends all servos to the user programmed position. It keeps receiving and the instant it gets good data it comes out of failsafe. By way of amusement, Mpx made a slight error!! Their 3030 Tx is capable of generating a signal outside the IPDs limits if you set it to some weird extreme values, and so is capable of sending the IPD into failsafe on a valid signal! hee hee! Mpx had to issue a tech notice to warn 3030 users about it and what conditions lead to it.

I hope this has helped to clear up a lot of confusion that still surrounds these IPD Rx. I use many of them, 7 channel single conversion, 9 channel d/c and 12 channel d/c (with built-in dual battery backer) since I like failsafe, and my Mpx Tx does PCM but Mpx stopped making PCM Rx in favour of IPD. They work superbly, only once in the last 3 years have I had an Rx go into full failsafe (throttle closed in my programming) for a fraction of a second. Woketman who posts regularly in the jets forum has used them in his jets without a hint of problem since they went on sale and I am about to entrust my 3k jet to the 12 channel IPD.

Harry
Old 06-03-2003, 09:02 PM
  #31  
DavidO
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Default Should PCM receivers really cost more?

Harry,
I have great problems with your first paragraph. What you have given is a not very good text book generalisation. Detail of how the receiver RF circuitry operates in conjunction to IPD receivers is flawed.

"FM Capture Theory" - Why quote this when AGC and a double tuned front end take care of this.

"quieter IMs," - 3rd order Intermodulation products are hard to measure with 4th order nigh impossible as the power reduces dramatically.

"So IPD widens its selectivity - that is it becomes less deaf to your frequency so it can still hear you in amongst all the other noise." - Do you mean digital variable bandwidth ? How the heck can IPD change RF selectivity ?

Your statement above is a contradiction in terms and shows you have no understanding regarding the action of Selectivity and Sensitivity in RF sections.

I have no problems with your second paragraph its typical digital signal processing.

Could be something has been lost in the translation of documents you have used for reference.

DavidO
Old 06-03-2003, 09:23 PM
  #32  
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Default Should PCM receivers really cost more?

The IPD appears to be a conventional Narrow band FM receiver with a microprocessor used to process the information out of the discriminator. The term selectivity is used to describe the bandwidth of the receiver and is determined by a fixed ceramic filter in the IF. Also NBFM has no ability to capture like wideband FM.
Some time ago I tested the IPD extensively and found it very stable in terms of pulse width jitter right up to the point where it went into failsafe. I saw no slowing down of the response as the signal got weaker. In other words there was no warning just like PCM.
The thing I didn't like was the resolution which seems to be limited to about 4 microseconds. Someone asked if it was worse than 512, well this is more like 256. I have a low end MPX transmitter with digital trims and one click equals 16 microseconds. I don't see how you could trim a pattern plane with that kind of resolution.
The update rate is determined in the transmitter and with my Futaba the frame rate is the same in PPM as PCM so it wouldn't be any faster. The Multiplex transmitters might be faster but not by much. If you are happy enjoy IPD but don't tell anyone it is better than PCM.
As far as the original question, PCM should not cost much more and I believe we can see prices coming down slowly.
Old 06-03-2003, 09:29 PM
  #33  
HarryC
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Default Should PCM receivers really cost more?

David, it's all terribly generalised and simplified because this forum is for a wide audience not just for radio techs, and simplifications always introduce problems. Judging by Mpx manuals their stuff on IPD may well have got a bit altered in translation. "Quieter IMs" referred to the 3oIM being quieter than the main signal, not all the umpteenth IMs. By all means pull it apart and put it back in proper tech language.

H
Old 06-03-2003, 10:04 PM
  #34  
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Default Should PCM receivers really cost more?

Originally posted by OhD
The update rate is determined in the transmitter
The Multiplex transmitters might be faster but not by much. If you are happy enjoy IPD but don't tell anyone it is better than PCM.
Mpx is not any faster as it uses standard PPM. Mpx claim that PPM is faster than PCM, I have always had doubts about that. Since each brand has its own PCM system which PCM are they comparing to? I guess it is to their own PCM which they no longer make. I believe early PCMs had a slower update but more modern versions give differing priorities to which channels get updated in each cycle and therefore can keep up the refresh rate on those channels that need it. I don't know for sure but I have a sneaking suspicion that the Mpx PCM was a bit outdated and therefore slower than the more modern systems, hence Mpx claim for PPM.

