[DECtalk] It's time to bust some DECtalk myths

[Nick Gawronski]

Hi, I read somewhere that screen readers would send the strings of rate 
and pitch to the dectalk every several lines to make sure that the 
device was at the correct settings.  I was using my dectalk express with 
speakup a lot and would often change the rate but when I set it the 
settings never went back to what the default was when the device was 
turned on.  I also would say that perhaps the dectalk is not remembering 
the settings that it was first given when speakup first loads and sends 
the string to the device.  I agree that the speakup developers should 
have test devices as this way they can fix bugs when they find an issue 
or just make sure there are no major bugs in a driver that they say to 
support.  Nick Gawronski

On 9/19/2022 12:06 AM, Don wrote:
> Chime,
>
> Looking through sources without a clear understanding of what is actually
> happening is an exercise in futility.  If it was that easy, the 
> maintainers
> would already have stumbled across it!
>
> Note that whenever you have two devices talking to each other, there
> are many variables that come into play.
>
> For example, if we assume the problem is an overrun in the DECtalk's
> input buffer (coupled with not being handled properly), one has
> to assume the code -- on both ends -- *tried* to work correctly.
>
> But, sending characters out of the PC involves some hardware buffering;
> while one character is "on the wire", another character (or characters(
> are sitting in the transmitter waiting to be sent.
>
> At the same time, the receiver in the DECtalk can be receiving the
> character that is on the wire, while still holding onto the character
> that was sent just prior -- waiting for the software to accept it.
>
> So, when the DECtalk sees that *prior* character as being received,
> it must be accommodated along with at least two more characters that
> are already queued up -- one that's on the wire and another waiting
> in the PC's transmitter.
>
> If the PC can queue more than one character (some can queue a dozen!),
> then these must also be accounted.  Likewise for the receiver in the
> DECtalk.
>
> If the DECtalk uses "hardware pacing" to inform the PC that it
> needs the PC to slow down, then it has to twiddle that signal and
> hope the PC sees it before it queue up yet another character.
>
> If, on the other hand, the DECtalk uses XON-XOFF signalling to
> convey this information, then the DECtalk has to queue the XOFF
> character for transmission -- which may have to wait for any other
> characters to be transmitted to the PC that have been enqueued,
> previously.  Once that character actually hits the wire, it
> has to be received by the PC and recognized as "stop transmitting".
>
> But, while it was "on the wire", another character from the PC
> could have been "on the wire" to the DECtalk.
>
> I.e., just "doing the right thing" is fraught with potential for
> slight misunderstandings -- that manifest only in circumstances
> of high traffic, etc.  You have to see that this is happening
> before you can look at where (and how) to prevent it.
>
> On 9/18/2022 7:04 PM, Chime Hart wrote:
>> Hi Don-and-All: Don, you are most likely correct on all your 
>> comments, but I
>> still think its an error in 1 or more of those Speakup drivers. Looks 
>> like
>> names are
>> speakup_dtlk.c
>> speakup_dtlk.h
>> speakup_dtlk.ko
>> speakup_dtlk.mod
>> speakup_dtlk.mod.c
>> speakup_dtlk.mod.o
>> speakup_dtlk.o
>> You could probably find them online, or I could send them off-line?
>> Please consider when I was in windows with this same device, I didn't 
>> have
>> these issues, but certainly there are windows drivers. So Speakup is 
>> the common
>> denominator. Thanks in advance
>> Chime
>>
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>
>
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