Demand Peripherals

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tonegen_design [2022/03/16 23:24]
dpisuperadmin [Minimal Tone Generator with Volume Control] 		tonegen_design [2022/03/20 03:45]
dpisuperadmin
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 ==== Minimal Tone Generator with Volume Control ==== ==== Minimal Tone Generator with Volume Control ====
-A full synthesizer, as described below, may take more that its fair share of FPGA logic.  This is fine if the goal of the project is a synthesizer but is not fine if the user wants something that just beeps.  This section describes something that beeps but incluees volume control.+A full synthesizer may take more that its fair share of FPGA logic.  This is fine if the goal of the project is a synthesizer but is not fine if the user wants something that just beeps.  This section describes something that beeps but includes a volume control.
  
 The API for a simple tone generator might specify the musical note to play, the volume in the range of 0 to 100, and the number of milliseconds to play the note.  We might also want to play a file of notes where each line the the file has the note, volume, and duration.  For example: The API for a simple tone generator might specify the musical note to play, the volume in the range of 0 to 100, and the number of milliseconds to play the note.  We might also want to play a file of notes where each line the the file has the note, volume, and duration.  For example:
-    dpset tonegen tone b4 30 200 +    dpset tonegen note b4 30 200 
-    dpset tonegen play mymelody.txt+    dpset tonegen melody mymelody.txt
  
 {{ wiki:audiotaper.png?200|}} Square waves are easy to generate and have a pleasing sound since they have lots of higher harmonics.  A more difficult question for a simple tone generator is how to control the volume.  Human hearing perceives audio volume on a logarithmic scale.  Electronics manufactures use what is called a 'audio taper' for potentiometers in audio applications. The diagram to the right shows an audio taper and we want our gain control to follow the same curve. {{ wiki:audiotaper.png?200|}} Square waves are easy to generate and have a pleasing sound since they have lots of higher harmonics.  A more difficult question for a simple tone generator is how to control the volume.  Human hearing perceives audio volume on a logarithmic scale.  Electronics manufactures use what is called a 'audio taper' for potentiometers in audio applications. The diagram to the right shows an audio taper and we want our gain control to follow the same curve.
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 The minimum output of the 2R-R circuit is 0.013 and the maximum value is 0.9935, a ratio of low to high of about 76.  While we have lost a great deal of resolution, we have gone from 4 bits of dynamic range for the linear R-2R network to over 6 bits of dynamic range using a 2R-R network. The minimum output of the 2R-R circuit is 0.013 and the maximum value is 0.9935, a ratio of low to high of about 76.  While we have lost a great deal of resolution, we have gone from 4 bits of dynamic range for the linear R-2R network to over 6 bits of dynamic range using a 2R-R network.
  
-What if we combined the linear pulse density modulation with the non-linear DAC?  We could PDM control each of the FPGA output pins with a separate 4 bit counter.  Doing this gives 64K possible gain settings. Since some combinations give the same gain there are only 14000 unique gain settings.  The minimum (one-sixteenth of the LSB) has a gain of 0.000817 and the maximum (all bits high) has a gain of 0.9314, giving us a dynamic range (log2(max/min)) of about 10 bits.  This is not too bad considering we have a 4 bit DAC. +{{ :wiki:tonegengain.png?200|}} What if we combined the linear pulse density modulation with the non-linear DAC?  We could PDM control each of the FPGA output pins with a separate 4 bit counter.  Doing this gives 64K possible gain settings. Since some combinations give the same gain there are only 14000 unique gain settings.  The minimum (one-sixteenth of the LSB) has a gain of 0.000817 and the maximum (all bits high) has a gain of 0.9314, giving us a dynamic range (log2(max/min)) of about 10 bits.  This is not too bad considering we have a 4 bit DAC. 
  
 Our design is now ready for a Verilog Wishbone implementation.  We should expect it to have  Our design is now ready for a Verilog Wishbone implementation.  We should expect it to have 
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 an 8 bit duration set by the host, an 8 bit duration set by the host,
  
-(The diagrams in this section were generated using Octave.  The sources are attached to the section in a wiki comment.  Edit the page or contact the author to get the sources for the diagrams.)+(The diagrams in this section were generated using Octave.  The sources are attached to this section in a wiki comment.  Edit the page or contact the author to get the sources for the diagrams.)
  
  
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 grid on; grid on;
 print -dpng '/tmp/r2-2.png'; print -dpng '/tmp/r2-2.png';
- 
-% Generate 100 points on an log curve and map the gain 
-% to setting in pwm - nonlinear DAC scheme.  This gives 
-% the actual table to use in the API driver modules. 
-% Manually add {0,0,0,0} and {15,15,15,15}. 
- 
-% Get the target gains 
-x = 1:1:100; 
-out = exp((5 .* x) ./ 100) ./ 150; 
-target_idx = 1; 
-% loop past all possible gains recording the first to pass out(target_idx) 
-idx = 0; 
- for i3 = 0:15; 
-  for i2 = 0:15; 
-   for i1 = 0:15; 
-    for i0 = 0:15; 
-     idx = idx +1; 
-     outval = ((i3 .* .73203) + (i2 .* .19608) + (i1 .* 0.05229) + (i0 .* 0.01307)) ./ 16; 
-     if outval > out(target_idx), 
-         %printf("%d %f {%d, %d, %d, %d},\n", target_idx, outval, i3, i2, i1, i0); 
-         printf("{%d, %d, %d, %d},\n", i3, i2, i1, i0); 
-         target_idx = target_idx + 1; 
-     end 
-    end 
-   end 
-  end 
- end 
  
 */ */
tonegen_design.txt · Last modified: 2022/03/20 03:51 by dpisuperadmin

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