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Greets!

I am finishing up a sound generator design and am now faced with the task of mixing 4 or more 8 bit "voice" registers into one single 8 bit "dac" output register.

Currently I use...

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mixer <= (voice1[15:8] + voice2[15:8] + voice3[15:8] + voice4[15:8]) /4;
dac <= mixer;


"mixer" is a 16 bit register.

which works, but does drop the output significantly as compared to assigning dac to a single voice.

I am looking for suggestions on a better way to mix together 8 bit voices into a single 8 bit output. I cannot change the number of bits in the dac or voices, but can certainly change the number of bits in any of the mixing math.

Cheers!
Brad

Hi,

I think, in general, you're already doing the best thing. But here's a couple of random suggestions...

1. If it's very unlikely that all voices will be playing at full volume at the same time, you might get away with dividing by 2 instead of 4, with clipping (but it will sound unpleasant when it clips):

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wire [9:0] sum = voice1[15:8] + voice2[15:8] + voice3[15:8] + voice4[15:8];
always @(posedge clk)
   dac <= (sum>=10'h300) ? 8'hFF : (sum<10'h100) ? 8'h00 : {~sum[8],sum[7:1]};

2. If you can run your DAC at a high sample-rate (e.g. >= 96kHz) you might be able to improve resolution by dithering (but this will add faint tones, and it only really works if voices go exactly to 16'h8000 when they're quiet):

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reg [9:0] remain;
wire [18:0] sum = voice1 + voice2 + voice3 + voice4 + remain;
always @(posedge dac_clk) begin
   dac    <= (sum[18]) ? 8'hFF : sum[17:10];
   remain <= sum[9:0];
end

There are more advanced kinds of dithering (see Noise Shaping, or the links on Jean's PWM page).
You could even try both tricks (clipping and dithering) at the same time.

BTW, I'm assuming that the signals are all unsigned / positive, and that the inputs are 16 bit.

Regards

Nick

Thanks for the info.

Update...

After much hacking, I found a better way to send multiple bytes into a single DAC.... time slicing.

At 40MHz, I just made set up a 3 bit counter and sent bytes to the DAC on each cycle so that all 8 bytes had 40MHz/8 of time.

This sounds perfect and has no fidelity loss as compared to the mathematical mixing.

I guess in a way, this is basically PWM, but since it sounds better, it is better.

Brad