Analog audio output produce noise around the sampling rate of the audio

Question

I connected the analog audio output to a speaker and played an audio file with sampling rate of 15625.

when I measure the sound coming out of the speaker I can see that the same sound is duplicated from the right and the left of the sampling rate.

the same phenomenon does NOT happen when I send audio through HDMI.

how can I fix the analog output?

in the image you can see in red the analog audio and blue the HDMI output audio:

Answer 1

I'm not sure this will answer your question but:

1. The analog audio output is very very noisy, probably one of the worst analog output I've heard since my first soundcard ever I had in the 90s (even Creative Labs Sound Blaster 8bits had a better SNR...)

2. I don't think you can fix the analog output to have less noise. I tried (by changing how to access the sound chip, with various libraries, linked to ALSA, I tried with different sampling rates, etc.) during some weeks, unsuccesfully.

3. In case you need a very good output, I highly suggest this non-expensive DAC (I'm not affiliated at all), based on Texas Instruments PCM2704 chip. That worked out-of-the-box on my Pi.