The Utah VHF Society - Using analog test equipment for evaluating D-Star system performance

The Utah VHF Society

Using conventional analog test gear to evaluate and test D-Star systems

Purpose of this page:

As new technologies come into use on the amateur bands, there is an increasing challenge to be able to evaluate and support these technologies. In the past, conventional test equipment has been used to maintain and diagnose such systems, but with these new technologies there is a challenge to be able to provide a means of being able to support such systems in a meaningful way.

An example of such a technology is D-Star. As this (and similar) systems become more widespread, the challenge to be able to design and maintain such systems increases. Using conventional test gear, one is limited in exactly how much diagnosis is possible - but there are still a few things that can be done to determine important aspects of the system's performance.

Important Notes:

This page deals only with the narrowband D-Star modes as found on the VHF and UHF U.S. amateur bands, and not the "high speed" modes that may be used on 23cm.
ONLY analysis of the disruption of voice transmission was considered. If the transmission of data is to be the primary concern rather than digital voice, it is possible that even more protection may be required to maximize performance.

A bit of background:

**Figure 1:**
Comparative spectra of D-Star (left) and typical analog (right) signals. In each case, vertical divisions are 10 dB and horizontal divisions are 2 kHz. An unmodulated carrier has been overlaid atop both images and the green line represents the level of the unmodulated carrier.
**Click on image for a larger version**

D-Star is simply FM. More specifically, it uses Frequency-Shift Keying (FSK) to convey data. By properly shaping the modulating waveform and appropriately choosing the amount of deviation, the transmitted spectrum can be adjusted to minimize the occupied bandwidth while still maintaining reasonable power efficiency in terms of being able to transmit data.

Figure 1 shows the typical transmit spectra of a D-Star signal and compares it with a typical analog NBFM signal, showing the "peak + average" power density. Because the data stream is fairly consistent in its spectral content, the spectral makeup of a transmitted D-Star signal is very consistent. Because it is 4800 baud, the highest modulated frequency is 2400 Hz, with numerous sidebands resulting from the modulation of the data stream. Another strong component of the D-Star voice signal, as can be seen in Figure 2, is that of the 50 Hz voice frame rate: It is this that causes the characteristic "buzzing" sound that is heard when a D-Star signal is monitored on an analog receiver.

As can be seen from Figure 1 the majority of the energy of the transmitted D-Star is constrained to within a few kHz of the carrier, with "nulls" at approximately +-3.6 kHz from the center frequency and sidebands of decreasing energy beyond that. It is through careful shaping of the modulated signal and the appropriate amount of deviation that this transmitted spectral shape is obtained.

Comment: Please take note of the resolution bandwidth of these analyzer plots and its effect on the relative power density of the modulated and unmodulated carriers.

D-Star baseband modulation:

The baseband modulation (that is, the signal being fed into the modulator) of the D-Star signal consists of 0's and 1's being modulated onto the carrier, but to simply throw a 0 or 1 (represented by a logic level) at the modulation would result in an abrupt frequency/phase change, causing the transmitted signal to occupy considerable bandwidth. It makes sense, then, to slow down the rate of change that can occur during modulation - but one can only go so far: With the 4800 bit-per-second D-Star signal, we could send alternate 0's and 1's. Without filtering, this would become a 2400 Hz square wave, but with filtering, this would turn into a 2400 Hz sine wave - a signal that would take a fairly minimal amount of bandwidth to modulate.

Filtering the original "square wave" data into something resembling a sine wave is rather tricky. If all that you wanted to do was to generate a 2400 Hz sine wave to transmit alternating 0's and 1's then it would be easy, but with data, you will have a combination of 0's and 1's - sometimes several of each in a row. When trying to filter the original data, one must make sure to minimize the filter's "memory": Suppose that you had been sending a bunch of 0's - but then a single "1" comes along, followed by a bunch of 0's. With a simple filter, everything will settle out to a "0" state - but when a "1" comes along, it has to be able to fully change to a 1 - and then fully change back to a 0. Improper filtering will tend to cause the previous state to "linger" and it can be more difficult to determine, upon decoding, if and when, exactly, that "1" began and ended.

To solve this problem, our single "1" is turned into a smooth pulse - one that can go from 0, to 1, and back again smoothly - and it so-happens that the filtering used to do this is Gaussian in nature - the name referring to a particular shape of the pulse and its properties. It also so-happens that with this sort of pulse filtering, if you were to alternate 0's and 1's, you would, in fact, end up with a nice sine wave as can be seen in Figure 3. Because the fastest that one could "change" the waveform with the 4800 baud D-Star signal is, in fact, 2400 Hz, that is the maximum peak frequency that can be modulated.

