G-tune Performance

G-tune has been designed to be a very accurate frequency measuring instrument. As such it is ideally suited to tuning musical instruments. However, the accuracy acheived in G-tune surpasses that of most other PC-based instrument tuners.

To test the accuracy of G-tune, sinusoidal signals were simulated and passed into G-tune. Various levels of signal were used with different levels of noise added to them. The plot below shows a typical measurement of G-tune’s accuracy. The x-axis shows the actual frequency that was used in the test, whilst the y-axis shows the error in G-tune’s measurement. Note that these tests only tested the performance of the G-tune algorithm, they did not test the accuracy of any sound cards.

From the above plot it is clear that G-tune is capable of very high accuracy indeed. The measurement range shown is only from 30 Hz to 40 Hz. It can be seen that the accuracy begins to degrade as the frequency reduces to 30 Hz, and this is to be expected as lower frequencies are more difficult to measure accurately. As the frequency increases, the measurement accuracy remains consistent up to 5500 Hz, with a maximum error of 0.003 Hz.

Thus it is clear that for a sinusoidal signal, the frequency measurement error is less than 0.01 Hz for all frequencies from 30 Hz to 5500 Hz. This error is very small indeed and it is likely that measurement errors introduced by the sound card will dominate the measurement process. Note that the maximum frequency G-tune can accurately measure will depend on the sound card used, but generally frequencies up to 5000 Hz are accurate.

Speed

The G-tune frequency measurement algorithm is also very fast. On a 233MHz AMD K6 processor, it can do more that 420 frequency measurements per second. In reality, G-tune does not run this fast because it must wait for data to arrive from the sound card. On most computers, G-tune will always provide 21.5 measurements per second, even on a computer that is 10 times slower than the 233MHz AMD K6. This is more than adequate for most purposes.

Questions & Answers

If G-tune only records audio at 11025Hz (and CD quality is 44100Hz), how can G-tune be accurate?

If you sample a bandwidth-limited signal at 11025 Hz, you preserve ALL the information up to frequencies of 5512.5 Hz (this is Nyquist’s theorem). Sampling at higher frequencies only means that you will gain information about higher frequency signals. Most instruments do not produce fundamental tones above 5512.5Hz, therefore there is no need to try and detect frequencies above this value. Therefore, it is better to sample at a lower frequency (11025 Hz) and use all the information you have than to sample at a higher frequency (e.g., 44100 Hz) and throw away three-quarters of the information. Do not listen to anyone who tells you otherwise!

Can I use G-tune to measure frequencies to an accuracy of 0.01 Hz?
Yes and no. Although G-tune is accurate to better than 0.005Hz, there may be a slight frequency offset caused by your sound card, which G-tune cannot compensate for unless you have access to expensive calibration equipment. Such offsets are due to slight differencies in the oscillators used in sound cards (with cheaper sound cards generally being less accurate). In addition, temperature variations within your computer will cause slight shifts in the measured frequencies.

So does that then mean that G-tune is not that accurate after all?
No. For tuning musical instruments, G-tune is extremely accurate and with typical sound cards, it it more accurate than many stand-alone instrument tuners.

Go back to main G-tune page.