Re: [Discuss-gnuradio] Help needed to obtain AM Depth of Modulation

I would start with

source -> low pass filter(complex) - rational-resampler(complex) -> am
demod -> number sink(float).

I have never used the AM Demod block and I have never calculated the
Modulation Index for AM.

But it does appear you have a plumbing problem.

I would start with changing the AM Demod so the channel capacity is
192000 Hz with Audio Rate of 480000 Hz.

Then set your re-sampler to decimate by 100000 Hz and interpolate by
192000 Hz.

And replace your number sink with Qt time sink and see what it looks like.

I haven't tried any of this - I'm making it up as I go along - based on
experience with FM.

-- Cinaed

On 05/10/2017 07:07 AM, Kevin Reid wrote:
> On Wed, May 10, 2017 at 3:31 AM, Matt Cooper <mattcoopernorton@gmail.com
> <mailto:mattcoopernorton@gmail.com>> wrote:
>
> Hi guys!
>
> I am trying to calculate the Depth of Modulation
> <https://en.wikipedia.org/wiki/Amplitude_modulation#Modulation_index
> <https://en.wikipedia.org/wiki/Amplitude_modulation#Modulation_index>>
> of
> an AM signal using GNU Radio, but I just can't make it work.
> ...
> - In GNU Radio, I'm using this diagram
> <http://i.imgur.com/YvYZtZ7.png <http://i.imgur.com/YvYZtZ7.png>> .
>
> I know the diagram is wrong (it's illustrative only), but it's
> exactly here
> where I need you help. What I'm trying to achieve is to obtain in
> GNU Radio
> the same depth of modulation as the one set in the signal generator. For
> instance, if the generator is set to a depth of 56% (just like in
> the first
> image), I'd like to get that exact(ish) same number in a GNU Radio sink.
>
>
> From one perspective, modulation index relates the strength of the
> carrier to the strength of the sidebands. Therefore, you must be trying
> to measure a /ratio/, but the flow graph you have there does not compute
> one — the output level will be proportional to the RF input.
>
> If you insert an AGC block before the AM Demod (which does not contain
> one; I just checked), then that will normalize the strength of the
> carrier, and the output level should be proportional to the modulation
> index.
>
> (Hm. That means that the AM Demod block does not do what its
> documentation claims it does unless the input is already normalized. I
> should file a bug about that.)
>
>
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