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Topic: Mirroring baseband using modulator? (Read 1639 times) previous topic - next topic
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Mirroring baseband using modulator?

Hello all,

I'm trying to acheive a reflection of baseband audio, similar to the picture below during playback.


Modulation appears to be the method creating this from what I've read, though there could be an alternative.
I obtained a modulator DSP, but have no idea how to replicate using it, any ideas?
Or perhaps use external dsp feedback loop in some way so that, the mp3 itself is untouched, but wav is placed on top?


example of dsp creating reflections, but 1st reflection too close to 22050.

If your curious as to why I want to do this,
This post is essentially a continuation this thread, more specifically this post.

Cheers

Mirroring baseband using modulator?

Reply #1 –
I believe the easiest way to recreate the aliasing is by just adding a zero for each X-1 that you do ( 2X? 1 zero each sample. 4X? 3 zeros each sample):
This is the first step of oversampling.

3, 5, 7, 9, 15  -> 4x -> 3, 0, 0, 0, 5, 0, 0, 0, 7, 0, 0, 0, 9, 0, 0, 0, 15, 0, 0, 0.


Intuition tells to just duplicate the previous value, as in 3,3,3,3,5,5,5,5.
The practice (and good theory) say that such thing is invalid, since those points were never sampled, so they are in fact zero. Once a filter is applied at the original sampling frequency, those values get the correct position, while if they had been just duplicates of the previous value, they would be causing distortion versus the place they really belong to.


Edit: related to the quotation in that post:

Quote
the image spectrum is folded which has the effect of de-correlating it from the baseband audio signal thus making it random for all practical purposes.
I guess it makes the implication that, since the signal is mirrored, it is not correlated in the sense that it doesn't change in the same way.
Then it mentions a gentle filter roll-off, in an attempt to use the aliasing as dither noise. It feels a bit weak, (but i don't have experience with hardware). What if I'm playing a bass? It's mirrored aliasing, if sampled at 44Khz, is around 43Khz.

Also, worrying about precision in the DAC on the LSB (least significant bit), but then allowing aliasing distortion to go to speakers, which are VERY prone to cause non-linearities and distortions *in the audible range* seems counterproductive.