Tombstone

Understanding triggering in TombStone

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TombStone is a free software that records transfer functions using sinusoidal sweeps. It is quite like the old-fashioned stand-alone sweep generators/recorders that was used before the computer age to do the same thing. The program has become widely used, it seems as if people like it, but it also has one major source of confusion. Almost all questions I receive are on the issue of triggering, and why the response curve sometimes becomes shifted sideways.

Triggering? One might ask why at all triggering is needed, and the answer is that Windows buffers the sound sent to the soundcard in order to maintain a continuous sound without interruptions. The problem with the buffering is that it introduces an unknown delay in the signal that is received by TombStone. Even worse, this delay varies a considerable amount between runs.

The way I have solved this is to let TombStone play a trigger signal before the sweep. This signal can be selected under Settings|Options under Trigger|Trigger signal, see Figure 1 (green circle).

Figure 1. The settings for triggering (green), and the checkbox for verifying proper trigger settings (red).

The default signal is white noise, but experience tells that this should be altered to a sinusoid of a frequency that easily passes the system. For example, if a tweeter is measured, the frequency should be set rather high, e.g. 2 kHz as in figure 2, whereas if a woofer is measured 100 Hz might be more appropriate.

Figure 2. The notch in the red circle appears at 1000 Hz, which indicates that the trigger settings were appropriate for this run. If the notch had appeared at some other frequency, the trigger settings are probably inappropriate. The green lines indicate a frequency range (~750-13000 Hz) where trigger level of -20 dB would be sufficient to detect the trigger signal correctly. In this case a sinusoid at 2000 Hz was used, which is well within the useable range.

This signal causes the level of the TombStone input to suddenly increase at a time that can be used as a reference for the coming sweep. So, TombStone waits for this signal, and as the level on the input goes above the trigger level, TombStone knows when to expect the sweep to start. Thus, the choice of trigger signal and level is important, and the appropriate values vary between measuring objects. If the trigger level is set too low, TombStone may recognize background noise as the trigger signal, and the response curve will be shifted to the right. If it is set too high, no trigger signal will be detected at all, and TombStone will give you the error message “no trigger signal detected” (figure 3).

Figure 3. This dialog appears if the trigger level is set too high or if the trigger signal does not pass the system at a sufficient amplitude.

The appropriateness of the trigger settings can be tested by letting TombStone switch the signal off for a short time when the signal is near 1000 Hz as shown in the red circle in Figure 1. If the trigger settings are appropriate, a notch will appear around 1000 Hz in the response curve, as shown in the red circle in Figure 2. If the notch appears at some other frequency, you should consider changing the trigger settings.

The triggering can and should also be monitored for each sweep by looking at the bottom left corner; as the trigger signal beeps before the sweep, the word “triggered” should appear at exactly the same time as you hear the start of the trigger signal, see figure 4. If it appears before or after the start of the onset of the trigger signal, the response curve will be shifted to the left or right, resulting in an erroneous curve

Figure 4. The word ”Triggered” should appear at the bottom left corner of the tombstone window in perfect synchrony with the trigger signal sound, otherwise the response curve will appear shifted to the left or right.

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Summary

-Do not set the trigger level too low, -20 to –30 dB is often appropriate.
-Select a sinusoid as the triggering signal, and make sure that the frequency easily passes the system that you are measuring.
-Verify that triggering occurs correctly by keeping an eye on the “triggered” panel while listening for the trigger signal, as in figure 4.