**Figure 2:**
Spectrum analysis of a baseband D-Star signal. There is a null at 4800 Hz correlating with the bit rate and there are strong spectral components at intervals of 50 Hz that correlate with the 20ms voice frame.
**Click on image for a larger version**

Because we aren't sending just a "01010101" all of the time, this nice, continuous sine wave is constantly being interrupted to form the data stream and in so-doing, multiple spectral sidebands result - which is why, in Figure 2, there is not just a peak at 2400 Hz. Instead, energy is spread around 2400 Hz but very little of it goes above 4800 Hz.

There is another important aspect of the D-Star modulation: The amount of deviation. For mathematical reasons, good spectral and power efficiency for this type of modulation occurs, with data, when one sets the total deviation to be one-half of the baud rate: Because the baud rate, the total deviation is 2400 Hz, or +- 1200 Hz, and this particular setting is referred to as Minimum Shift Keying. Because we have already pre-filtered our data with a "Gaussian" filter, the combination is called Gaussian Minimum Shift Keying, or GMSK.

Don't let all of this scare you: All one really needs to remember is that the D-Star's baseband modulation consists of bits of 2400 Hz sine waves.

Another important aspect of the D-Star's baseband modulation is the limitation of low-frequency components. If too-many 0's or 1's were transmitted sequentially, the low-frequency content of the baseband would increase and the DC level representing a 0 or a 1 could become indistinct - particularly if capacitive coupling were used. Another problem with low frequency content is that the radio's synthesizer works by locking the "average" frequency. Because this is frequency modulation, the synthesizer was designed to avoid canceling out the modulation - but this is typically done by preventing the synthesizer from responding to any by the slowest changes in frequency. If the baseband modulation has too much low-frequency content, the synthesizer will attempt to track it and cancel it tout. As it turns out, the data stream used for D-Star voice has some fairly low-frequency components, most notably the 50 Hz "voice frame" rate. Because of this, the frequency response of the baseband must extend down well below 50 Hz to avoid distortion of the waveform.

D-Star transmitter and receiver:

As it turns out, the D-Star transmitter is just an FM transmitter - with a few special considerations given to assuring that it will properly pass both low (<30 Hz) and high frequency (to 4800 Hz) energy with a minimum of distortion. Likewise, for reception, a D-Star receiver simply takes the demodulated signal from the discriminator and passes it to the modem board.

Because D-Star is just FM, it follows that standard test gear designed for use with FM communications gear may be useful in the evaluation and diagnosis of D-Star equipment - although some of the techniques for doing so are different for standard analog voice.

Comment: For the purposes of this discussion, we are ignoring the fact that some Icom radios - specifically their repeaters and the ID-1 - generate their D-Star modulation through baseband/quadrature methods rather than direct FM.

Tests using analog test gear:

There are a number of tests that one may do using normal analog test gear to verify performance of a D-Star radio. To some extent, these rely on the assumption that the codec in the radio is working properly, but if there are other problems with the system, one may be able to determine what they are.

Transmit power test:

Being that the D-Star transmitter is simply an FM receiver with a digital codec, one can perform normal tests for forward and reflected power: No surprise there. Like analog FM, there is no amplitude component present, amplifier linearity and measurements of PEP are irrelevant.

Frequency test:

It is possible to use ordinary means to determine whether or not a D-Star transmitter is on-frequency: The modulation should not skew readings to any significant degree. Don't forget that most D-Star radios are capable of analog modes as well so one may simply switch the radio to an analog mode to check to see if the radio is within specifications.

SINAD test:

One common test of receive system performance is the SINAD test. For this test, a single, precise tone is generated - usually 1 kHz - at a standard deviation - usually +-3 kHz in the U.S. The level of this tone is then compared to the amount of noise that is NOT at 1 kHz. For a full-quieting signal, a SINAD reading of over 30 dB may be expected for most radios, while a SINAD of just 12 dB sounds quite noisy, but is still easily intelligible to most people.

Switching a D-Star capable radio to analog and running a SINAD test is a convenient way to verify its performance. Note, however, that for the D-Star digital modes, FM-Narrow mode is used. Because the nominal peak deviation in "narrow" mode is +-2.5 kHz, a deviation of +-1.5 kHz is often used instead of +-3 kHz for the 1 kHz test tone.

