Suggestion for depth control for amplitude modulation?

I'm using a multiplying DAC in order to apply amplitude modulation to an audio signal. The DAC is 8bit so I am not sure this is something I can do in the digital domain with good results, perhaps in analog? Please give me your suggestions.

The output code to the DAC is generated using the top 8 MSB from a 32bit phase accumulator which is added a certain value at a fixed time interval. The added values corespondent to different modulation freq. The said top 8 bits are used as index in an 8bit LUt with sine wave values between 0 and 255.

I used an analog multiplier chip back in college:
http://www.analog.com/en/special-linear-functions/analog-multipliersdividers/products/index.html

Are you suggesting using it in conjunction with the MDAC? It seems like it can do without the MDAC altogether... however I do have the circuit setup using it. So I do appreciate your comment but I would prefer suggestions using my existing setup.

It would replace the DAC. You are the one who asked for suggestions "perhaps in analog", not me.

The DAC is 8bit so I am not sure this is something I can do in the digital domain with good results

In the digital domain it would require you to do a multiplication every sample. This will slow down the carrier frequency greatly.

The other option is to have another D/A being fed with the modulation waveform and feeding that into the reference of the first.

This is normally the preserve of high speed signal processing CPUs, of which the Arduino is not a member.

I actually do have a second DAC available (it's dual channel device). So you are suggesting to feed the signal into one channel and feed that into the 2nd channel? Won't that simply act as a volume control?

If you use the audio output as the Vref into the 8-bit DAC, won't that yield the desired amplitude modulation of the audio signal?

Won't that simply act as a volume control?

Yep that is what amplitude modulation is.

If you can waggle the volume control fast enough you get a more recognizable image of amplitude modulation.

Applying the modulation itself to the signal is not the problem... The challenge is controlling the depth of the said modulation (in % if you wish).

Jonny_O:
Applying the modulation itself to the signal is not the problem... The challenge is controlling the depth of the said modulation (in % if you wish).

So why did you not say that in the first place?

It is trivial, you change the amplitude of the modulating signal.

Yes, that is the goal. The problem is that the modulating signal's amplitude is set at 100% because it is done digitally by loading values from a sinus look-up table. I could reduce the actual value using arithmetic but if there is an analog way to do it might be better.

because it is done digitally by loading values from a sinus look-up table.

A sinus table is nothing to be sniffed at. ( spelling joke from a dyslexic :slight_smile: )

Just use another table for the modulation's look up table!

I'm not clear what the signal path is here. You are synthesizing an audio
signal in software and passing that to the DAC? In which case what is the
analog modulating signal coming from? Is that the envelope or are you
ring-modulating or what?

A diagram is worth 2^10 words you know!

A diagram is worth 2^10 words you know!

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