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  • MOS1 versus MOS2 versus ADC versus MDS

    Could someone knowledgeable detail out the major differences in tone generation between the Allen MOS1, MOS2, ADC, and MDS organs? For instance, what was the evolution of chiff generation between the systems? The MOS1 didn't have it, whereas ADC had it on a few major stops along with the infamous "Chiff" stop, and MDS seemed to have it on all flue stops, as well as an evolved envelope generator.

    Also, were there variations within ADC and MDS in terms of developments and changes throughout the years within the series?

    Any nerdy stuff like that, I would love!

  • #2
    I'll answer to "nerdy" though others are more "qualified" than I in the nitty-gritty of these organs. I think I can summarize some of the evolution for you, and others can fill in the blanks.

    MOS1 - the original digital used a primitive one-wave-shape recipe for each stop and the tuning of all stops and divisions was locked to a single clock. The multiple computer models had each computer individually tuned, so there were subtle offsets, and a given computer could become "celeste tuned" which is to say "sharp" and create genuine celestes. While there are obvious limitations and drawbacks, these old organs sounded amazing (to me) at the time, more like pipes than any non-pipe organ I'd ever heard. Some that I service today still sound incredibly good in lively acoustical environments.

    MOS2 - upgraded MOS design with certain of the MOS1 options now being standard, such as "frequency separation" -- a subtle tuning offset between the two channels of a single computer, not as interesting as the multi-computer models, but made the entry level organs sound better than in MOS1. By this time, the DAC board had been refined and produced less noise in the output. Other enhancements as well, but nothing tremendous.

    ADC - radical new design that sounds far better to my ears than the best MOS. Still just a single-wave per stop, but each group of 6 or 8 stops has its own tuning reference for a much more lively ensemble. Each stop group has its own tone and level controls for much more flexible voicing. Envelope generator boards provide something of a unique attack/decay for individual stops. Improved "random motion" adds a believable "jitter" to each pitch to suggest that you are hearing "long" samples, though you are not. Also, divided expression appears on all but the dinkiest models, since expression is now done in the cage instead of at the amps. Decent chimes and other percussion effects are now available. The card reader is more useful, becomes a "floating division" of sorts.

    There are several versions and iterations within the ADC era, and at the end there are models with "articulate voicing" which is to say air sounds and attack sounds that are separately generated and much more realistic than anything previously used.

    MDS - began as an updating of ADC, with the capture action and keying control moved to a single board called the "MN" board, which had ribbon cables and other connectors running to the entire system. This new integration of console functions allowed MIDI to be fully implemented for the first time. The cage and cards were almost identical to late-ADC. Before long, MDS was further updated and completely new tone generator systems such as "W-4" and "W-5" appeared that displaced the old ADC-style cages. These systems had better-sounding samples (true long samples?) and "wind" that was somewhat better than the "articulation" of ADC, but still a separately generated effect.

    RE: "chiff" over the years -- MOS1 had a very primitive chiff "stop" on the swell that was a sort of percussive whistle-like tone. Larger MOS organs used the percussion effect to "pulse" a high harmonic on each attack for a little better effect. MOS2 has a better chiff, but still quite phony-sounding. ADC started out with a chiff not much better, but the "articulate voicing" of late ADC and early MDS was a lot better and completely adjustable. Later MDS was simply a refinement of this, but sounded better and better as they worked on it.

    Only with Renaissance did truly realistic attack sounds appear, and that is because they are actually recorded attack sounds (at least I think they are real). Finally we have actual long-sample recording and playback, and the sample includes the attack. The only drawback is that the chiff is no longer adjustable, it just is what it is.

    Somebody else needs to correct and flesh out this poor description. I know that several forum members are very knowledgeable about the technical details and can explain these things much better than I can.

    John
    John
    ----------
    *** Please post your questions about technical service or repair matters ON THE FORUM. Do not send your questions to me or another member by private message. Information shared is for the benefit of the entire organ community, but other folks will not be helped by information we exchange in private messages!

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    • #3
      Quoting from the MOS 2 service manual:

      "The MOS 2 system is a refinement of the original MOS system with updated hardware. The theory of both systems is the same..."