"Equivalent SINAD" test:

It is possible to relate the SINAD in FM-Narrow mode to the performance in D-Star digital voice mode. This is is possible because the SINAD measurement tells us something about the amount of extraneous noise in the receiver's baseband - something that correlates well with data errors. This test is handy as it requires no special test gear at all, other than what would be used for SINAD measurement.

**Figure 3:**
Baseband waveform of a D-Star signal. In this image can be seen a period of alternating 0's and 1's (toward the right.) Also evident from this picture is a bit of DC level (or low frequency) shift caused by the IC-91AD's synthesizer attempting to track the data.
**Click on image for a larger version**

Notes:

For the measurements below, an Icom IC-91AD was used.
It is unknown at the time of this writing what sort of analog baseband test signal is readily available from the receiver module of Icom repeaters.

For this test, three levels of D-Star signal disruption were investigated:

"Clean" audio decoding: No bit errors were observed over a period of 60 seconds or so.
"Mostly clean" decoding: One "bloop" (an unrecoverable bit error) occurred every 10 seconds or so.
Loss of D-Star sync: At this error rate, not only has recovered speech become unintelligible, but the receiver can no longer maintain synchronization on the D-Star signal.

For this test, two types of situations were simulated using test equipment:

Weak signal degradation: For this test, the signal level of a D-Star signal was reduced until each of the 3 levels of D-Star signal disruption were achieved.
D-Star adjacent channel degradation: For this test, another D-Star signal was generated 8 kHz offset from the one being received. With the test signal set at -90 dBm, the level of the interfering signal was increased until each of the three levels of D-Star signal disruption were achieved.

When each of the three levels of disruption were reached, the IC-91AD was switched to FM-Narrow mode while, at the same time, the test generator was switched from generating a D-Star signal to generating an FM signal modulated with a 1 kHz tone at +-1.5 kHz deviation: At this point, an un-weighted SINAD measurement was taken using the audio from the IC-91AD's speaker connector.

As it turned out the SINAD readings for each of the "D-Star" signal disruption levels were the same whether the degradation was due to a weak signal or adjacent-channel interference. The correlating SINAD levels were:

Table 1: Comparing SINAD of an analog signal (as received in FM-Narrow mode) to the perceived quality of a D-Star signal.
Quality of digital signal	SINAD in "Narrow FM" mode	Additional comments
"Clean" D-Star decoding achieved	17-18dB SINAD	No audible decoding errors of digital audio
"Mostly clean" decoding	15.5-16dB SINAD	Occasional "bloops" in audio (approx. one every 10 seconds)
"Ratty, but mostly copyable"	12dB SINAD	Considerable degradation in the digital signal, but mostly copyable by an experienced operator. Synchronization to a received signal typically took 2-5 seconds.
"Loss of sync"	9dB SINAD	The D-Star decoder would not maintain reliable lock of signal and no intelligible audio was recovered

Comment:

With the narrower bandwidth used for D-Star recording, a theoretical 2-2.5 dB weak-signal gain should be obtained due to the reduction in detection bandwidth, as compared to the "normal" (+-5 kHz) FM mode. In reality, this difference is closer to 1 dB owing to some S/N gain in the wider bandwidth due to the wider deviation.
The above test requires that one has onhand a D-Star receiver that is capable of analog (FM) reception in the "FM-Narrow" mode.

Comparison of Analog and Digital signals of equal levels:

In this test, we decided to see how a D-Star signal of various strengths correlated with a conventional "wide" analog FM signal in terms of copyability. This test can be useful in that, using one's own experience as well as conventional test gear, get a general idea as to how a D-Star system might perform under similar circumstances.

It is important to remember that this test may not be entirely valid in the presence of adjacent-channel interference as a typical D-Star receiver has somewhat narrower bandwidth and it may be somewhat less-susceptible such interference: If done over the air, this sort of testing should be done while potential adjacent-channel signal sources are not transmitting.

For this test, the following configuration was used:

An Icom IC-91AD was used to generate a signal in FM-Wide (+-5kHz) mode.
Both analog and digital signals were received using an Icom IC-2200H.
The signal/noise of the received was reduced and SINAD measurements were taken using a 1 kHz tone modulated to +-3kHz.
SINAD readings were measured using the external speaker connector of the IC-2200H using both "unweighted" (unfiltered) and CCITT weighting as noted.
At each "step" of SINAD readings, the 1 kHz tone used for the SINAD measurements was sent, analog voice was sent, and then both the IC-91AD and IC-2200H were switched to D-Star voice (DV) mode.
For each test, the audio from the IC-2200H was recorded.

