Sampling Rate of Screen Chart

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elfallero
Posts: 4
Joined: Thu Dec 02, 2004 8:20 pm

Sampling Rate of Screen Chart

Post by elfallero »

1. What is the sampling rate of the data that is drawn to the screen. i.e. the output of the DeciScreen.vi?

2. Does the data come out of this VI in fixed or variable length packets?

Coen
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Post by Coen »

Each chart always displays a fixed number of samples: 1024 for the low resolution versions, and 2048 for the high resolution versions of ActiView. So, the amount of downsampling depends on the selected sample rate (combination of speedmode and decimation), and the selected screen length in seconds. For example: on speedmode 4 and decimation = 1 (resulting sample rate is 2048 Hz), and a screen length of 8 seconds, only 1 of each 8 samples is actually plotted on screen in the Hires version (1 of each 16 in the Lores version). However, the same sample rate and a screen length of 1 second leads to plotting of every sample in the Hires version (and 1 of each 2 in the Lores version).

Downsampling of the original data to the lower sampling rate on screen is done by simply deleting of sample points (only the first of a group of samples is plotted). For most users, this is the preferred display mode. There are two alternatives:

- proper decimation with anti-aliasing filter before plotting to screen. The disadvantage is that the bandwidth on screen becomes very limited, higher frequencies information is not displayed anymore. The simple throwing aways method makes the higher frequencies visible due to aliasing (for example: 50/60 Hz interference is clearly displayed as a lower frequency).
- plotting all sample points. This is arguable the best method. However, because of the limited screen resolution, this leads to a group of samples being plotted on the same column of pixels. Therefore noise and interference on the signal leads to the signal trace becoming a thick line. Most customers experience this display method as "strange". It makes noise in the signal more difficult to interpret (50/60 Hz interference is not quickly distinguished from electrode noise). Plotting all points also requires extra processing power.

The length (number of points to screen) of the Deciscreen.vi output array varies. ActiView continuously adjusts the stepsize on screen to the available processor time (small steps for smooth scrolling if a lot of processor time is available, large steps for effective plotting if less processor time is available, or if the display is lagging the writing in the ringbuffer).

Best regards, Coen (BioSemi)

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