Custom Products for the Digital Radio Amateur Enthusiast
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Custom Products for the Digital Radio Amateur Enthusiast |
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By Kevin Custer W3KKC
Simple Explanation:
C-Media Hardware PTT is a logic level change in the radio interface (directly at the C-Media soundcard chip) and results in a push-to-talk signal that is used to key the radio. All RA's, RL's and DRA's have C-Media Hardware PTT capability because we use C-Media chipsets in all of our radio interfaces. Some application authors use the term "CM108" or a different reference to CM(some number), but functionally all current and legacy C-Media sound card chips can produce a compatible PTT signal from its GPIO 3 logic lead.
Some radio interfaces derive the PTT by detecting the presence of audio from the application. That audio is detected by a VOX circuit that generates the logic that actuates a relay to make the radio transmit. Some digital programs use a form of Serial communication (RTS/CTS - DTR/DSR) thats decoded by a serial chip (FTDI) and one of these signals are used to key the transmitter. CAT and COM-0-COM are additional protocols that can be used to key the radio. Each / any of these could be referred to as "Hardware PTT" but they are all significantly different from one another.
DRA's don't use real or virtual Serial or CAT signaling, and they don't create or use a real or virtual Comm Port. They appear in Windows (under Device Manager) as a sound card. Reading this article in its entirety details what's different.
The programs listed below are directly compatible with C-Media Hardware PTT, and do NOT require a VOX capable DRA. By default, no program automatically configures C-Media PTT - it must be configured manually in the software to properly assert the push to talk and actually key the radio.
If you have a VOX capable DRA - it's best to Disable VOX and configure the software to
generate the PTT using the C-Media Hardware PTT function. Programs need to be configured to
do that - they don't sense the hardware and do it automatically. For instruction on how to enable C-Media
Hardware PTT for a particular application, follow the hyperlinks for the specific software program
you are using. IE: click its link, when there is one.
This VARA PTT support article details how to set up VARA to use what José calls "RA-Board PTT". References to RA-Board also include/mean DRA-Board.
This SoundModem PTT support article was written by Scott Currie NS7C. Again - references to RA Board also include/mean DRA-Board. At a minimum, you'll need to edit "PTT Port" by adding EXT and then "Advanced PTT settings" and choose "USB PnP Sound Device". You may find the "TEST" button very helpful.
This FLDIGI PTT support article details
how to set up FLDIGI to use "C-Media GPIO PTT" (Hardware PTT means something different in fldigi!)
NOTE: Hardware PTT in fldigi refers to a different method of keying the radio and won't
work with DRA's.
Use the information called "C-Media GPIO PTT" by scrolling down in the page.
Additionally - Be sure to go through the other PTT settings in fldigi and make sure C-Media PTT
is the only option selected. You can run into trouble switching rigs where you may have had a different
PTT mode selected at the same time as C-Media PTT. It can be done and it causes some sort of conflict.
We've reported it to Dave, but he said that the program was working as designed.
Additionally - Be aware that the rate of transition between the transmit and receive is VERY
quick when the TEST button is actuated. Some radios cannot follow this rate of transition and it gives
the appearance that the radio isn't transmitting. However - when the program is used normally - everything
works as expected.
For Direwolf, you'll need to download version 1.7 (or later) and add "PTT CM108" to the direwolf.conf file. While DRA's use a CM119A soundcard chip, the CM108 PTT method works identically for DRA's.
Detailed Information - for those willing to continue reading:
You'll see several references to articles on our FAQ page explaining how different programs can
directly assert the PTT signal to key the radio. In addition to C-Media Hardware PTT, our DRA-65,
'70, '80, '100 and 'SR also have VOX, but this document focuses on the C-Media method of producing
the PTT signal.
Not all digital programs and applications have the C-Media Hardware PTT, but more and more are updating to include it. For those programs that don't, our DRA's with VOX allow you to use applications that don't have Hardware PTT. If the program you're wanting to use isn't listed above, you'll probably need a VOX capable DRA or another method of asserting the PTT. FT8 (WSJT-X) is one that currently requires VOX or CAT.
Technical Explanation:
The C-Media Hardware PTT operation on the RA, RL, and DRA Series was originally designed for
AllStar Link. AllStarlink or "ASL" is a software
analog VoIP repeater linking and hotspot application and platform. Think of it as a computer (PC
or Raspberry Pi) based repeater controller. Under this application, when PTT is asserted, the
application toggles GPIO3 on the big 48 pin C-Media chip producing a logic change in conjunction
when it's necessary to make the radio/repeater transmit. The PTT logic developed at pin 13 of the CM1XX
is buffered by a 2N2222 bipolar transistor to make the radio transmit. "Hardware PTT" is a function
asserted from software that results in a logic change at the hardware level for the purpose of making a
radio transmit.
The advantages of Hardware PTT over VOX:
VOX detection inherently takes time, both to detect and create a PTT, and time to decide when to
stop producing a PTT signal. This lag adds time to the back and forth transition between receive and
transmit and transmit and receive. Hardware PTT is commanded directly by the software, and is not related
to the presence of audio. In other words, the keying of the radio is totally independent of audio.
As such - traffic can be passed more efficiently. This can be a big advantage when passing a lot of
traffic, especially on faster negotiated connections (VARA FM Wide).
Proper VOX operation requires the operator to set audio levels sufficiently high to produce reliable PTT operation. Because of the level of audio required for VOX, you may find it difficult to set the modulation level on the radio - because of how high the levels are set. The result is a touchy TX level pot. However - Hardware PTT doesn't rely on the presence or amount of audio being produced by the radio interface, and you're free to reduce the Windows speaker level or TX level in the application (or both) for a more comfortable TX level (control pot) position.
History:
The DRA Series radio interfaces (sound cards) are an evolution of the RA Series AllStar Link radio
adapters. RA's were the first Masters Communications devices to have this type of PTT generation
- back in 2014. As such, DRA use the same hardware design. All RA RL and DRA Series radio interfaces
have C-Media Hardware PTT functionality and will work with several digital applications updated to
provide this type of operation.
The use of our radio interfaces with digital applications was a vision of Scott Currie NS7C. He is the first person I know of to use our RA Series to convey high-speed digital data. You will see references to "RA Board" in the various programs. This is because the Hardware PTT functionality was incorporated into some of these programs before the DRA Series was produced. In these programs "RA Board" also means "DRA Board" or even RL Board.
Van Malan NH7IT is the person mainly responsible for the addition of C-Media Hardware PTT being added into several digital data programs. The DRA Series is an evolutionary product allowing for easier, faster, better (and possibly cheaper) digital data deployments.
Enabling Hardware PTT Only - (Disabling VOX PTT):
If you have a VOX capable DRA, you should disable VOX when using Hardware PTT.
The time required to switch between transmit and receive is faster when using Hardware PTT, and
results in faster back and forth negotiations and throughput - as long as VOX is disabled by removing
the VOX enable jumper on the DRA. Disabling VOX in software does NOT actually disable it on the DRA.
You MUST remove the VOX enable jumper on the RED 2-pin header. Yes - the VOX enable/disable "header"
is always red on any DRA, even though the JU jumper number may be different.
Email Kevin Custer for additional support.