Audio recordings made of these of the received signals consist of three parts:

About 10 seconds of 1 kHz tone as received in analog mode used for measuring the SINAD.
A voice recording transmitted and received in analog with the peak deviation set to +-5 kHz, the standard for analog FM use with the IC-2200H set for "Wide" FM mode (e.g. standard for +-5kHz deviation.)
One or more repetitions of a voice recording as transmitted and received in D-Star mode.

Table 2: Comparing SINAD of an analog signal (as received in "Wide" FM mode) with
Quality of analog signal	Link to recording	Comments about analog signal quality	Comments about digital signal quality
12dB unweighted SINAD (13dB CCITT)	12dB SINAD Test	Analog signal is copyable by the majority of listeners with little or no difficulty.	Noticeable degradation of the digital stream, but still generally copyable speech. At this level, it takes 2-5 seconds before signal lock is achieved.
7dB unweighted SINAD (10dB CCITT)	7dB SINAD Test	Analog signal is quite noisy: Copyable by experienced operators with little or no difficulty and with only minor difficulty by inexperienced listeners.	There was considerable degradation of the digital stream resulting in "recognizable but mostly uncopyable" speech. At this level, it takes 5-7 seconds before signal lock is achieved.
3dB unweighted SINAD (5dB CCITT)	3dB SINAD Test	Analog signal is very noisy: Generally copyable by experienced listeners, with some difficulty by inexperienced listeners.	The receiver would not lock on digital signal: Signal was briefly boosted 10dB to force lock (during the "This is K7" portion) and then reduced to the original level.

Comments:

The above recordings have been MP3 compressed to reduce file size and the fidelity of the analog portions, especially in the presence of noise, may suffer somewhat. (Uncompressed versions of the above files may be obtained by changing the .mp3 suffix in the above links to .wav).
At the 12dB SINAD level, it usually took 2-5 seconds for the digital voice stream to acquire lock. Users should keep this property in mind when making short transmissions, or if important information is placed at the very beginning of a transmission.
When a D-Star transmission begins, it is preceded by a short preamble that is used by the D-Star decoder to rapidly recognize and acquire lock onto the signal. If this preamble is missed, as may be the case when signals are weak and/or multipathy, it can take several additional seconds for D-Star decoder to lock onto the signal and produce audio.
At the 7dB SINAD level, the D-Star decoder usually locked within 5-7 seconds, but only a few bits of the speech were recognizable.
As mentioned above, the D-Star decoder would not reliably lock onto the D-Star signal at the 3dB SINAD level: The D-Star signal was briefly boosted (during "This is K7") by 10dB to allow the receiver to lock onto the signal and then reduced again to the 3dB SINAD level.
At weaker signal levels (7-12dB SINAD) slightly better results (1dB or so) were obtained with the digital signal when the deviation was artificially boosted to the 3-4 kHz range, well above the recommended 1.2kHz setting. Note: This is not a recommended practice as it causes the transmitted signal to significantly exceed the design bandwidth of D-Star.

"Why are your results different from those obtained by the ARRL?"

In the June, 2005 issue of QST, there was a review of the Icom IC-V82 HT. Associated with this review was a brief overview comparing D-Star and Analog FM signal performance. ARRL members may read this article here: http://www.arrl.org/members-only/prodrev/pdf/pr0506.pdf

In this article the ARRL lab reports that a D-Star signal maintained "...solid, virtually noise-free communication, equivalent to 'full-quieting' at any analog SINAD above 6dB." Our results do not reflect this and we thought that the discrepancy was likely a result of possibly different methodologies used in measuring SINAD. Fortunately, the ARRL has put their "Test Procedures Manual" (available online to ARRL members at this URL: http://www.arrl.org/members-only/prodrev/testproc.pdf ).

Having reviewed the ARRL's procedures for measuring SINAD and determined that our methods are equivalent to theirs, we are at a loss to explain the discrepancy between our readings and those stated in the June 2005 article, or why the results obtained by the ARRL lab do not correlate with Icom's own specifications: If you conduct similar measurements, please inform us of your results!

Comment: It is suspected that some of the signal/noise readings mentioned in the ARRL article were observed at the modem's input (e.g. discriminator audio) rather than the radio's audio (speaker) output. SINAD measurements taken at this point would, in fact, reflect a much lower reading that those obtained after de-emphasis - such as those at the speaker terminal.

