Introduction to APL TDA IM
APL TDA IM, adds intermodulation distorting testing to the time domain analysis capabilities of APL TDA. Its full spectrum IMD testing gives greater insight into non-linear distortion performance than conventional THD and fixed 2-tone testing alone, especially in the HF band. With it, IMD performance of both speakers and microphones can be tested, as well as that of any component in the signal transmission path.
All of the features of APL TDA are preserved in the new software and controlled in the same way from the familiar control panel screen that has some new buttons and boxes added to support the new features. There are now 2 columns of buttons to the left of the graph display area for calling up different result graphs. Controls for the added modes are just above the large green RUN button.
APL TDA IM has two new test modes: one with a fixed LF tone and a second with two tones that remain relatively close together during the sweep. A strong LF tone stimulates both motor non-linearity and Doppler effects due to cone movement. The motor non-linearities result in amplitude modulation sidebands at sum and different frequencies while cone movement causes frequency modulation sidebands at the same sideband frequencies. A close tone test on the other hand creates distortion products across the entire audible spectrum. Its important to be able to judge the audibility of these during speaker development and evaluation.
The frequency spectrum created by the IM distortion in these tests is described in Appendix 1: Frequency Spectrum Due to IMD. Details of how to control the new features is provided in Appendix 2: Test Modes and How to Control Them.
IMD Measurement Graphs
Three different graphs can be displayed from the results of a close tones test via the buttons to the left of the graph display region on the control panel. In the graphs, ΔF is the difference between the two tones. F2 + ΔF is labelled on the first two graphs as “IM Component” and in most cases, will be the strongest IM distortion product. The IM distortion level trace is computed as the difference between the clear signal curve and the graph’s IM component curve. The noise level is shown on the curves as an indication of measurement quality.
Figure 4 Close Tones Test Displaying F2 + ΔF
Figure 4 shows the primary sum frequency distortion product, F2 + ΔF. In Figure 4:
- Clear Signal is the recorded signal level of the F2 test tone
- IM Component is the recorded signal level at frequency F2+ ΔF
- IM Distortion level is the difference betwean Clear signal and IMD compnent F2+ΔF
- Noise level is shown as an indication of measurement quality
Figure 5 Close Tones Test Displaying F2-F1 = ΔF IMD component
In the close tones test, a fixed ratio of 1.1487:1 is maintained between F2 and F1 throughout the sweep and (F2-F1) tracks at about 3 octaves below F2. Both the level of distortion produced at a particular frequency and the level of IM distortion caused by a particular test tone frequency pair are of interest.
Therefore, Figure 5 shows the level of the F2-F1 IM distortion product two ways:
- The “IM at product fr” trace shows the level of the F2-F1 distortion product versus frequency. The IM level shown at any particular frequency occurs while F2 and F1 are approximately 3 octaves higher.
- The “IM at source fr” shows the same data but shifted so that the F2-F1 distortion level shown at a particular frequency is the distortion created when the source of it, F2, is at that frequency.
As with Figure 4, F2 is the Clear Signal and the noise level in the measurement is shown.
Figure 6 Close Tones Test Showing High Order Sum Products
Figure 6 shows higher order sum products: F1+F2, F1+2F2, 2F1+2F2, 2F1+3F2, 3F1+3F2, 3F1+4F2, 4F1+4F2, 4F1+5F2, 5F1+5F2, for the close tones test. This graph is obtained by pressing either the “HNLDA” button on the control panel. These results can also be displayed as percentages via the “HNLDA %” button.
Figure 7 Far Tones Test Results Graph
Figure 7 is an example of the single results graph available for a far tones test – one with a fixed frequency LF tone plus a sweeping tone - and is obtained by pressing the “IMD FF” button. F1 is the LF fixed lower frequency tone and F2 is the sweeping tone. Curves of intermodulation distortion components F2+F1, F2+2F1, F2+3F1, F2+4F1, F2+5F1 are shown.
Appendix 1: Frequency Spectrum Due to IMD
The conventional practice in intermodulation distortion testing is the spectrum analysis of the response to two fixed test tones. APL TDA IM supports testing with two tones that are close together in frequency or far apart.
The frequency spectrum of a two-tone test with widely separated tones is shown in Figures 1 and 2.
Figure 1 Conventional Fixed Two Tone Test Spectrum
The lower test tone F1 is at 500 Hz and F2 is 10 kHz. Distortion products can be see clustered around the 10 Khz tone and its harmonics. More precisely, distortion products are produced at nF1 +/- mF2 for all m and n, with progressively lower amplitude as the order increases.
Figure 2 Two Tone Test Spectrum Centered on HF Tone
In Figure2, the display is centered at 10 kHz to highlight the distortion products are at offsets of +/- n*500 Hz from the 10 kHz tone.
Figure 3 Test Spectrum with Two Tones Close Together in Frequency
A different picture results when the two test tones are close together in frequency.
In Figure 3, F1= 10 khz and F2 = 11 Khz. Intermodulation produces a 1 Khz tone as F2-F1. A 2 Khz tone results from 2F2-2F1, a 3 kHz tone from 3F2-3F1, and so forth. IMD from close tones thus can spread across the entire audible spectrum with different potentially audible effects than the doppler and amplitude modulation distortion from a strong LF tone.
Appendix 2: Test Modes and How to Control Them
APL TDA IM can be run in modes corresponding to the two examples above, producing sweeps with two tones close together or far apart. Test mode specific time conversion processing of the recorded output signal from the system under test picks out distortion products whose level is then evaluated and displayed as a frequency response.
The test mode is selected via the measurement mode box just above the green RUN button on the control panel. Time domain analysis is one of the options there and is configured and runs just as in APL TDA. Results graphs are selected via a new column of buttons just to the left of the graph display area.
The output levels of both tones can be controlled to produce the desired level of stress. Output level is controlled via boxes located just below those that select the input and output devices, which are in the same locations as in APL TDA.
Between the new mode box and the output level controls are several buttons for controlling a tweeter friendly mode. F1 and F2 entries define a region in which sweep amplitude is gradually reduced to the HF Gain Level entry, intended to protect a tweeter. Similarly, a high pass effect can be configured to protect a woofer from over-excursion, which may be necessary when testing a bare driver since the sweep starts at 0 Hz.
In “far” mode, one tone is swept in the presence of a fixed frequency, ostensibly LF, tone whose frequency in Hz is specified in the “F1 for IM” box only in the far tone mode.
In “close” mode, the frequency sweep of the 2nd tone starts after a time delay from that of the first tone, resulting in a constant frequency ratio of 1.1487, which corresponds to 0.2 octaves, between the two tones during the log sweep.
These two-tone sweeps can be used to test loudspeaker IMD when both tones play through the same speaker or microphone IMD when each tone is played in a separate speaker. To do a microphone test, select “2 ch” output mode in the output box.