I, for one, have not said that IPD is better than PCM, though some others have said it. We went through this a while back (on the jet forum I think it was) and I pointed out that PCM does have an advantage over IPD. Since IPD is PPM it relies on a synchro pulse and uncorrupted data all the way through each cycle. Any corruption loses all subsequent data in that cycle even if it was good data since the Rx loses where it is in the cycle and needs to wait for the next synchro pulse. If the synchro pulse alone is hit and the Rx can't find the start of the cycle, all the good data for all the servos is lost for that entire cycle. Some PCM systems break the cycle into two halves each with its own PCM version of synchro so you don't lose as much data if it cam't synchronise, and if each servo's data is individually identified rather than just relying upon the sequence, any little hits on one servo don't necessarily scupper the data for all the subsequent servos in that cycle. So when interference is short in time and repeating rapidly, such as spark ignition, PCM still has an advantage as although it experiences the same hits on an individual servo, it does not tend to lose all the following servos' data in that cycle like PPM does.

H
Old 06-03-2003, 10:31 PM
  #35  
radray
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Default Should PCM receivers really cost more?

OHD,

I believe that your measurement was flawed as the resolution of the system is indeed quite good, at least as good as my Futaba 1024 PCM by feel in the air where it really counts. I have flown Futaba 512 a long, long time ago and it is certainly much finer than that. I routinely set my trim authority (MPX 4000) to 10% which has about 40 clicks of trim full travel and moves the servo very little from one extreme to the other and I can feel (see) a change of one click of trim in the air. There is no problem with resolution with IPD (PS. I use only high quality digital servos).
Old 06-03-2003, 11:15 PM
  #36  
Kris^
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Default Should PCM receivers really cost more?

Harry, interference is interference. . be it a resonance from two nearby sources, hi-energy saturation from a bad ignition, or just being too far away from the transmitter with a low-power transmitter on the same frequency near the plane. When a receiver attempts to improve it's sensitivity by "widening" it's receptive window (as you described) it also opens the possibility of interference from adjacent frequencies. I don't like that idea, as it sounds like an accident waiting to happen. I look at it this way. . if your signal to the receiver is that weak where an IPD receiver has to widen it's bandwidth (or any other receiver has to do it either), then you are too far out anyway. If you are being jammed by another transmitter on the same frequency, no amount of "sensitivity increase' is going to matter. Every receiver can be blanked by RF interference from ignitions and other sources. No amount of special circuits or different types of modulation/encryption can change that.

By the very nature of PPM coding (as admitted by yourself) all information is lost or garbled when the receiver loses signal or is jammed, even partially. PCM (yes I know how it works), however, still gets part of the signal and can "function" up to the limit of being totally blanked out. IPD, as you described it, is still limited by the characteristics of FM/PPM (PWM???), and then throws in a "failsafe" on top of it all.

It may work, you may like it, but I consider the shortcomings (as described by you) to be less desirable than PCM's foibles (lockout can be a pain) I didn't like IPD when it first came out. . I still don't like it.
Old 06-04-2003, 05:59 AM
  #37  
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Default Should PCM receivers really cost more?

Originally posted by Lynx
Rodney.. PCM receivers can cost over 150 dollars and all it does is receive and decode a 9600bit/sec RF signal at 72MHz... Now lets compare that to a moment to say a USB wireless card. Which can transmit, recieve, decode, encode, encrypt and route a 11mbit/sec signal at 2GHz which (checking on pricewatch) can go for as little as 33 dollars... Justify the cost of a PCM receiver knowing that...
If there's a USB wireless card that can survive in the same conditions as our receivers, is as small and light, and doesn't weigh any more, I'll take fifty for $33 a pop.
Old 06-04-2003, 10:49 AM
  #38  
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Default Should PCM receivers really cost more?

