Amplitude Modulation and Carrier Wave Addition

Special Interest Groups Audio Processing
1

Hi,

I want to reconstruct what is described in this section of a paper and because I don’t want to buy a signal generator I want to use Audacity to test for feasibility.

I have a mono track and used these two Nyquist Codes:
AM:

(mult s (hzosc 28000 *table* 90))

f_c is then 28000Hz and n_1 is 1 (looks good to me, nothing gets cropped), right?
CWA:

(mult 0.5 (sum s (hzosc 28000 *table* 90)))

f_c is then 28000Hz and n_2 is 2, right?

Somehow the resulting signal does not do what it is supposed to do.
Am I doing everything correct here? Some tutorials state you need to add 1 and multiply by 0.5 before applying AM.

Thank you!

2

(https://en.wikipedia.org/wiki/Amplitude_modulation)

If you want 100% modulation, then the “message” needs to have a range of 0 to 1.

If this is about “Silent Subliminals”, then this topic may help: https://forum.audacityteam.org/t/silent-subliminals-solved/20100/1
Also, note that sound cards are designed for humanly audible sound, so they are often limited to a maximum analog in / out frequency of not much more than 20 kHz (even if they support sample rates of 192 kHz or more).

3

Ok, so the nyquist code should be:

(mult (mult 0.5 (sum 1 s)) (hzosc 28000 *table* 90))

for a 0.5 modulation index?

4

With a 0 dB (full scale) “message” envelope, the modulation will be 100%
tracks000.png
By the way, in the current “version 4” syntax, that code could be written as:

(mult 0.5 (sum 1 *track*)(hzosc 28000))