Sound Card Performance Technical Benchmarks
TechTalk
What is IM?
IM is short for IMD which stands for "Intermodulation Distortion".
Intermodulation Distortion is a means for measuring Nonlinear Distortion . Nonlinear Distortion is a form of signal processing error that creates signals at frequencies that are not necessarily present in the input. It contrasts with Linear Distortion, which is a form of error that creates no new frequencies beyond those that are present in the input signal. Linear Distortion changes the relationship of the size and timing of the frequencies that compose the input signal. Linear and Nonlinear distortion are directly based on frequencies present in the input signal. Noise is a form of error that is not as directly related to the input signal.
IMD is determined by measuring the size of each of the new frequencies that are created by the source of the distortion. The new frequencies are called "beats" because they exist at frequencies that are sums and differernces and multiples thereof, the two tones in the input signal. For example, the beats of 14 kHz and 15 kHz are at 1 kHz, 29 kHz, 13 kHz, and so on.
The beat at the sum and difference frequency (1 kHz in the example, above) is due to "Even Order Distortion" and is caused by errors that are not symmetrical. The remaning beats are due to "Odd Order Distortion" and are caused by symmetrical errors. Push-pull or balanced circiuts are symmetrical and therefore reduce Even Order Distortion.
Historically, IM has been more difficult to measure, but modern FFT's and digital signal generators make measuring IM very feasible.
IM has the advantage of not requiring the measurment of harmonics that are at frequencies that are many multiples of the frequency of the signal.. Therefore, IM is good for measuring nonlinearity at high frequencies. One disadvantage of IM is that it is harder to determine whether the errors it portrays are symmetrical or not.
IM distortion's audibility depends on the difference between the tones that are intermodulating with each other. Thus, nonlinear distortion of high frequency tones can create intermodudation products at lower frequencies, where they are more audible.
From: J. Robert Stuart(Meridian Audio), Digital Audio for the Future, Audio, 3/98 pp 30-37.
Go to PC Sound
Card Technical Talk Selection page
Go to PC Sound
Card Technical Performance Benchmarks Report Selection page
Go to PC
Sound Card Technical Performance Benchmarks Comparisons menu
Go to PC Sound Card
Technical Performance Benchmarks home page
Go to PCAVTECH home page