Harry,
From reading your interpretation of a german technical translation again it isn't any wonder that things come out back to front. A friend who speaks German has great difficulty in reading/interpreting technical German - it's another ball game altogether.

The front end signal processing you have partly described is in fact ABC&W. Yes, it's JR's principle and baby of many years. Looks as though the Patent has run out and its open season on ABC&W.
The front end schematic for 1995/7 Mini 9's and 1997 Micro 5/7's show identical circuitry to JR's ABC&W. Even the Multiplex 12ch DC receiver uses it.

It isn't possible for the IPD principle to control Sensitivity, Selectivity, Dynamic Range or Front End Bandwidth. Any reference to this must be taken as translation error.

ABC&W is an active form of monitoribg frequency selection whereas Dual Conversion is a fixed form of selection. An ABC&W receiver is very good at moving its centre frequency (Window) while still retaining its selectivity - ie. no need to retune when changing JR crystals, about fifteen spots either way (even at 20khz) and your still safe.

All other frequencies which will come in along with the one you want are all ATTENUATED, which means their strength will be very much reduced leaving the one you want shouting head and shoulders above the rest.

IPD and PCM are only as good as the FM circuitry in front of it.
DavidO
Old 06-04-2003, 11:44 AM
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onewasp
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Default Should PCM receivers really cost more?

The original post was " Should PCM cost more ?"

Cost has nothing to do with selling price, only the P & L statement.

In a free market price is set by the competition PERIOD . You either compete or you do not compete .

There are ways around this, ---- they just aren't legal ! Collusion and price fixing are examples there are others .
Old 06-04-2003, 12:39 PM
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David Cutler
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Default Should PCM receivers really cost more?

Originally posted by onewasp
The original post was " Should PCM cost more ?"

Cost has nothing to do with selling price, only the P & L statement.

In a free market price is set by the competition PERIOD . You either compete or you do not compete .

There are ways around this, ---- they just aren't legal ! Collusion and price fixing are examples there are others .
In that case, why aren't Ferraris sold at, say, $29 each to get more sales volume?

It's a lot more accurate to say the top boundary of the price is set by competition, and the bottom boundary by cost.

This especially applies to electronics, where the unit cost is almost negligible. For example, if you buy a faulty calculator and you send it back to the manufacturer, they will simply throw it away and give you a new one. It's cheaper for them to do that than to fix it.

If a manufacturer wants to produce a new product, using new technology, the finance man (who has the final word!) will ask only three questions. How much will it cost, how many will they sell, and what price can they get. If one of those doesn't fit the equation, it won't fly, no matter how cool or accessible the technology is.

In the case of using LAN technology in RC, all three answers will stop the project dead in it's tracks.

There are two provisos to this. If the technology can be applied from another area (where the costs have been covered already) and the system is backward compatible to some extent (that is, people can use some of their existing equipment) things might change.

If the technology is desirable, it might end up on the wish list, but it won't come true without acceptable answers to all three of the above questions.

-David C.
Old 06-04-2003, 03:48 PM
  #41  
onewasp
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You apparently don't understand P & Ls or their effect on product lines and profit planning as well as market share .

I spent a lifetime in this position for Fortune 300 Companies.

End of story .

Incidentally, as a Mfgr., Ferrari or otherwise , you can control demand to meet your production capabilities simply with pricing policy . We managed to stay that way with high ROI, ROE and Net Profit for years .
Old 06-04-2003, 04:08 PM
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Default Should PCM receivers really cost more?

wow
Old 06-04-2003, 04:20 PM
  #43  
David Cutler
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Default Should PCM receivers really cost more?

Originally posted by onewasp
You apparently don't understand P & Ls or their effect on product lines and profit planning as well as market share .

I spent a lifetime in this position for Fortune 300 Companies.

End of story .

Incidentally, as a Mfgr., Ferrari or otherwise , you can control demand to meet your production capabilities simply with pricing policy . We manager to stay that way with high ROI, ROE and Net Profit for years .
I most certainly do understand them. You must have misread my posting. We could trade experiences if you like. I'll start by saying I worked in manufacturing for 1/3 of a century, Does that give me a start?