      The following extra cost features of MOS 1 are built into the MOS 2 computer:
      • Delay - the Main channel can be delayed relative the Flute channel. The delay is continuously adjustable instead of the two positions of the previous delay.
      • Frequency Separation - is on all boards with the Main channel voices being tuned slightly sharp relative the Flute channel. The degree of separation is adjustable by means of jumpers
      • E-Proms - formally optional for the 16' chip are standard for it and the Mixture chip as well allowing custom programming of the voices specified by the customer.
      • Slow Pedal - slows the attack rise time on all pedal voices.
      • All MOS 2 organs are equiped with Card Readers.
      Other board changes:
      • The Stopboard Array (SBA) combines the function of the Stop Board Array and the former Clock and Card Reader logic boards. The tuning control is located on the SBA. Isolation between computers is provided on the SBA so that Tone Strip diode isolation is no longer required.
      • Keyboard Array (KBA) The former Tremulant Gate and Random Motion boards are eliminated. The Random Motion function is included on the KBA. Trem Gate boards are only required on multiple computer models. Vibrato and sustain length controls are included as previously. Vibrato amplitude can be adjusted via jumpers. A plug-in chip is now provided for tuning functions. Stretch tuning is standard, with Tempered Tuning provided detuning for the Celeste function.
      • Speech Articulation is no longer continuously variable but is settable via jumpers on the KBA
      • On multiple computer models the KBA 2A board is used with the multiplexing circuitry being moved to the SDDS MUX and SDDS MOS boards.
      • Delayed Vibrato Unit (DVU-2) is used on Theatre models.
      • Card Reader uses -16 VDC rather the -5 VDC and is not interchangable with MOS 1 models.
      • DAC 4 includes an Attenuator circuit which was a separate unit in previous models.
      • Audio Line Driver is a standard item on larger models serving as an audio buffer to minimize high frequency loss between the console and amplifier rack by providing a 1K output impedence as opposed to the 22 K impedence of the DAC.
      -Admin

      Allen 965
      Zuma Group Midi Keyboard Encoder
      Zuma Group DM Midi Stop Controller
      Hauptwerk 4.2

      Comment


      • #4
        allen tech progression

        The main thing I'd correct is you almost make it sound like there might have been very early MDS organs that were ADC boards under MN control. Although that theoretically would have been possible, I doubt there were such organs as the main point of MDS was to compete with the primitive 8 bit multisampled organs flooding the market at that time, like the A-G Praeludium series. When I first heard one, which was probably around 1990 I thought to myself how it sounded more realistic than a typical ADC Allen. They had recorded a lot of pipey articulation sounds, and Allen had to have something on the market to match it in a standard model and not the custom TT-4 boards of the 1980s. Hence the accusation that the early MDS (which were 16 bit...burr brown PCM53JP chips) sounded too articulate. But the MDS-45 for example did have a single ADC (TG10) to provide chiff _even beyond the chiff built into the multisample_! If you think...that must have made the organ insanely chiffy...you're right. It would have been almost as annoying (even fully turned down) on hi-rez speakers in a small living room as the ADC series 8 bit issues.* However, in a church setting I'm sure it sounds wonderful if adjusted correctly. That's one reason why I passed on the organ. The quality of the samples, though, is definitely superior to ADC as they are 16 bit, very clean, with none of the phase distortion issues we are currently discussing in another thread. Read more about early MDS here: http://www.organforum.com/forums/sho...357#post212357


        * this delta (as we say in the IT field when we are being pretentious) between a digi-org in its intended installation space and one in a home setting is one I'm realizing is incredibly important, yet something we haven't discussed much on the organforum. When I played the MDS I looked at, it had already been removed from a performance space and I listened to the organ's output directly via headphones coming through a Mackie mixer I brought for audition & testing. (speaker wires had been cut and it would have been too tedious to get everything hooked back up correctly) But I've heard an MDS organ in a church and it sounded great...as have various ADCs I've heard over the years. The experience of your ear being 5 feet away from even a mediocre speaker is just going to be different from your ear being 50 feet way. I'm now thinking I'd be cautious about even going the Hauptwerk route w/o hearing at least one home installation and being impressed with it. The issue with HW being that they usually close sample the pipes, which, again, puts their virtual placement way closer to your ear than they'd actually be in reality. Actually I think what would be ideal for me would be a used early Renn. organ, where, I believe, DOVE will allow the voicer to "dial down" the chiff to a point where its still noticeable, but not annoying. I don't think you can do something like that on HW, but maybe you can now. My general point here is that having a fantastic sounding home installation in a 18X27 living room with an 8' ceiling is probably a far greater acoustic challenge than making a church space sound good.
        Last edited by circa1949; 08-01-2010, 08:42 AM.

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        • #5
          "Audio Line Driver is a standard item on larger models serving as an audio buffer to minimize high frequency loss between the console and amplifier rack by providing a 1K output impedence as opposed to the 22 K impedence of the DAC."
          Interesting. I wonder if this means large MOS-2 organs could use standard line level reverb units without the issues that other MOS organs have using them.