Checking the deviation of a D-Star transmitter:

To create an MSK signal, the deviation of a D-Star transmitter should be set to +-1.2 kHz: As mentioned above, this value is chosen so that the total amount of deviation (2.4 kHz) is equal to half of the bit rate of 4.8 kbps to generate an optimum signal in terms of occupied bandwidth and BER performance.

To verify that a D-Star transmitter is set up properly, one may use the same methods used for setting the deviation of any FM transmitter. An important note here: For this test, one must make sure that the test equipment is measuring "flat" FM rather than PM, or FM with some sort of filtering switched in (e.g. CCITT, etc.)

"Excess" deviation due to "PLL Wander":

There is a caveat with this measurement, however: Some of the Icom radios (such as the IC-91AD and IC-2200H) tend to suffer from "PLL Wander" as can be seen by observing the low-frequency shift in Figure 3. This effect is caused by the radio's synthesizer trying to track low-frequency components (such as the 50 Hz "voice frame rate") of the D-Star waveform with the result of the transmitter wandering up and down several hundred Hz about the center frequency. The result of this is that the "deviation meter" on many pieces of test equipment may read an amount of deviation higher than that of the D-Star's modulation. If this occurs - and the deviation is set to +-1.2 kHz, this could result in the actual D-Star deviation being set a bit too low, causing a slight amount of degradation of the signal.

The amount of "excess" deviation seems to vary from radio to radio and it probably varies with operating frequency band (e.g. VHF or UHF) and the temperature and age of the radio as well. In our tests, the amount of deviation for the same radio also varied, depending on which deviation meter we looked at and how it was able to track the low-frequency components: Some deviation meters were fast enough in responding that this "frequency wobble" caused the meter to read only slightly high - that is, about +-1.4 to 1.5 kHz for a signal modulated to +-1.2 kHz, while others seemed to accurately read the total amount of frequency swing, which caused readings as high as +-1.7 kHz.

There is a solution to this: The use of the monitor scope. Many service monitors or communications test sets include an oscilloscope (either analog or digital) that may be read to determine the precise deviation of a signal being received. On these scopes, one can see the "frequency wobble" - but, if the scope is correctly adjusted, you can also make out the modulation waveform itself, apart from the "wobble" and determine the true amount of deviation from the data.

Some Icom transmitters (such as those used in various Icom repeaters and in the ID-1) do not directly modulate their synthesizer but, instead, perform quadrature modulation at an IF: These radios have far less "wobble" in their carrier frequency as the signal that the PLL itself is unmodulated.

Generating D-Star signals with analog test gear:

Because a D-Star signal is simply a special case of an FM signal generated by applying an appropriate baseband signal to an FM transmitter, it would make sense that one could apply this same type of baseband signal to a good-quality frequency modulator and create a D-Star signal. Some intrepid hombrewers have done this by adapting an Icom D-Star module for their own use and interfacing it with their own transmitter: This method works well, but it can be rather complicated and expensive.

There is another way: Using a "canned" D-Star transmission.

Because the baseband is simply audio, it would make sense that one could simply "record" this audio from a D-Star transmitter and play it back later - and this is, in fact, true! There are several caveats:

The source baseband audio must be "flat." What this means is that "discriminator audio" is required as this has no audio bandpass filtering or de-emphasis. Many service monitors or test sets have "demod" outputs, directly from the discriminator that have excellent frequency response - from near DC to well over 10 kHz. Note that many test sets also have various audio filters (such as CCITT or some type of equalization) that should be disabled.
The source baseband audio must be "clean." For a faithful recording to be made, it should be as free of noise and distortion as possible. If one is using a service monitor, this is easily accomplished by connecting the transmitter directly to the service monitor (as one would do to measure transmitter power) and make a recording. The fact that the bandwidth of the receiver in the service monitor is wider than that of a D-Star receiver is of little consequence if the signal fed to the service monitor in this way.

It has been reported that some radios with 9600 baud packet capability (such as the FT-817) may be used to demodulate a D-Star signal for recording, provided that the received signal is strong enough to be noise and interference-free.

The recording system must be capable of frequency response from a few 10's of Hz to at least 10 kHz. Fortunately, most computer sound cards fit the bill very nicely!
The playback system must be capable of frequency response from a few 10's of Hz to at least 10 kHz. Again, most computer sound cards work well for this.
The modulator must be a "flat" FM with no pre-emphasis, filtering or equalization of any kind. It must be capable of flat frequency response from a few 10's of Hz to about 10 kHz. Examples of of suitable modulators include:

Many service monitors have suitable "External Modulation" inputs. Just make sure that this equipment is configured for "flat" FM, without any filtering or equalization (such as CCITT) or pre-emphasis.
It has also been reported that some radios capable of 9600 baud packet operation (such as the Yaesu FT-817) may also be modulated with a D-Star baseband with excellent results.