Of course you can control demand. That's obvious.

What happens, however, if the price that the market can stand, at the required volume, isn't high enough to sustain the profitability?

That is, when your ROI is too small?

THAT'S the problem that many of the old established industries have been up against in the past few years. I know many people who would love to have the easy situation you describe (about controlling volume with price and still staying profitable) but the extreme competitive situation has driven them way past that ideal position.

It comes to us all.

-David C.
Old 06-04-2003, 05:06 PM
  #44  
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Default Should PCM receivers really cost more?

With regard to the resolution of an IPD receiver, I have evidence here that is does resolve as low as 1 microsecond.

The signal was generated by an Mpx 4000, with the standard Mpx 1100microsecond end to end travel. Mechanical trim sensitivity set to 2% end to end thus covering 22microseconds, with 41 steps thus each click of trim is worth 0.53microseconds. In a perfect world anyway.

Receiver used is an IPD 12 channel.

Measurement of the signal time being output to a servo from the IPD comes from the TEMS ECU of a PST gas turbine.

With stick at centre and trim to one side the time reading was jittering between 1536 and 1537 microseconds. One click of trim made no change, next click jittered 1537-1538microseconds, next was steady on 1538 microseconds, then 38-39 microseconds, then 39-40microseconds, steady on 40 microseconds, jitter 40-41 microseconds, jitter 41-42 microseconds, and so on.

Harry
Old 06-04-2003, 05:26 PM
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Default Should PCM receivers really cost more?

DavidO, I most definitely give way to your up to date and much superior knowledge of radios than mine. My last formal lessons were at the end of the 70s - technology, fashions and concepts have moved on much since then (in the opposite direction to my memory!), and I have no doubt that in me trying to re-interpret the already translated Mpx brochure I have managed to come up with some junk.

To Kris and everyone I would say that given PCM's superior way of holding on to good data where PPM's need for a complete clean cycle can lose a lot of good data, I would prefer PCM. My Tx can do PCM but since each PCM system is proprietary, and my proprietor no longer makes PCM Rx, I am a bit short of options. If a PCM system can get as good refresh rates now as PPM, and can come out of failsafe the moment it gets good signals again like IPD does, I would prefer PCM. But whereas there is a clear difference between plain PPM and PCM, IPD closes up the gap considerably and in the real world away from laboratory numbers, flying at the airfield there is very little practical difference between them in the users hands.

Harry
Old 06-04-2003, 06:12 PM
  #46  
Luke 3D
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Default Should PCM receivers really cost more?

Harry C
Sorry, i bet this sounds very stupid, but if it resolves as low as one microsecond does it make it good or bad in comparison to PCM?
The only reason i am asking this is because i am planning on fitting my Mini DS IPD reciever to my funtana in conjunction with really expensive Hitec digital servos. I want to get the best out of these servos obviously, so will IPD affect their preciceness? If this is the case i think i might go back to PPM, because of being told to stay away from PCM.
Thanks
Luke
Old 06-04-2003, 06:42 PM
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HarryC
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Default Should PCM receivers really cost more?

Luke, I may get corrected for being too simplistic but in my calculation that makes it fairly much equivalent in resolution to a 1024 step PCM unit.

The servo travel covers 1100 micro seconds using Mpx Tx normal 100% timing. My Tx appears to be capable of sending at least 1 micro second steps, judging by the way the times did not always make a full 1 microsecond increase. The Rx certainly appeared to resolve in 1 microsecond steps. Therefore it does appear to resolve every single one of those 1100 microsecond steps from one end of travel to the other.

That is normal Mpx travel, an Mpx, JR or Futaba in PPM at max travel covers 1200 microseconds, and the Mpx Tx and Rx seem to resolve to each 1 of those 1200 steps in comparison to the 1024 steps of the best PCM. If it is that simple, then the PPM system is giving nearly 20% finer resolution than 10 bit PCM.

H

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