How Does Usb Speed Affect Performance

How Does Usb Speed Affect Performance, a forum discussion on Cleverscope Mixed Signal USB Oscilloscopes. Join us for more discussions on How Does Usb Speed Affect Performance on our Interesting Questions forum.

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10 Apr 2006
Posts: 481

Not a lot. Cleverscope keeps all the samples in the Cleverscope Acquisition Unit, and only transfers the samples needed to fill out the pixels currently being displayed. To make sure peak capturing yields both the mininium and maximum values for each pixel, Cleverscope transfers eight times the number of pixels needed. So with a typical 300 - 600 pixel display, 2400 - 4800 samples are transferred. The USB transfer rate is 200K samples (each sample is 32 bits) per second. So 4800 samples takes about 24 msecs. The user does not see a large delay in zooming or panning the graph. The normal update rate with a single standard size display is about 16 frames per second.

20 May 2006

I understand that it is not necessary to download all of the buffer during real-time viewing, but how long will it take to download all of the data if I want to save it to disk?

30 May 2006
Posts: 481

It takes about 20 seconds to transfer 2M samples (ie the average transfer rate is actually 100k samples/sec). A progress indicator shows progress.

22 Jun 2006

Poor Question: Where/when is the spectrum analysis being performed? Is it performed in CS328 and only the results are transfered or is it performed in the PC?

It seems 6.4Mbps (200K sample by 32bits) is being used in current interface. What if remote control is being requested on E1(2.0Mbps)? I think it is not possible to transfer all the data. Is it possible to transfer some selected data only, like spectrum analysis data only?

22 Jun 2006
Posts: 481

Hello PantsOn,
The spectrum analysis is performed in PC. When you choose a particular frequency bandwidth we calculate the effective sample rate required to get that bandwidth = 2 x Bandwidth.

Next we use the frequency resolution you chose to calculate the number of samples required, from a set of 1024, 2048, 4196, 8192, or 16384 samples equating to min frequency resolution to maximum frequency resolution. Next we decimate the samples in the CS328 sample buffer, across the time duration specified by the scope graph, to get the required effective sample rate. These samples are transported to the PC.

In the PC we use an FFT to convert the time information to frequency information. (Note that there is a possibility of aliasing here if the signal contains information at a higher frequency than half the sample frequency). Next we convert the FFT output to amplitude, or gain/phase values and take the log to get dB if required. Finally we display the spectrum graph.

The current interface operates at 12 Mbit/sec. There is throughput loss due to framing and turn around time around in the PC driver and application interface which reduce the effective throughput rate to 6.4 Mbit/sec.

If you customer is using an E1 interface to transport TCP information you can expect a data rate a bit slower than the 2.048 Mbit/sec E1 transport rate.

23 Jun 2006

""using an E1 interface to transport TCP information you can expect a data rate a bit slower than the 2.048 Mbit/sec E1 transport rate."" means lower performance ? Affect spectrum analysis performance?

If yes, what is the way to solve or avoid this problem?

1. One more PC near CS328 to do analysis?
1-1. remote desktop program?

2. Faster TCP/IP?

1. What if we use one more PC with CS328 to analyze and transfer the analyzed-reslults over TCP/IP (E1 speed)? Is it possible? 1-1. What if we use remote desktop program?

2. Do we have to propose faster TCP/IP? If yes, is it possible to do remote control with current hardware? If yes, how long it will take?

*. Is it possible to perform spectrum analysis in the CS328 and transfer the results only? so, we can have results over TCP/IP (E1 Speed)

Thanks in advance.

27 Jun 2006
Posts: 481

Yes, a rate slower than 2 Mbit/sec will affect performance - the update rate will be slowed. In general we transfer around 1920 samples (for a small scope display) and 1024 samples (for the frequency display), a total of 2944 samples. At 32 bits/sample, this is 94,208 bits/transfer. At 2 Mbit/sec, we probably manage 1 Mbit/sec after framing, TCP and turn around losses. So we can manage around 10 frames/sec.
If the spectrum resolution has to be higher (eg with a 50 Mhz bandwidth you want 6.1 kHz resolution) you will need to transfer 16384 samples per frequency display, or (16384+1920)*32 = 585,728 bits per update. The update rate would be slightly less than 2 per second.
The way to solve this is to have a faster link.
Doing the processing near to the Cleverscope will not increase performance, as the data set being transferred is about the same size.
Using a remote desk top will decrease performance, as much more information needs to be transferred to update a visual display than the actual data required to make the display (eg changing area is 400 x 600 pixels = 240,000 pixels, at 24 bits/pixel = 5,760,000 bits. You will get an update once every 5 seconds or so.
There is very little difference between the data set required to make a spectrum (1024 samples) and the spectrum output (2 channels of 512 reals). There is no gain in doing the spectrum analysis in the acquisition unit.
We have thought about modifications, and we will do them, on this basis:
a. They go on a list with all the other modifications proposed by our users, ranked in order of the number of people that want them, and the number of unit sales that will happen as a result of the modification.
b. We work our way through the list.
c. We provide updates via our website when they get done.
d. We provide no estimates as to when modifications are done - they are done when they are done. In making a buying decision it should be made on the basis of what is available today.

We hope this helps with your deliberations.
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