For our initial test we simply connected the DEMOD output of a service monitor (a Schlumberger 4031, for the majority of our tests) tuned to the transmit frequency of the D-Star transmitter (an IC-91AD) to the Line Input of a laptop computer. Using a program such as Audacity, we then recorded the audio from the D-Star transmission to a .WAV file. We made sure to start the recording just before the transmitter was keyed up and to stop the recording after the transmitter was unkeyed to be sure to capture the "key" and "unkey" portions of the D-Star transmission.

For playback, we simply connected to the Line Output of the sound card to the external modulation input of the service monitor. We then played back the D-Star waveform, adjusting the deviation to +-1.2 kHz as described above.

Sample rates and encoding of baseband D-Star audio files:

For our initial recording, we set the sound card to a sample rate of 44.1 kHz with 16 bit audio to generate an uncompressed .WAV file. In later tests, we found that a sample rate of 22.05 kHz at 16 bits was also adequate with only a very slight (and probably insignificant) degradation in the baseband waveform.

We also experimented with resampling of the 44.1kHz/16 bit waveform down to an 8 kHz/8bit waveform using an audio editing program and found that, although the baseband waveform became slightly "ringy" owing to a slight amount of aliasing, there was little degradation in the ability of the D-Star receiver to decode the signal under poor conditions. Note that recording and then down-sampling to 8 kHz/8bit is likely to yield better results than recording at 8kHz/8 bits owing to the fact that the software resampling is likely to be of higher quality than "capturing" a signal live at 8 kHz and relying on the sound card's hardware and drivers to do the appropriate filtering "on the fly."

Comments:

The user should be aware that many sound cards (especially USB-types) can have sample rates that may differ from the nominal rate: Even the record and playback rates may be different! These differences can result in slightly different bit rate of the played-back "canned" D-Star audio, causing receive bit errors. It is unknown how much bit-rate difference the D-Star decoders can typically tolerate.
Some operating systems (such as XP and Vista) can cause slight sampling-rate errors, especially at rates other that 48kHz. This is usually due to the fact that the hardware is always operating at just one sample rate (such as 48 kHz) and is converted to other rates, on-the-fly, as needed - but this conversion may not be very precise. This, too, can cause errors due to slight differences in the bit rate.

D-Star MP3 files:

Later, we took the 44.1 kHz 16 bit audio file and used WinLame - a freeware program - to encode the original .WAV file to MP3. Through experimentation, we observed that recoding this .WAV file to 128kbit/second mono (with 44.1kHz sampling) produced a fairly good replica of the original D-Star baseband waveform and rates of lower than 64kbps (in mono) produced usable (although somewhat degraded) results. If stereo coding is used, a bitrate of 192 kbps or higher is recommended.

Note: Most MP3 encoding utilities do not offer the users specific options for encoding, such as the selection of sample rate and whether the result should be a stereo or mono .MP3 file. If this is the case, simply select the "highest" quality mode available until the quality of the playback waveform can be closely analyzed.

We then loaded the .MP3 files into a number of different portable audio players - some of them fairly expensive, and one of them extremely cheap (e.g. <$20) and we found that they all worked fine - as long as the equalization was disabled and any "inversion" (see below) was accommodated!

Playing back "canned" D-Star baseband recordings:

When doing a playback of a "canned" D-Star recording, there are a number of things to remember:

The "keyup" and "unkey" portions of the transmissions should be preserved to allow the D-Star receiver's codec to gracefully detect the beginning and ending of the transmissions. This means that the recording should be started before the transmitter being recorded is keyed up and stopped after the transmitter is unkeyed. In some cases, such as long-duration test tones, etc. where the speed of acquisition is unimportant, one may not need to worry about this.
When playing back, be absolutely certain to disable all audio equalization and special effects!

Many sound cards have treble, bass, equalization, reverb, echo, and/or "3D" effects - all of which should be set to zero or disabled before playback as any one of these can wreck the D-Star waveform!

Like sound cards, many portable audio players also have settings for equalization, and some may even have some other fancy audio effects - all of which should be disabled or set to a "flat" response.
You may need to do an audio phase inversion in playback to be able to decode the D-Star waveform.

