Audio frequency issue

[andreondra]

First of all, thank you for such a great project!

I have a strange issue with the sound output. The image works perfectly without any problems, both in my CPU project and in the hdmi-demo project. I use Nexys Video devboard.

However, in both projects I have an issue with the audio frequency. In both projects a sawtooth waveform with a certain frequency is generated: in my project I tried various freqs, in the hdmi-demo it is 480 Hz if I see correctly. For debugging, I use my HDMI monitor with an audio output (via 3.5mm jack). I measured the frequency of the signal coming from the jack in two ways to mitigate a measurement error:

• I connected the speakers, recorded the audio and then measured the frequency,
• I connected the oscilloscope and measured the frequency directly in the instrument.

I found out that in both projects the frequency is always approximately double. See the measurement of the waveform generated by the hdmi-utils demo:
.

I also measured the frequency of the generated audio_clk, it seems correct.

Do you have any idea what the issue may be? Is this a known issue, or maybe I do something wrong? If this occurred only in my project I wouldn't be surprised but the same issue is also in the demo project. And my monitor (which I also suspected) is working correctly, because if I connect a computer and generate an arbitrary waveform, the frequency is OK.

Edit: so it seems that modifying the audio_clk generator in the demo like this:

// from this
assign clk_audio = clk_pixel && counter == 11'd1546;
// to this
assign clk_audio = counter == 11'd1546;

Fixes the issue. In the Questa Sim I saw a glitch caused by the clk_pixel &&. So -- in my design a have probably the same issue, will investigate.

[andreondra]

I just found out that in my project it can be fixed by handling the audio_clk correctly -- not using timer's value directly as a clock, but passing the value to a clock buffer with CE:

  always_ff @( posedge hdmi_clk ) begin : audio_clk_gen_proc
      if (reset || audio_clk_timer == 524) begin
          audio_clk_timer <= 'h0;
      end else begin
          audio_clk_timer <= audio_clk_timer + 1;
      end
  end

  BUFGCE #(
      .CE_TYPE("SYNC"),
      .IS_CE_INVERTED(1'b0),
      .IS_I_INVERTED(1'b0),
      .SIM_DEVICE("VERSAL_PRIME")
  )
  BUFGCE_inst (
      .O(audio_clk),
      .CE(audio_clk_timer == 0),
      .I(hdmi_clk)
  );

The first approach (without buffer) probably introduced glitches which triggered the clock multiple times.