What has to be done in order to multiplex several oscillators' outputs onto one line? Do you just feed them to the line one at a time - really fast?

For example, let's say I have four signals, all oscillating at different frequencies, completely independent of each other. If I connect the oscillators' outputs to the collectors of four respective transistors, the bases to the outputs of a 2-4 decoder (the inputs of which are connected to a binary counter which is constantly counting), and the emitters to a common line...

Then (I'm pretty sure) the line would first send the first signal, then the second, third, fourth, and finally, loop back to the first in an endless cycle. My questions are:

1. If I hooked this all up to a speaker driver, would I hear all of the frequencies at once, overlapping each other in polyphony?

2. Let's say I'm using voltage controlled oscillators. Is there any reason I can't just multiplex the inputs of each oscillator onto one line instead of the outputs? - That way I will only need one VCO.

3. How do I determine the frequency to multiplex the signals? - That is, the frequency of the binary counter's clock. How fast do I switch from signal A to B to C to D, and back again? If that rate is higher than 20 khz (the highest audible frequency), will I still be able to hear the sound?

 

Why multiplex them? It sounds like you simply want to mix them.

Bob

 

Why multiplex them? It sounds like you simply want to mix them.

Bob

What is "mixing" and how is it typically done?

 

Mixing is the addition or summing of your four audio frequency signals. Typically this is done with an op amp configured for addition, assuming that the level of each of the AF signals is no more than a few volts. That is, each signal is present on the common line simultaneously and constantly.

 

Mixing is the addition or summing of your four audio frequency signals. Typically this is done with an op amp configured for addition, assuming that the level of each of the AF signals is no more than a few volts. That is, each signal is present on the common line simultaneously and constantly.

Wouldn't feeding the signals to an op amp give me the difference between the two signals? Two signals being combined into a monophonic one?

I mean, yes both signals would be present, but I want them to sound as if they are independent of each other, as if I held down two keys on a piano at the same time.

 

This is the diagram of a summing amplifier:

http://www.electronics-tutorials.ws/opamp/opamp_4.html

You can connect an arbitrary number of inputs to the summing junction.

A multiplexer would select one of N signals to be passed to the output. Phase coherence is a problem with such a circuit unless the switching occurs at the zero crossings. If it occurs at other places you can hear audible clicks and pops.

 

but I want them to sound as if they are independent of each other, as if I held down two keys on a piano at the same time.

Trust me, there's no multiplexing in that scenario. Simple analog summing (mixing) like the others have described is what you want.

 

Wouldn't feeding the signals to an op amp give me the difference between the two signals? Two signals being combined into a monophonic one?

No.
An op amp does the linear addition of the two frequencies, which creates no new frequencies. The mixed signal would sound the same from one speaker as if you sent each frequency to separate side-by-side (identical) speakers
To get a sum or difference frequency requires a nonlinear circuit, such as a multiplier or diode mixer.

Unfortunately "mixing" has two different and diametric meanings in electronics.
In audio circuits it refers to the adding of the signals in a linear manner, such as in an audio mixing console.
In RF circuits it refers to the non-linear multiplication of the signals to get sum and difference frequencies.

 

Wouldn't feeding the signals to an op amp give me the difference between the two signals?

It would if the opamp were configured as a difference amplifier, amplifying the difference between the two signals.

I mean, yes both signals would be present, but I want them to sound as if they are independent of each other, as if I held down two keys on a piano at the same time.

Another opamp configuration is a summing amplifier, adding one signal to the other *continuously*. This is exactly what your ear does when presented with the continuous signals from two piano keys simultaneously.

To your original question, yes, multiplexing two signals very quickly can sound like they are summed, but the multiplexing must be very rapid, at least twice the highest audio frequency or harmonic (Shannon Sampling Theorem). Much more rapid multiplexing, such as 5 or 10 times the audio frequencies, will reduce the audible distortion caused by the rapid switching. However, this method never will sound as natural as simple analog summing.

A more common use for analog multiplexing is to send multiple analog signals over a single wire, but keep them separate. For example, hundreds of separate telephone signals can be multiplexed onto a single wire or channel on one end, and separated into the original separate signals on the other end of the channel, in a way that makes it virtually undetectable by the user. Harry Nyquist and Claude Shannon, both mathematicians for Bell Labs, worked out the theory and design math in the early 20th century. That theory is the core of all modern communications systems.

ak

 

A more common use for analog multiplexing is to send multiple analog signals over a single wire, but keep them separate. For example, hundreds of separate telephone signals can be multiplexed onto a single wire or channel on one end, and separated into the original separate signals on the other end of the channel, in a way that makes it virtually undetectable by the user. Harry Nyquist and Claude Shannon, both mathematicians for Bell Labs, worked out the theory and design math in the early 20th century. That theory is the core of all modern communications systems.

ak

That's wild! What kind of frequencies do they switch at, and how many separate analog signals can they typically squeeze out of each line with this multiplexing arrangement?

 

It would if the opamp were configured as a difference amplifier, amplifying the difference between the two signals.

Another opamp configuration is a summing amplifier, adding one signal to the other *continuously*. This is exactly what your ear does when presented with the continuous signals from two piano keys simultaneously.

To your original question, yes, multiplexing two signals very quickly can sound like they are summed, but the multiplexing must be very rapid, at least twice the highest audio frequency or harmonic (Shannon Sampling Theorem). Much more rapid multiplexing, such as 5 or 10 times the audio frequencies, will reduce the audible distortion caused by the rapid switching. However, this method never will sound as natural as simple analog summing.

A more common use for analog multiplexing is to send multiple analog signals over a single wire, but keep them separate. For example, hundreds of separate telephone signals can be multiplexed onto a single wire or channel on one end, and separated into the original separate signals on the other end of the channel, in a way that makes it virtually undetectable by the user. Harry Nyquist and Claude Shannon, both mathematicians for Bell Labs, worked out the theory and design math in the early 20th century. That theory is the core of all modern communications systems.

ak

Ok, now I understand, thank you!

 

That's wild! What kind of frequencies do they switch at, and how many separate analog signals can they typically squeeze out of each line with this multiplexing arrangement?

The direct multiplexing of analog telephone signals was around from the '20s to the '60's, but I don't remember the stats.

https://en.wikipedia.org/wiki/Time-division_multiplexing



ak

 

Why multiplax them?

 

Why multiplax them?

To transmit more than one signal over a single wire.