If you experiment with multiple audio playback devices (e.g. different computers, portable audio players, etc.) or different service monitors, you should remember that each of these may or may not require a phase inversion. Because most audio players do not have a way to flip the audio phase, you must take this into account! Remember: If you use a compressed audio format, you may have to flip the phase before compressing it from the original .WAV file as the MP3 conversion itself *may* cause its own phase inversion.
Most audio editing software packages (like Audacity) can be used to "flip" the phase using the "invert" function
You may build a simple op-amp circuit that will allow you to reverse the phase with the flip of a switch.

Any data input or callsigns programmed into the transmitter at the time of the recording will be maintained through the recording. If you plan to use the "canned" D-Star recording as a test signal, make sure that your callsign and other configurations are set appropriately! If this original recording is made using a digital data mode, it may be possible to generate a rudimentary BER test system by transmitting this "canned" file through the system and analyzing the results.

What might be put on the "canned" recordings?

Some obvious examples are:

Plain speech announcing the test. This is a good test to see how things sound and to determine if decoding errors are occurring.
Standard tones. Using the procedures in the Icom service manual, one can feed a standard tone into the microphone connector at a known level. This is particularly useful in a system where D-Star audio may be converted to analog, as might be the case on an D-Star<>Analog gateway. (Note: Under certain conditions, the D-Star audio codec may produce unexpected results with a constant tone!)
If the transmitter was in data mode, known data may be transmitted for the purpose of analysis to determine the magnitude of data corruption or loss.

What can you do with the "canned" recordings?

Using a standard piece of analog test gear and a portable audio recorder, it is possible to generate a standard D-Star test signal to test the performance of a D-Star system in much the same way as one can test an analog radio system. This can include tests such as:

Receiver sensitivity. One can see if the receiver is working as well as it should! It is also possible to remotely check a repeater using test gear from a remote location to see if it is performing as it has in the past.
Desense. This is particularly important in a repeater system to determine if its own transmitter (or another transmitter) is reducing receive system sensitivity.
Interference. One can verify the performance of a system to see if other signals (adjacent, on-channel, or those resulting from intermodulation distortion) may be causing a problem.
Audio level tests - particularly if there is an interface to the "analog" world somewhere.
BER tests: If the "canned" recording includes known data, this may be analyzed to determine the error rate of the system, otherwise one may simply listen to a known-good recording and listen for errors.

Analyzing received transmissions with analog test gear:

Unfortunately, receiving a D-Star signal and decoding it back to audio with test gear is not so easy. Again, it may be possible to interface an Icom D-Star module or a so-called "D-Star Dongle" to a service monitor or test set - provided that a means of generating a GMSK baseband signal is provided - but these alternatives will likely require homebrewing and/or the necessity to lug a laptop around.

In many cases, your D-Star radio may be able to serve as a piece of test gear: With it, you can monitor the transmission to see if it seems to decode properly, and you may even be able to run rudimentary BER tests using the data mode.

One of the ways that an analog test set may be useful is to demodulate the receive signal and analyze the baseband waveform with the monitor scope. With it, one can see if the baseband waveform appears to be correct and if the "eye" pattern looks clean.

(More about analyzing the pattern on the monitor's scope will be added later.)

Two simple tests that can be performed are:

Frequency measurement. A standard test set should be able to accurately read the transmitted frequency.
Deviation. As mentioned above, the deviation meter - and especially the modulation scope - can be used to see if the D-Star signal is being modulated as it should.

Some caveats:

If an off-air signal is being monitored, remember that the test set likely has a much wider bandwidth than a typical D-Star receiver. This means that the received signal may be being degraded by adjacent-channel interference that would not bother a D-Star receiver.
Many test sets and service monitors are not particularly sensitive when used as receivers for off-air signals. If this is the case - or even if there is interference - additional noise may "fuzz up" the received signal, making measurements difficult.

BER Testing:

One aspect of the D-Star technology is that even though it is a digital system, there are very few tools available to the D-Star system designer that are available to the designer of almost any other digital wireless system. It is somewhat alarming that even the most basic of these tools - a Bit Error Rate (BER) indication is sadly lacking: This is somewhat surprising, as even the most inexpensive digital wireless devices - such as 802.x wireless cards, most cell phones, and satellite receivers, just to name a few, have available (albeit sometimes obscured) indicators of bit error rate.

At the time of this writing, Icom has published very little pertaining to D-Star system design and provided minuscule resources in the form of tools to allow the design, analysis, maintenance and diagnosis of problems in their D-Star products: It would have taken very little extra effort on their part to provide even rudimentary tools to the D-Star user and system designer!

