having some free days, continuing my work on an audio signal generator on a CY8CKIT-059. With help of Odissey and his components (DDS24 24-bit DDS arbitrary frequency generator component and hardware multiplier), I can output a waveform with variable amplitude. So, first many many thanks to Odissey and his valuable contributions to the community.
Now, I have some noise in output when using the headphones, but when connected to soundcard input, there's no noise. The noise sounds like some sort of ripple related to the amplitude control, but it's hard to describe. I'm not sure if it also can be some sort of overtones.
Current test setup is 1kHz sine wave, with an triangle amplitude control waveform of ~0.5Hz. Sample frequency is 512kHz. Output device is 16 Ohm headphone. Connection from two internal OpAmps (one for left & right each) to headphone is done by 100uF in series for each channel, nothing else.
To verify my output stage, I used the PSoC 5LP audio example, which makes the PSoC appear as an USB soundcard, modified the example to my output stage and played some songs. Here, the sample frequency of the device is 32kHz. Using the headphones, there's not the same noise I can hear with the PSoC generated sound. There's only some...hmm, I think it's called "pink" noise, which is what you can expect from low-cost casette playback devices from the 80's/90's So, this let me assume that my output stage is okay.
Next idea was to connect the device to the soundcard and record the input to have a test sample, showing the problem - but interestingly there's no "ripple" noise So I can't show you the problem. I've also written a small C# application, generating the same sound as described above (1kHz sine with 0.5Hz triangle amplitude) and used it with the USB soundcard application, no ripple noise, even when changing the sample rate and/or bit depth of the generated sound... I also changed the headphones, no effect.
- signal generator application & headphone: not OK
- signal generator application & soundcard: OK
- USB soundcard application & headphone: OK
So, I assume there's some sort of filtering and/or audio processing when using the soundcard, which is missing when using the headphones. How can I figure out where the error is and how to solve it?