          Comment


          • #6
            Originally posted by circa1949 View Post
            "Audio Line Driver is a standard item on larger models serving as an audio buffer to minimize high frequency loss between the console and amplifier rack by providing a 1K output impedence as opposed to the 22 K impedence of the DAC."
            Interesting. I wonder if this means large MOS-2 organs could use standard line level reverb units without the issues that other MOS organs have using them.
            The typical input impedence of most audio gear is 10K or greater. Rule of thumb is that input impedence should be 10x or more greater than the output impedence, so there's no problem interfacing to other gear with this driver.
            -Admin

            Allen 965
            Zuma Group Midi Keyboard Encoder
            Zuma Group DM Midi Stop Controller
            Hauptwerk 4.2

            Comment


            • #7
              Circa,

              As always, you have a much larger grasp of the details that I'd ever hope to. Thanks for the additional info. Admin, I'm glad you copied that material from the MOS2 manual. I had forgotten just how many of the MOS1 options became standard on MOS2. When I service and play a MOS2, I generally feel that it sounds and works somewhat better than a MOS1, though the differences are fairly minor.

              I agree that hearing an organ in a home setting is far different from hearing the same organ in a church or hall. The contributions of natural reverb, random delays, and speaker-distance from the ears are huge. Headphone listening would seem to be the worst possible situation, as there is nothing between the speakers and the eardrums to mellow and flatter the sounds. I've had pretty good luck using Alesis reverb units in my various home installations, but there really isn't any replacement for the real thing when it comes to acoustics. But we can still enjoy a home organ if we try!

              The other thread you mentioned is one I had not read, and I enjoyed the observations about early MDS vs. ADC organs. I certainly don't know much about the technology, but the early MDS (larger models anyway) have a cage that looks just like ADC and some of the boards appear to be very similar. Reading the sales literature, though, it seems that Allen was definitely trying to give the impression that MDS was an all-new technology, and perhaps it was. In church installations, I don't hear much difference between late ADC and early MDS, but by the W-5 era MDS really came into its own and some of those organs still sound so grand that one would be hard-pressed to hear anything better from even the current Renaissance and Quantum models.

              BTW, I think the card reader did finally disappear with the W-5 models. IIRC, both early MDS with big cages and the W-4 models had card readers, or at least the option. I think you are correct that the W-4 card readers used MADC cards while the big MDS models used ADC cards.

              John
              John
              ----------
              *** Please post your questions about technical service or repair matters ON THE FORUM. Do not send your questions to me or another member by private message. Information shared is for the benefit of the entire organ community, but other folks will not be helped by information we exchange in private messages!

              https://www.facebook.com/pages/Birds...97551893588434

              Comment


              • #8
                Originally posted by jbird604 View Post

                I agree that hearing an organ in a home setting is far different from hearing the same organ in a church or hall. The contributions of natural reverb, random delays, and speaker-distance from the ears are huge. Headphone listening would seem to be the worst possible situation, as there is nothing between the speakers and the eardrums to mellow and flatter the sounds.
                True indeed. Although I had so much experience w/listening to my ADC-1140 through both headphones and speakers that, when it came to listening to an MDS, I could tell...ok...the sounds are much much cleaner, however the chiff even turned down all the way was louder than an MADC3 with the chiff stop turned "on". And more realistic...yes...but only marginally more "pleasant". Listening w/o reverb was the acid test to really assess the tone quality of the instrument. It is a definite improvement over ADC but I heard the excess articulation issue and knew I'd find it distracting.

                Originally posted by jbird604 View Post
                The other thread you mentioned is one I had not read, and I enjoyed the observations about early MDS vs. ADC organs. I certainly don't know much about the technology, but the early MDS (larger models anyway) have a cage that looks just like ADC and some of the boards appear to be very similar. Reading the sales literature, though, it seems that Allen was definitely trying to give the impression that MDS was an all-new technology, and perhaps it was. In church installations, I don't hear much difference between late ADC and early MDS, but by the W-5 era MDS really came into its own and some of those organs still sound so grand that one would be hard-pressed to hear anything better from even the current Renaissance and Quantum models.