Fortunately, all is not lost: It is possible that the intrepid homebrewer can devise a means to glean this information from the innards of their D-Star radio - provided that they aren't afraid to do a bit of hacking.

The GMSK modem used in the ID-1 and IC-91AD and its "BER" indicator:

The ID-1 uses a CMX589A GMSK modem chip for recovering data from the GMSK baseband signal from the MC3356 demodulator in both the low-speed (DV) and high-speed (DD) modes: The ID-1 uses a separate modulator to generate I/Q signals for transmit, leaving half of this chip unused. The ID-91AD, on the other hand, uses this chip for both reception of and generating the GMSK baseband waveforms.

The CMX589A is an integrated receiver/transmitter that is designed to receive and generate GMSK baseband waveforms. (A data sheet for this chip may be found here.) Interestingly, this chip has an "RX S/N" pin that outputs a signal that can be used to approximately estimate the signal-noise ratio of the received signal but alas, this connection (pin 23) is left disconnected in the ID-1 and IC-91AD: This is a pity, as the use of this pin might have proven helpful in determining optimal signal quality when setting up D-Star links - not to mention in everyday use by the casual user!

How would one use this signal? In the simplest form, simply "listening" to it with an audio amplifier gives a rough indication of the signal quality. The use of a few components (resistors, capacitors, etc.) can also be used to develop a voltage that would provide a repeatable, consistent indication of the received signal quality and this information could then be translated into a form that the system designer/maintainer or even the casual could use.

Comment: The ID-1 uses this chip only for receive while the IC-91AD uses it for both receive and transmit. It is worth noting that BT (the ratio of the transmit filter's -3dB bandwidth and the bit rate) is set for 0.5 in the IC-91AD's modulator, a reasonable compromise between occupied bandwidth and ISI.

BER output indication from the AMBE 2020 codec:

In perusing the datasheet of the AMBE 2020, one of the audio codecs that can be used in D-Star, one can see that the codec produces a word in its output stream (word 7, to be precise) that can be used to determine the BER. This is the same bitstream that is used by the radio to determine the status of the codec - among other things - but it would seem that this data BER is simply thrown away by the radio rather than being made available to the user. It may be possible that, in the future, Icom may make this data visible in some way, but in the meantime, one could, theoretically, "eavesdrop" on this bitstream with another microcontroller and bring this data out of the radio in a usable form.

There is good news, however, for the would-be programmers of the DV dongle: It is there in the code, available for the developer. (e.g. "BitErrors" in the structure "tOutFrame").

(The datasheet for the AMBE codec used in the Icom radios and the nature of this BER indication can be found on DVSI's website.)

Conclusions:

Even with "conventional" gear such as a service monitor or a communications test set, it is possible to use it to assess and troubleshoot a D-Star radio system with little extra equipment.

This page is a work in progress and is constantly being updated.

Disclaimers:

The above procedures have been tested using available test gear and Icom D-Star radios and are believed to be valid. It is likely that this information will, in the future, be updated and techniques refined.
It is up to you, the reader, to verify that this information is, in fact, correct and suitable for your needs. We cannot be held responsible for the use of the above information!
If you find that the above information is incorrect or incomplete, please contact the frequency coordinator using the link below.
Your mileage may vary!

Other Utah VHF Society links related to D-Star:

Utah VHF Society - D-Star Channel Spacing recommendations - Recommendations of channel for D-Star analog channels.
Observations of the codec used for D-Star - How does the codec used for D-Star respond if subjected to sounds other than those of the human voice? We decided to find out.

The following are FAQ's provided by the Utah VHF society. Note that these may topically overlap the links above:

Misc. links related to D-Star:

http://en.wikipedia.org/wiki/D-STAR - This has a general overview of D-Star.
http://www.arrl.org/FandES/field/regulations/techchar/D-STAR.pdf - This document specifies various aspects of D-Star and its protocols.
http://www.ccarc.net/images/CCARC-Spectrum%20Committee%20Report-%20Rev%203.pdf - This is a document produced by the Colorado frequency coordination body discussing D-Star channel spacing.
http://groups.yahoo.com/group/dstar_digital - This group harbors discussions and information about D-Star.
http://dstarutah.org - The Utah D-Star group

The above list is, by no means, exhaustive: Other information may be found via web searches.

This matter is open for discussion: If you have concerns or opinions one way or another, please make them known to the frequency coordinator at the email address below.

Questions, updates, or comments pertaining to this web page may be directed to the frequency coordinator.

Return to the Utah VHF Society home page.

Updated 20080909