[RTTY] TinyFSK modem for 45 and 75 baud

[Joe Subich, W4TV]

> However, many people use standard consumer-grade USB-Serial
> converters without true UARTs and they therefore require the EXTFSK
> optional module for MMTTY to provide correct Serial timing.

Not quite correct ... most of the commercial USB-Serial converters
are true UARTs.  The problem is that the chip foundries have increased
the internal clock speeds in order to operate at higher data rates for
mass storage applications, processor interfacing, etc.

These higher maximum data rates create a problem because of a fixed
ratio of the maximum to minimum data rate.  UART data rates are set
by a 14 bit "data rate divisor" which means (other than certain
special cases) the maximum rate can't be more than 2^14 (16,384) times
the lowest supported rate.  As the USB specification has expanded to
support ever higher data rates and equipment designers have demanded
hardware support (e.g., transfer 64 GB of data to/from a camera,
MP3 player, phone, tablet by USB) for faster data rates, the chip
fabricators have been forced to abandon support for the "obsolete"
data rates.  Most new USB UARTs are now limited to 300 baud minimum
and I don't know of any designs introduced since the advent of the
USB 2.0 standards that support a minimum data rate rate less than
110 baud.

73,

    ... Joe, W4TV


On 10/11/2013 3:00 AM, Ed Muns wrote:
> Nothing ... IF your computer has real, i.e., with good UARTs, hardware
> serial ports., or you use a USB-Serial converter such as the Edgeport units
> with real UART components.  However, many people use standard consumer-grade
> USB-Serial converters without true UARTs and they therefore require the
> EXTFSK optional module for MMTTY to provide correct Serial timing.  Turns
> out that MMTTY's EXTFSK's "correct Serial timing" is not so correct.  Thus,
> a solution like Andy's is warranted to obtain perfect RTTY timing via USB.
>
> The Edgeport converters are commercial products retailing for over $300, but
> I've bought lots of them for $30 or less (4-port) and $60 or less (8-port)
> on eBay.  Just two different approaches to obtain proper Serial timing.
>
> Ed W0YK
>
> Andy K0SM/2 wrote:
>>> For those of you looking for a relatively simple and
>> inexpensive method of doing 45 and 75 baud RTTY using logical
>> keying ("FSK") you can have a look at a TX-only modem that I
>> built that will accomplish this.
>
> Bill W6WRT wrote:
>>
>> REPLY:
>>
>> What does this do that MMTTY and a pair of one-transistor
>> drivers for FSK and PTT does not do?
>
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