I know that ladder filter existed for a while before Bob Moog was a student. I know that he patented his design of ladder type filter. I know that multiple manufacturers were either ripping off his patented design or building very close visually at least own variants. What I do not know is why there are barely any other ladder filters (I only know about Parker-Steiner filter that has at least some ladder like structure) being used in analog sound synthesizers? Was it because of lazy engineers of other companies or because it is hard to build ladder filters for analog sound synthesizers that do not look like design patented by Bob Moog? I have seen some old AT&T ladder filters with vacuum tubes that were used for telephone signals and they visually on schematics seems to have at least some similarity to Moog's filter. I however have no knowledge on other old designs of ladder filters prior to Moog's. I also have no knowledge on designing analog ladder filters. I am not familiar with published pre Moog's filter construction literature on analog filters. I don't know how obvious or how non trivial it was to design Moog's filter. Could you provide any comment on such topic? Any suggestions or other thoughts on that topic would be greatly appreciated.

 

To me it is not clear what you are asking for. Which "topic" are you interested in?

 

Please post a schematic or a link to a reference article or something else so we have some idea of what the topic is.

ak

 

It is hard to obtain a historical perspective on things that happened before our time (ca. 1966). The concept of using a ladder topology was not the key idea in my limited understanding of the matter, rather it was using the resistance of the base emitter-diode of a bipolar transistor. There is also the matter of analytical and computation resources available at the time. Today there are many alternative designs that produce arguably better results. As always, the "golden ears" will NEVER be satisfied

I said in a previous thread, that getting legal advice here is a suboptimal proposition. You need to consult a patent attorney if you're as worried about it as you seem to be.

 

OK, here is an article that contains schematics of Moog's filter:
https://www.allaboutcircuits.com/technical-articles/analyzing-the-moog-filter/

One of patent's: https://patents.google.com/patent/US3475623?oq=3,475,623

As for patent - I'm not worried about, neither interested in legal matters. I'm interested on this topic in purely technical engineering level. I'm not going to start a company building filters in nearest months and my current job is in completely different domain.

 

Looks like I should ask elsewhere.

 

The circuit is basically a string of single-pole R-C filters in series. This has a very rounded attenuation curve, versus sharper cutoff corners with other filter types. Another reason for this not being a popular approach is that it has a lot of parts compared to other filter topologies. Also, it has issues.

One, there is bleed-through from the control signal to the audio signal. As the control voltage varies, it modulates both the signal amplitude and its baseline. To keep this from affecting the output signal, there must be frequency separation between the audio signal bandwidth and the harmonics of the changing control signal.

Two, harmonic distortion. The circuit uses the *effective* resistance of a diode (a transistor base-emitter junction) as the variable resistor in each filter stage. At the time the circuit was developed, diode switches and attenuators had been around long enough to be well understood. This "resistance" is a function of the instantaneous voltage across and current through the diode. That means that the resistance changes not only with changes in the control voltage, but also as the audio signal goes up and down. Now there is a resistance in the circuit that varies throughout each cycle of the audio, distorting the original waveform. This might be OK for synthesizer tones, but not for real audio.

ak

 

Thank you for your comment.