Subversion Repositories wbpwmaudio

# A demonstration of the Improved PWM waveform

This directory contains the files necessary to build a demonstration of the

[improved PWM](../rtl/wbpwmaudio.v) waveform used in this repository.  The

demonstration is designed to work both in a Verilator based simulation

(pdmdemo.cpp), as well as with a board having a single PMod AMP2, switch, LED,

and a 100MHz clock input.

The [toplevel Verilog file](toplevel.v) should be easily recognizable as

[toplevel.v](toplevel.v).  This contains the [cordic.v](cordic.v) component to

generate a sine wave as the test signal, as well as components to turn this

test signal into both a [traditional PWM](traditionalpwm.v) output as well as

the [improved version](../rtl/wbpwmaudio.v) found within this repository.

The [toplevel design](toplevel.v) will either output a 440 Hz tone, or a swept

tone running from 110Hz to about 27.3kHz, depending upon which part of the

test sequence is being run.

The `DEF_RELOAD` parameter within toplevel.v can be used to adjust the PWM

interval for the [traditional PWM](traditionalpwm.v), or sample interval for

the [improved version](../rtl/wbpwmaudio.v).  Likewise, when run on actual

hardware, the switch input will control which

waveform is produced--either the

[traditional PWM](traditionalpwm.v) or the

[improved waveform](../rtl/wbpwmaudio.v).

These files can also be composed within a

[Verilator](https://www.veripool.org/wiki/verilator) based simulation.  In this

case [pdmdemo.cpp](pdmdemo.cpp) forms the main simulation component.  A boolean

value within it, `traditional_pwm`, can be used to control whether or not the

file downsamples and outputs a [traditional PWM](traditionalpwm.v) signal or

the [improved version](../rtl/wbpwmaudio.v).  Likewise,

there's a commented line that you can use to generate a VCD file which can be

used to view traces of the logic within this demo.  This demonstration file

will also produce a file of 64-bit doubles (overkill, I know) containing

downsampled audio samples at 44.1kHz.  This audio file, named wavfp.dbl, can

be processed within

[Octave](https://www.gnu.org/software/octave)

by using [showspectrogram](showspectrogram.m), to view a spectrogram of the

data.

If you take the time to run the [simulation](pdmdemo.cpp), you'll clearly see

that the [improved PWM](../rtl/wbpwmaudio.v) waveform is much cleaner:

it doesn't include unwanted harmonics, nor does it include any artificial

features (beyond any expected aliasing products, present in both).

The [traditional PWM](traditionalpwm.v) waveform, on the other hand, cannot

seem to be able to produce a clear tone without also creating unwanted

harmonics.  This effect is only worsened when the PWM period is within

hearing range, such as when the period is the reciprocal of 8kHz.