Masters Communications Custom Products for the Digital Radio Amateur Enthusiast

Background:
You may wonder what connection speeds and data transfer rates are possible with your radio and from the DRA Series radio adapters from Masters Communications.

Generally - the radio dictates what speeds are possible. Radios that don't have a dedicated data connector (and only a MIC and speaker jack) are only capable of lower connection speeds. This is because of the type of audio that's available at the microphone and speaker. The bandwidth of the audio here is limited by several things including some aggressive filters:
1 - The high-pass CTCSS filtering in both the receive and transmit audio paths.
2 - The the low-pass (splatter) filtering of the transmit audio.

Even though these filters are not necessary for digital applications, they are still present and they limit the throughput, because you can't turn them off. The high-pass CTCSS filtering in the receiver eliminates the "PL" buzz in the speaker. The PL filtering in the transmitter eliminates voice talk-off because of low fundamental frequencies often contained in the male voice. The splatter filter is needed to clean up the harmonics that are created by the audio (speech) processing (clipping) that's used with a microphone to give the transmit audio some punch. The response of the filters mentioned above is generally 300Hz to 3000Hz, but is often somewhat less depending on the radio manufacturer. In addition to filtering, the audio is purposely tilted. This is called "emphasis". Transmit audio at the microphone adds "pre-emphasis" and audio at the speaker adds "de-emphasis". It's beyond the scope of this document to explain the reasons behind why FM two-way radios use emphasis stages, but let's just say that emphasis is not necessarily a good thing when conveying digital data.

Radios that are designed for 9600 baud, like, for example the Yaesu FT-8800 or Kenwood TM-V71A, have a data port that's designed to work at higher speeds. For 9600 (or VARA FM Wide operation), receive audio is sourced from the discriminator or detector and minimally buffered and filtered. The 9600 audio is not de-emphasized, squelch gated or have its level adjusted by the volume control. Audio for the transmitter is applied to the FM modulator with some modest filtering, but does not add pre-emphasis. As such, these connections bypass the emphasis and aggressive filters I mentioned earlier for CTCSS and splatter filtering for MIC and SPKR audio. While there may be some filtering in-place, the audio bandwidth is much wider for both the receive and transmit audio paths at these connections, and its frequency response is much flatter. It's because of the type of audio (and the resulting wider audio bandwidth and flatter response) that's available here that higher connection speeds are possible.

You can expect Level 13 connectivity in VARA FM Wide (licensed) using the DRA Series interfaces with radios that are 9600 baud (wide-bandwidth) capable. Radios that have a dedicated data port usually provide both 1200 baud receive audio (that's de-emphasized and otherwise filtered) but is not squelch gated or controlled in level by the volume control. It's usually best to use the 9600 audio regardless of actual throughput of the application. For best overall compatibility, all stations should be able to provide the same amount of throughput. The chain is only as strong as the weakest link.

These days, the references to "1200 and 9600" (baud) are a bit of a misnomer. Programs like VARA don't have a static transfer rate. VARA is dynamic and will change its connection speed based upon several things like band conditions, audio level and quality. VARA FM Wide can transfer data much faster than 9600, even though that's the data rate specified by the radio manufacturer and what's printed in their manuals. Using terms like "wide-band" and "narrow-band" in reference to the audio types are actually more accurate descriptions, but don't expect the radio manufacturers to adopt something different (and more technically correct) any time soon. Just realize that there are different types of audio available from data capable radios, and for the fastest connection speeds, you'll want to use the widest ones that are available.

What about portable radios? For the most part, Hand-held radios are limited to lower baud rates because you only have the MIC and Speaker signals to connect the sound card interface to. While there are some portable radios that do have a dedicated data port, most do not. Receiver audio is de-emphasized, filtered, and is subject to the level of the volume control. Transmitter audio is pre-emphasized (tilted), processed (clipped) and heavily filtered on the top end by the splatter filter. Getting past Level 7 or so in VARA FM Wide is difficult when using this type of audio, but it usually works fine with 1200 baud packet and most levels of VARA FM Narrow.

So what about the DRA? What determines its fastest possible connection speed?
The DRA Series radio interfaces (all of them - no matter their cost) use a C-Media CODEC that is capable of generating and processing an audio bandwidth from 20 cycles to 20kHz. The audio paths are kept low in noise and distortion because of its design, the choice of components, and the PC board layout. In the case of digital data conveyance, the DRA interfaces exceed the capabilities of (any) two-way radio by a lot. They can certainly operate beyond the limits of what's currently legal from Part 97. Don't let that worry you though, most radios are incapable of being operated beyond the FCC rules because of inherent safety-net filtering.

While I don't keep track of the capability of every radio, this article should help you determine what's possible with most radios. Remember - radios with a dedicated data (modem) connection with "9600" baud audio type allow for the fastest data transfer speeds.

Email Kevin Custer for additional support.