Latest
Test Results
by Brian Crosby
12/08/02
The latest test result have been divide into three sections, Measurements of input signals, Threshold voltages, and linearity between the frequency and the rotation of the guitars tuning peg. All of the testing was done on a Gibson SG, electric guitar at VTC on December 12, 2002.
The first section has to do with measuring the period of each separate string on the oscilloscope. This information was then used to see if it is at all possible to calculate the frequency from the results. This was done with great success. The scope shows large spikes of each note pluck and also shows its sub-harmonics. The measurements were taken from similar positions on each of the spikes this gives a 1\4 of the period. This measurement was then inverted and multiplied by 4 to obtain the final frequency. The picture below is a snap shot of the “D” string being pluck on the oscilloscope. The resolution from the digital camera is not the greatest but you can at least see the shape of the waveform.
Measurements were taken from peak-to-peak of the
largest waves.
Sub Harmonics
“D” String Measured in lab
The snap shot of the oscilloscope shows three large voltage spikes on the screen. To obtain the frequency, the measurement of time must be taken between each spike. In this case above, the time of the period is approximately 6.88ms. This time is then inverted and multiplied by 4. The result is 581.2hz. The expected standard frequency tuning for a “D” string is 587.33hz. This is close enough for the sampling needed for the Auto Tuner.
The second type of testing done was to find the voltage thresholds for each of the strings plucked. The results of this test would find a max and min voltages needed to change the input sign wave into a square wave. This wave conversion will be done with the use of an Schmitt trigger. Table 1 below shows the test results of voltage thresholds obtained in the lab.
Table 1. Voltage Thresholds
In the table above, the highest top threshold is at 151.6mv and the highest bottom threshold is at 64.2mv. This sets the upper and lower voltage limits. The next step when designing the Schmitt trigger circuit is to choose a voltage in between the upper and lower limits, preferable in the middle.
Schmitt Trigger:
Schmitt Trigger at
work:
The last part of testing done was to check and see if the circular rotation of the tuning peg on the head of the guitar is linier to the change in pitch, degrees Vs frequency. This test was done on the two different strings, the “A” string and the “D” string. The results are in table 2, and graph 1 below.
Table 2. Degrees to Pitch change
Graph 1.
The graph above shows for the most part the pitch to degrees is pretty linier. The “D” string shows a slight variation, this is due to the rotation of the gear head not being measured with a measuring device. The rotation turns were eye balled.