                BTW, I think the card reader did finally disappear with the W-5 models. IIRC, both early MDS with big cages and the W-4 models had card readers, or at least the option. I think you are correct that the W-4 card readers used MADC cards while the big MDS models used ADC cards.
                Yeah I think you are probably right on all these points...if there was negative reaction to the excessive chiffiness and sag of the W4 models, and I'm told there was, it makes sense they would have refined things a bit for the W5 version. What I couldn't know is whether the organ I saw had the amended memory chips or not...everything was dated 1991 even though the copyrights on the boards (W4) was 1989, so it might have been integrated into the line by then.
                I've been told by an Allen tech who covers a large geographical area that the best sounding Allen he's ever heard was a 3 cage, W5 based custom model with many channels of audio. This corroborates your point about the best of the MDS line holding their own against the current Quantum organs.
                Last edited by circa1949; 08-01-2010, 05:25 PM.

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                • #9
                  You guys are great: this is really good stuff so far!

                  A couple questions:

                  1. How do you tell which MDS organs are W5 from model numbers? When did that generation take over? I tend to think that the early MDS was almost more a marketing response to the new digital organs coming from Rodgers and the Europeans in 1989-91 than it was a bonafide generational technological advance.

                  2. Can someone elaborate on the ADC-era TT4-based Classic I and custom organs, and how they differed from stock ADC in terms of tone production and audio output and channelization? How many voices per card, and how many audio channels did this typically mix to?

                  I played a 40-stop 2-manual TT4 custom as a sub a bunch of times at an Episcopal church in Suffern, NY about 10 years ago. It was a remarkable sounding organ. The only place it really sounded "electronic" was the chiff and wind generation on the principals when you played them solo. The ensemble was pretty amazing.
                  Last edited by michaelhoddy; 08-01-2010, 08:59 PM.

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                  • #10
                    John,

                    Regarding the ADC technology, didn't Allen go to 4 waveshapes per stop across the keyboard. I realize it was still single cycle like MOS I and II. If they didn't go with 4 waveshapes with the ADC series, at what point did they?

                    AV

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                    • #11
                      Michael,

                      I can't help you on your question #1.

                      On #2, I don't know everything either. But, I have read this. The TT4 based organs, had 2 to 4 stops per board. I believe each stop had 4 waveforms (at least the manual stops) across the keyboard. The articulation
                      was sort of a primitive sampling. Each board would have been fairly independent, so, there would have been frequency separation between stops, moreso than standard models. Typically, the audio was more than double standard models. Also, the data, the clocking, the implementation in the card cage was different from standard organs. So, if you found the TT4 based Custom sounding much better than a typical Allen, you heard correctly. I don't think Allen really made very many of these organs though. I would think that some of these organs would still sound very respectable today, although technology supposedly has advanced and today's instruments should sound much better, many of them don't sound any better than one of these well installed, well adjusted organs.

                      AV

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                      • #12
                        Originally posted by arie v View Post
                        John,

                        Regarding the ADC technology, didn't Allen go to 4 waveshapes per stop across the keyboard. I realize it was still single cycle like MOS I and II. If they didn't go with 4 waveshapes with the ADC series, at what point did they?

                        AV
                        Only on certain stops where it was deemed necessary. Even on a large, late model like the ADC-5300 it wasn't the majority of the stops by any means. You can tell by the # of memory slots a voice takes on the voicing chart.
                        Still, they sounded pretty good. ( BTW I've had this assumption corroborated by a still current tech.)

                        " I tend to think that the early MDS was almost more a marketing response to the new digital organs coming from Rodgers and the Europeans in 1989-91 than it was a bonafide generational technological advance."

                        No, see my other posts. Real multisamples, much more memory, 16 bit burr brown DACs versus 8 bit PMIs. Full MIDI with a real computer controlling the console versus the "discrete logic" setup of an ADC organ. As for how things advanced between W4 & W5, it was much less a change than late ADC to MDS. The quantum leap...no pun intended...was Renaissance with full software synthesis on SHARC or possibly Motorola RISC (early) chips. Given the main complaint - so I'm told - about MDS at the time was being too articulate, you'd assumed they tamed that in W5 or made it somehow more sophisticated sounding.

                        W4 boards would be labeled W4 & W5 would be labeled W5. I think the "Girandot e-org list" split some of the MDS series by system...or did it? I think about the first 3-4 years was W4...so 89 to 93?
                        Last edited by circa1949; 08-02-2010, 08:41 PM.

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                        • #13
                          Arie,
                          I remember briefly getting to play a TT4 based Classic I in the late 80s. This was not when I visited Allen but at JK's Washington DC showroom that had a huge room devoted to Allens. (now they have a room about the size of a Dr.'s office waiting room! Sign of the times!) Man, that thing was chiffy! And buzzy! But it sounded incredible. I think a bit of JK marketing lit. said it had the same amt. of memory as an ADC-6000. So probably roughly 4X the memory on a per stop basis. It also had something like 16 channels of audio right?

