# Sampling slowly **URL:** \ **Category:** GNU Radio\ **Created:** [November 30, 2014, 4:13pm UTC](https://www.ruby-forum.com/t/sampling-slowly/234864 "2014-11-30T16:13:12Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![alcina](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/a587f6/32.png) [@alcina](https://www.ruby-forum.com/u/alcina)\ **Post date:** [November 30, 2014, 4:13pm UTC](https://www.ruby-forum.com/t/sampling-slowly/234864/1 "2014-11-30T16:13:12Z")
Looking for help I have written a source block to do satellite predictions. It acts as a source of a float representing the input parameter - e.g. elevation. The block probes predict via UDP, looks at the string returned and selects the asked for parameter. So far fine, but I would like to probe it slowly, not on every sample. This appears to be impossible. If I use a probe and function probe and ask it to update each second, the probe grabs data from the source runs at full sample rate. Unfortunately probe seems to have almost no documentation and I don’t understand the source but I assume it goes as fast as it can get input samples. If I try combining say an add block with a decimated version of the sample rate (coming from a USRP) and probe again, it still runs too fast. It is as if it is trying to cache a set values. Logically I can see that might be useful to maintain a constant rate where blocks take different times. Is there a simple fix or do I need to implement some form of trigger port - and if I do will it still cache data? Mike ---
**Author:** ![vh1](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/v/278dde/32.png) [@vh1](https://www.ruby-forum.com/u/vh1)\ **Post date:** [November 30, 2014, 4:36pm UTC](https://www.ruby-forum.com/t/sampling-slowly/234864/2 "2014-11-30T16:36:51Z")
I would suggest looking at the message-strobe block and using messages. - Seth Sent from my iPhone ---
**Author:** ![vh1](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/v/278dde/32.png) [@vh1](https://www.ruby-forum.com/u/vh1)\ **Post date:** [November 30, 2014, 5:26pm UTC](https://www.ruby-forum.com/t/sampling-slowly/234864/3 "2014-11-30T17:26:35Z")
That may work, but it may cause problems with the scheduler if you are using stream ports. - Seth Sent from my iPhone ---
**Author:** ![vh1](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/v/278dde/32.png) [@vh1](https://www.ruby-forum.com/u/vh1)\ **Post date:** [November 30, 2014, 4:48pm UTC](https://www.ruby-forum.com/t/sampling-slowly/234864/4 "2014-11-30T16:48:59Z")
I looked at that but I don’t know how to and it’s extremely difficult to understand. It is probably possible to bodge using the system timer to avoid a re-calculation if called within less than a specific time, e.g. 10 seconds. This is what I am thinking at the moment. Basically reduce the workload. Mike From: Seth Hitefield [[mailto:removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid)] Sent: 30 November 2014 15:35 To: Mike W. Cc: [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) Subject: Re: [Discuss-gnuradio] Sampling slowly I would suggest looking at the message-strobe block and using messages. - Seth Sent from my iPhone On Nov 30, 2014, at 10:11 AM, Mike W. \ wrote: Looking for help I have written a source block to do satellite predictions. It acts as a source of a float representing the input parameter – e.g. elevation. The block probes predict via UDP, looks at the string returned and selects the asked for parameter. So far fine, but I would like to probe it slowly, not on every sample. This appears to be impossible. If I use a probe and function probe and ask it to update each second, the probe grabs data from the source runs at full sample rate. Unfortunately probe seems to have almost no documentation and I don’t understand the source but I assume it goes as fast as it can get input samples. If I try combining say an add block with a decimated version of the sample rate (coming from a USRP) and probe again, it still runs too fast. It is as if it is trying to cache a set values. Logically I can see that might be useful to maintain a constant rate where blocks take different times. Is there a simple fix or do I need to implement some form of trigger port – and if I do will it still cache data? Mike ---
**Author:** ![vh1](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/v/278dde/32.png) [@vh1](https://www.ruby-forum.com/u/vh1)\ **Post date:** [November 30, 2014, 6:34pm UTC](https://www.ruby-forum.com/t/sampling-slowly/234864/5 "2014-11-30T18:34:33Z")
Yes, it worked but the CPU load is immense as the function probe is probing about as fast as it can and of course there is still overhead in looking up the time. Basically if the time in seconds is the same as last time it just returns the previous sample without repeating any calculations. I will try and sync it to the sample rate to see if that slows it down. Mike From: Seth Hitefield [[mailto:removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid)] Sent: 30 November 2014 16:25 To: Mike W. Cc: [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) Subject: Re: [Discuss-gnuradio] Sampling slowly That may work, but it may cause problems with the scheduler if you are using stream ports. - Seth Sent from my iPhone On Nov 30, 2014, at 10:47 AM, Mike W. \ wrote: I looked at that but I don’t know how to and it’s extremely difficult to understand. It is probably possible to bodge using the system timer to avoid a re-calculation if called within less than a specific time, e.g. 10 seconds. This is what I am thinking at the moment. Basically reduce the workload. Mike From: Seth Hitefield [[mailto:removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid)] Sent: 30 November 2014 15:35 To: Mike W. Cc: \ Subject: Re: [Discuss-gnuradio] Sampling slowly I would suggest looking at the message-strobe block and using messages. - Seth Sent from my iPhone On Nov 30, 2014, at 10:11 AM, Mike W. \ wrote: Looking for help I have written a source block to do satellite predictions. It acts as a source of a float representing the input parameter – e.g. elevation. The block probes predict via UDP, looks at the string returned and selects the asked for parameter. So far fine, but I would like to probe it slowly, not on every sample. This appears to be impossible. If I use a probe and function probe and ask it to update each second, the probe grabs data from the source runs at full sample rate. Unfortunately probe seems to have almost no documentation and I don’t understand the source but I assume it goes as fast as it can get input samples. If I try combining say an add block with a decimated version of the sample rate (coming from a USRP) and probe again, it still runs too fast. It is as if it is trying to cache a set values. Logically I can see that might be useful to maintain a constant rate where blocks take different times. Is there a simple fix or do I need to implement some form of trigger port – and if I do will it still cache data? Mike