Timing send interval

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Hi I have developed a software that sends media using the RTP protocol using C++, the packets is sent every 20ms, and the files are read from discs. My problem is timing the send inteval, I have not found a good way to wait exactly 20 ms without overusing CPU resources.
We all now that Sleep(20) could return any time after 20ms and can not be used. The way I do it now is by polling a timer, and that works perfectly, but a hate the solution cause it is using to much cpu and it is ugly.
Does anyone has a suggestion?

Thanks in advance.

Posted 19-Apr-12 1:51am

Mikael Segerstein

Updated 19-Apr-12 4:11am

v2

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Resmi Anna 19-Apr-12 7:55am

You can use the function WaiteForSingleObject(), if you can fire some event from the place where you finish sending the media

Mikael Segerstein 19-Apr-12 8:53am

Thanks for your suggesion. It will still not work, sending the next packet to soon will flood the receiving buffer on the receiving part, I need a short delay. The receiving part is usually a softphone or asterix switch.

3 solutions

Solution 1

Windows is not a "real time operating system". There are no guarantees on timing of any event.

Even if you sat in a loop reading the cycle counter to determine exactly when 20ms has passed, your process may still be preempted by some obscure Windows process that may take *SECONDS* to complete and return to you.

You can't get that accurate with Windows.

Now granted that you did not specify the operating system and "sleep()" does exist on many other operating systems (although you did seem to evaluate a solution based on WaitForSingleObject(), a Windows call). But most generat purpose operating systems still do not provide a complete "real time kernel" effect that would give you the precise timing you are looking for.

Posted 19-Apr-12 4:02am

Chuck O'Toole

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Mikael Segerstein 19-Apr-12 10:19am

Your assumption that it is Windows is correct, I have updated the tags, thanks for pointing it out. The solution I have now is working surprisingly well but since it it using way to much cpu, I can not have as many simultaneous thread as I would like.

JackDingler 19-Apr-12 11:47am

Are you putting any Sleep calls in between the timer calls?

If Sleep(20) is too inaccurate, how about Sleep(5)?

Mikael Segerstein 19-Apr-12 17:43pm

No, I don't. If I do, the media transfer is slowed down.
Can't use Sleep(), not even with lower values like you suggest. It can still wake up after 50 ms or so.

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Jochen Arndt · Accepted Answer · 2012-04-19T04:10:00

Solution 2

You may use a WaitableTimer. See CreateWaitableTimer()[^] at MSDN and the example link [^].

If this is still too inaccurate you may tweak the timer resolution using the undocumented function NtSetTimerResolution() (use GetProcAddress() to get the function address from the Ntdll). See http://www.decuslib.com/decus/vmslt97a/ntstuff/timer.txt[^].

See also the Timers Tutorial[^].

Posted 19-Apr-12 4:10am

Jochen Arndt

Comments

Mikael Segerstein 19-Apr-12 10:29am

Thanks for the links, I'll have a look at them.
Thank you for your effort.

Mikael Segerstein 19-Apr-12 17:52pm

The solution was also found in one of the links you sent so thank you for pointing them out.

nv3 · Accepted Answer · 2012-04-19T04:11:00

Solution 3

You should take a look at Windows Multimedia Timers, thus the function in the group:

timeSetEvent
timeKillEvent

timeGetDevCaps

I personally haven't worked with them, but MS promises that they are way better for the precise timing required in multi-media applications.

Posted 19-Apr-12 4:11am

nv3

Comments

Mikael Segerstein 19-Apr-12 17:49pm

I have been doing some test with your suggestion and I belive I can make this work. It is not totally accurate but it will be a big improvment.
Thanks.