                          "Also, the data, the clocking, the implementation in the card cage was different from standard organs." You could be right but I know I've read that standard ADC models could be augmented with TT4 boards for special stops (Walter said so, I think)...so there must have been an "ADC backplane compatible TT4" system at some point.
                          Last edited by circa1949; 08-02-2010, 08:45 PM.

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                          • #14
                            You guys know more than I do about the numbers of wave-shapes per stop, and so on. My statement about the ADC was only meant to say that ADC was not that different from MOS in the way sounds are reconstructed. Far as I know, an ADC stop is stored as the same general kind of sample as a MOS stop, which is to say a numeric description of a single wave. Some of the more complex stops probably had more than one wave to give the sound more appropriate character along the ranges of the keyboard. I could be wrong about that, as I don't have the technical background that circa and some other posters have. I'm not sure at all about MDS, whether the one-wave model continued to be used or if there was some sort of true audio sample with looping. I don't actually hear that much difference, and I assume that the wind noise and "chiff" of MDS continued to be an artificial effect as it was in ADC, since it is controllable by a simple pot on the generator board.

                            I'm quite sure that Renaissance technology is Allen's first usage of real "sampling" (by which I mean recording the actual start-up sound of a pipe as wind enters the foot, then the steady-state tone for a significant period of time, followed by the decay as the wind is stopped). These raw samples are processed to regularize tuning and tone color, but are much less "artificial" than the single-waveform recipes of the older technologies. The stored samples, normaly between 5 and 10 per stop spread across the keyboard range, are then pitch-shifted and properly looped by the organ's playback circuits to create the necessary 61 notes of a rank.

                            Does this different technology make Renaissance organs sound far more realistic than MDS? Not necessarily. I have heard examples of all the Allen technologies in highly-reverberant churches that could fool almost anybody (other than the members of this august forum) into thinking they were hearing pipes. When you get right down to it, the "source" of the tones are not of extreme importance. Even the old MOS technology, when properly applied in a well-designed and properly installed organ, can create the requisite tone colors. If the audio equipment is adequate to the task and the room is extremely favorable, you will hear amazingly "pipe-like" sounds!

                            I hope this thread continues to generate interest. I am enjoying every bit of new knowledge. Thanks, all!
                            John
                            John
                            ----------
                            *** Please post your questions about technical service or repair matters ON THE FORUM. Do not send your questions to me or another member by private message. Information shared is for the benefit of the entire organ community, but other folks will not be helped by information we exchange in private messages!

                            https://www.facebook.com/pages/Birds...97551893588434

                            Comment


                            • #15
                              John,

                              I doubt there are many or even any that know as much about these older Allen technologies. Anyways, I do believe you are correct in most of what you say.

                              Allen's various improvements with new technologies, seemed to me to be rather minor until the late ADC series. MOS-1 and MOS-2 were pretty much the same, with Allen cleaning up the tone somewhat over time from 1971 to 1983. I believe MOS-2 came out in 1981, but was short lived as Allen ran into problems with RF emissions with the FCC. They had to redesign, and came out with the ADC organs, which were packaged in metal cages. They were better, but to me they had the "Allen" sound to them. When they went to the late ADC, the organs suddenly sounded much more "organ-like" rather than "computer like" or "Allen digital". Especially if they had a fair number of resources. To me the MDS series was sonically not much of an improvement, although again Allen improved them over time. To me though, even the MDS organs sounded like Allen organs, as opposed to say a Rodgers or a Galanti or whatever. There was an almost signature sound to them. When Allen went to Rennaisance technology, things changed again, and they went to software control. Still, in many cases, you could hear a lot of stops that sounded very much like MOS-1/2, ADC organs. To me the Allen folks seemed content in regurgitating their sound library with the same old root samples, just processing them differently.

                              Some things I admired about Allen organs over the years, was their good build quality, and with the MOS-1 and MOS-2 organs, they got the relative balances correct for the stops. Good thing too, as really they were not voiceable. Considering how primitive the tone generation and how small the wave shape data was, it is amazing the MOS-1 and MOS-2 and early ADC organs sounded as good as they did.

                              I believe when the Galanti Praeludium II came out in late '86, that is when people for the first time heard the potential of what sample playback was all about. The basic tone, especially of the Principals and Flutes improved dramatically. I recently serviced a bunch of them, and am surprised still at the pipe-like quality of the basic tone.

                              Of course today, most products sound even better, and digital organs, when properly setup are more pleasant on the ear and a better substitute for a pipe organ.

                              AV

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