Slow (Decimate) Controls

Controls: X-Axis Dialog >> Slow

• Decimate On/Off
• Factor (X)
• Rate (Hz or sec)
• Envelope
• Demodulate
• Signed

These controls allow you to reduce the effective display sample rate by an arbitrary Decimation Factor. This allows very low rates (down to one sample per day or slower) with high precision, unlike the normal rates allowed by Windows and the sound card.

Better yet, it does not affect the actual Generator output signal, allowing it to maintain a high sample rate for good high-frequency response without aliasing. You can thus monitor a low-frequency or long-duration response to a high-frequency or short-duration stimulus signal.

For example, suppose you are measuring auditory evoked potentials from a guinea pig. You use the Generator to create 30 kHz tone bursts, and want to observe neural responses up to (say) 40 msec from the burst. You set the sample rate to 96000 Hz, since the highest output frequency is limited to half the sample rate (the so-called Nyquist frequency). Half of 96000 is 48000, so your 30 kHz signal will have no aliasing.

At 96000 Hz, each sample is only a bit more than 10 microseconds in duration. The waveform display can show up to 1024 samples, or about 10 msec... only a quarter of the desired 40 msec span. You could use Trigger Delay to see the whole range, in 4 views of 10 msec each, but these evoked potentials require thousands of frames to be averaged for each view, so the Trigger Delay approach would take 4 times longer to observe the full range.

But with Decimate active, you can set the Factor to 4x and see the 40 msec span as if the input had been sampled at a 24000 Hz rate, while the output is still running at 96000 Hz. The slow effective input sample rate is no problem for the evoked potentials, which are typically below 1 kHz.

You can also use Decimate in conjunction with a (modified) sound card that can respond to DC inputs, to view and/or record very slow signals such as temperature or pressure. With Factor set to 48000, the effective sample rate is one per second. At 2880000 it is one per minute, and at 172800000 it is one per hour.

However, you don't need to compute these values; you can use the Rate control to automatically set the Factor for any given effective rate. You can toggle the default Hz mode to seconds, and set 1, 60, 3600, or whatever sample period you desire.

For these very slow rates (anything below an effective rate of about 500 Hz or so), it is usually best to run with Trigger off. This will result in a scrolling waveform to emulate a conventional chart recorder or data logger.

The Envelope button toggles a "fat trace" view that shows the maximum and minimum values encountered in obtaining each decimated sample.

The Demodulate button is similar to Envelope, but it shows only the difference between the maximum and minimum values, displayed as an outline waveform. This can be used with a simple external "chopper" circuit to allow any sound card to accept DC signals.

The Signed button is for use with a special external chopper circuit that encodes the sign of the signal, as well as its magnitude. (The simple chopper suitable for use with Demodulate doesn't preserve the sign.)

Saving to Files:

If you save a single-screen .DQA file (such as via Save Trace as .DQA Data File in the File Menu), the file will contain 1024 samples of decimated data, just as viewed on the screen, not the full-speed raw data such as the 30 kHz tone bursts in the above evoked potentials example.

However, if you save an arbitrary-length DDisk file it will contain the raw data by default. This will, of course, make the file larger... 4 times larger (the Decimate Factor) in the above example. But you then have the option of changing the Factor when viewing the file, or toggling Decimate off completely, or even selecting different Envelope, Demodulate, or Signed modes.

On the other hand, if Decimate Factor is large the file size can become unmanageable, or even exceed the 4 GB limit imposed by the Microsoft RIFF .WAV format (which is also used by Daqarta's own .DQA files). In typical slow-scrolling "chart recorder" applications, you won't need to change Decimate settings after the recording anyway. In such cases you can record with the Decimate Lock option selected in the DDisk Controls dialog. This will save only the decimated data, just the way it appeared on-screen during the recording.

Note that when saving decimated DDisk files, the file position readout will indicate progress in Bytes, Samples, or one of four different time modes. The Bytes and Samples modes will show actual bytes or samples saved, so if Decimate Lock is not used these will advance at the full sample rate. (Time modes are unaffected by Decimate Lock.)

See also X-Axis Control Dialog