[time-nuts] RS 232

Didier Juges shalimr9 at gmail.com
Fri Jul 26 15:50:26 EDT 2013


There is a difference between managing the latency (as in ensuring that sound and video are synchronized, but latency itself is acceptable) and minimizing the latency as in a Morse code keyer where the operator has to manually control the generation of elements that can be as narrow as 20mS (one dit at 60 words per minute) while getting timely aural feedback. That means you need the sound to start and stop within less than about 5 mS following the key closing and opening.

It is trivial to do on a microcontroller running at 1MHz but surprisingly harder to do on a 2GHz Windows machine.

It is not just a matter of time stamping the key closure, you have to get the sound system starting and stopping.

Didier KO4BB




Jim Lux <jimlux at earthlink.net> wrote:
>On 7/26/13 4:41 AM, briana wrote:
>> Ever since WINxp arrived on the scene hams who send code  via
>computer
>> to radios via parallel, serial or usb ports (with serial port
>converters
>> following) have seen the latency issue in spades.  We're talking
>about
>> effective baud rates less that 50.   3-4 milliseoond variable latency
>> changes making the code nearly unreadable.   The killer is that the
>> latency changes randomly.
>>
>> Previous to WINXP one could do direct writes to the ports under
>software
>> controlled timing.  All was good.
>>
>> The solution for WINXP  was to bypass WINDOWS handling of port data
>via
>> a DLL called DLPORTIO
>> There is a similar one for WIN7.   I haven't timed how accurate it
>is.
>> However 65 words per min (6 characters/second) code can be sent with
>no
>> detectable timing problems.
>>
>> The simple act of open and closing a set of contacts at precise times
>> now requires a huge, faxt machine, tons of software and software to
>work
>> around the normal software.   That's progress?
>>
>>
>no.. what it means is that you have a bad system architecture.  You 
>shouldn't be trying to do hard real time on a machine primarily
>designed 
>for user interface experience and running office automation tools like 
>word and excel.  It is no more reasonable to expect a modern PC 
>(primarily a media display device) to do hard real time control than it
>
>is to expect an iPhone to do so, or even a mainframe computer.  The
>days 
>are long past when a PC is basically a microcomputer with a few limited
>
>peripherals.
>
>Hams have problems because the developers have insufficient budget or 
>resources to devote the time to writing appropriate code using the 
>Windows media stack to get the synchronization performance desired. 
>They're looking for the "quick fix", a'la direct port writes.
>
>
>Note that real time action in games works fairly well, as does keeping 
>the video and audio synchronized in various and sundry media players, 
>even when playing through USB connected devices.  Midi sequencers run 
>just fine with Windows.
>
>So, it's more a matter of spending the enormous amount of time needed
>to 
>become facile with the entire multimedia API, all the various hooks and
>
>widgets in Windows, etc.   This is a non trivial task, and one that 
>cannot be done in spare time on the weekends for most people. You 
>generally need to be immersed in the "windows way" of doing things 
>(which is exceedingly different from DOS, microprocessor, or Unix) and 
>understand how it works, and then you need to be using Windows 
>development tools and have the appropriate libraries, etc.  The Windows
>
>ecosystem is quite powerful, but it's different than other ones, so a 
>life of Unix kernel hacking might give you the general background, but 
>will result in you fighting with Windows at every turn, because it's 
>just different.
>
>And, in some cases, moving the timing critical operations off to a 
>separate device is going to be the wisest plan.  The Roland MPU401 Midi
>
>box was one of the first to do this, back in the DOS days.
>
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-- 
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